android小技巧之不缓存的ViewPager

前言

在开发中我们会经常用到ViewPager这个类，比如引导页的啦，主页啦，等等之类的。

一般情况下，我在ViewPger中都是放的Framgnt,这样简单有方便，但是ViewPager却会默认的缓存当前页面的最近两个页面，于是问题就产生了，我们的需求是只有切换到哪页，哪页才创建或者加载布局，但是谷歌官方的ViewPager却没有自带这个选项,就算我们用setOffscreenPageLimit(0)也不能解决这个问题：

private static final int DEFAULT_OFFSCREEN_PAGES = 1;

public void setOffscreenPageLimit(int limit) {
if (limit < DEFAULT_OFFSCREEN_PAGES) {
Log.w(TAG, "Requested offscreen page limit " + limit + " too small; defaulting to " +
DEFAULT_OFFSCREEN_PAGES);
limit = DEFAULT_OFFSCREEN_PAGES;
}
if (limit != mOffscreenPageLimit) {
mOffscreenPageLimit = limit;
populate();
}
}

从上面的代码可以看到，就算设置0、负数，它也会默认为1，为了解决这个问题，有两个方法：

第一种：重写整个ViewPager

第二种：使用Fragment的setUserVisibleHint方法（相当于Activity中的onResume方法）

第一种方法

重写整个ViewPager，下面直接上代码，需要的直接拷贝过去就好

import android.content.Context;
import android.database.DataSetObserver;
import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.os.Parcel;
import android.os.Parcelable;
import android.os.SystemClock;
import android.support.v4.os.ParcelableCompat;
import android.support.v4.os.ParcelableCompatCreatorCallbacks;
import android.support.v4.view.KeyEventCompat;
import android.support.v4.view.MotionEventCompat;
import android.support.v4.view.PagerAdapter;
import android.support.v4.view.VelocityTrackerCompat;
import android.support.v4.view.ViewCompat;
import android.support.v4.view.ViewConfigurationCompat;
import android.support.v4.widget.EdgeEffectCompat;
import android.util.AttributeSet;
import android.util.Log;
import android.view.FocusFinder;
import android.view.KeyEvent;
import android.view.MotionEvent;
import android.view.SoundEffectConstants;
import android.view.VelocityTracker;
import android.view.View;
import android.view.ViewConfiguration;
import android.view.ViewGroup;
import android.view.ViewParent;
import android.view.accessibility.AccessibilityEvent;
import android.view.animation.Interpolator;
import android.widget.Scroller;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

/**
* 不缓存的ViewPager
* Created by Administrator on 2016/1/26.
*/
public class NoCacheViewPager extends ViewGroup {private static final String TAG = "LazyViewPager";
private static final boolean DEBUG = false;

private static final boolean USE_CACHE = false;

private static final int DEFAULT_OFFSCREEN_PAGES = 0;// 默认的加载页面,ViewPager是1个,所以会加载两个Fragment
private static final int MAX_SETTLE_DURATION = 600; // ms
static class ItemInfo {
Object object;
int position;
boolean scrolling;
}

private static final Comparator<ItemInfo> COMPARATOR = new Comparator<ItemInfo>() {
@Override
public int compare(ItemInfo lhs, ItemInfo rhs) {
return lhs.position - rhs.position;
}
};

private static final Interpolator sInterpolator = new Interpolator() {
public float getInterpolation(float t) {
// _o(t) = t * t * ((tension + 1) * t + tension)
// o(t) = _o(t - 1) + 1
t -= 1.0f;
return t * t * t + 1.0f;
}
};

private final ArrayList<ItemInfo> mItems = new ArrayList<ItemInfo>();

private PagerAdapter mAdapter;
private int mCurItem; // Index of currently displayed page.
private int mRestoredCurItem = -1;
private Parcelable mRestoredAdapterState = null;
private ClassLoader mRestoredClassLoader = null;
private Scroller mScroller;
private PagerObserver mObserver;

private int mPageMargin;
private Drawable mMarginDrawable;

private int mChildWidthMeasureSpec;
private int mChildHeightMeasureSpec;
private boolean mInLayout;

private boolean mScrollingCacheEnabled;

private boolean mPopulatePending;
private boolean mScrolling;
private int mOffscreenPageLimit = DEFAULT_OFFSCREEN_PAGES;

private boolean mIsBeingDragged;
private boolean mIsUnableToDrag;
private int mTouchSlop;
private float mInitialMotionX;
/**
* Position of the last motion event.
*/
private float mLastMotionX;
private float mLastMotionY;
/**
* ID of the active pointer. This is used to retain consistency during
* drags/flings if multiple pointers are used.
*/
private int mActivePointerId = INVALID_POINTER;
/**
* Sentinel value for no current active pointer. Used by
* {@link #mActivePointerId}.
*/
private static final int INVALID_POINTER = -1;

/**
* Determines speed during touch scrolling
*/
private VelocityTracker mVelocityTracker;
private int mMinimumVelocity;
private int mMaximumVelocity;
private float mBaseLineFlingVelocity;
private float mFlingVelocityInfluence;

private boolean mFakeDragging;
private long mFakeDragBeginTime;

private EdgeEffectCompat mLeftEdge;
private EdgeEffectCompat mRightEdge;

private boolean mFirstLayout = true;

private OnPageChangeListener mOnPageChangeListener;

/**
* Indicates that the pager is in an idle, settled state. The current page
* is fully in view and no animation is in progress.
*/
public static final int SCROLL_STATE_IDLE = 0;

/**
* Indicates that the pager is currently being dragged by the user.
*/
public static final int SCROLL_STATE_DRAGGING = 1;

/**
* Indicates that the pager is in the process of settling to a final
* position.
*/
public static final int SCROLL_STATE_SETTLING = 2;

private int mScrollState = SCROLL_STATE_IDLE;

/**
* Callback interface for responding to changing state of the selected page.
*/
public interface OnPageChangeListener {

/**
* This method will be invoked when the current page is scrolled, either
* as part of a programmatically initiated smooth scroll or a user
* initiated touch scroll.
*
* @param position
*            Position index of the first page currently being
*            displayed. Page position+1 will be visible if
*            positionOffset is nonzero.
* @param positionOffset
*            Value from [0, 1) indicating the offset from the page at
*            position.
* @param positionOffsetPixels
*            Value in pixels indicating the offset from position.
*/
public void onPageScrolled(int position, float positionOffset,
int positionOffsetPixels);

/**
* This method will be invoked when a new page becomes selected.
* Animation is not necessarily complete.
*
* @param position
*            Position index of the new selected page.
*/
public void onPageSelected(int position);

/**
* Called when the scroll state changes. Useful for discovering when the
* user begins dragging, when the pager is automatically settling to the
* current page, or when it is fully stopped/idle.
*
* @param state
*            The new scroll state.
* @see android.support.v4.view.ViewPager#SCROLL_STATE_IDLE
* @see android.support.v4.view.ViewPager#SCROLL_STATE_DRAGGING
* @see android.support.v4.view.ViewPager#SCROLL_STATE_SETTLING
*/
public void onPageScrollStateChanged(int state);
}

/**
* Simple implementation of the
* interface with stub implementations of each method. Extend this if you do
* not intend to override every method of
*/
public static class SimpleOnPageChangeListener implements
OnPageChangeListener {
@Override
public void onPageScrolled(int position, float positionOffset,
int positionOffsetPixels) {
// This space for rent
}

@Override
public void onPageSelected(int position) {
// This space for rent
}

@Override
public void onPageScrollStateChanged(int state) {
// This space for rent
}
}

public NoCacheViewPager(Context context) {
super(context);
initViewPager();
}

public NoCacheViewPager(Context context, AttributeSet attrs) {
super(context, attrs);
initViewPager();
}

void initViewPager() {
setWillNotDraw(false);
setDescendantFocusability(FOCUS_AFTER_DESCENDANTS);
setFocusable(true);
final Context context = getContext();
mScroller = new Scroller(context, sInterpolator);
final ViewConfiguration configuration = ViewConfiguration.get(context);
mTouchSlop = ViewConfigurationCompat
.getScaledPagingTouchSlop(configuration);
mMinimumVelocity = configuration.getScaledMinimumFlingVelocity();
mMaximumVelocity = configuration.getScaledMaximumFlingVelocity();
mLeftEdge = new EdgeEffectCompat(context);
mRightEdge = new EdgeEffectCompat(context);

float density = context.getResources().getDisplayMetrics().density;
mBaseLineFlingVelocity = 2500.0f * density;
mFlingVelocityInfluence = 0.4f;
}

private void setScrollState(int newState) {
if (mScrollState == newState) {
return;
}

mScrollState = newState;
if (mOnPageChangeListener != null) {
mOnPageChangeListener.onPageScrollStateChanged(newState);
}
}

public void setAdapter(PagerAdapter adapter) {
if (mAdapter != null) {
mAdapter.unregisterDataSetObserver(mObserver);
mAdapter.startUpdate(this);
for (int i = 0; i < mItems.size(); i++) {
final ItemInfo ii = mItems.get(i);
mAdapter.destroyItem(this, ii.position, ii.object);
}
mAdapter.finishUpdate(this);
mItems.clear();
removeAllViews();
mCurItem = 0;
scrollTo(0, 0);
}

mAdapter = adapter;

if (mAdapter != null) {
if (mObserver == null) {
mObserver = new PagerObserver();
}
mAdapter.registerDataSetObserver(mObserver);
mPopulatePending = false;
if (mRestoredCurItem >= 0) {
mAdapter.restoreState(mRestoredAdapterState,
mRestoredClassLoader);
setCurrentItemInternal(mRestoredCurItem, false, true);
mRestoredCurItem = -1;
mRestoredAdapterState = null;
mRestoredClassLoader = null;
} else {
populate();
}
}
}

public PagerAdapter getAdapter() {
return mAdapter;
}

/**
* Set the currently selected page. If the ViewPager has already been
* through its first layout there will be a smooth animated transition
* between the current item and the specified item.
*
* @param item
*            Item index to select
*/
public void setCurrentItem(int item) {
mPopulatePending = false;
setCurrentItemInternal(item, !mFirstLayout, false);
}

/**
* Set the currently selected page.
*
* @param item
*            Item index to select
* @param smoothScroll
*            True to smoothly scroll to the new item, false to transition
*            immediately
*/
public void setCurrentItem(int item, boolean smoothScroll) {
mPopulatePending = false;
setCurrentItemInternal(item, smoothScroll, false);
}

public int getCurrentItem() {
return mCurItem;
}

void setCurrentItemInternal(int item, boolean smoothScroll, boolean always) {
setCurrentItemInternal(item, smoothScroll, always, 0);
}

void setCurrentItemInternal(int item, boolean smoothScroll, boolean always,
int velocity) {
if (mAdapter == null || mAdapter.getCount() <= 0) {
setScrollingCacheEnabled(false);
return;
}
if (!always && mCurItem == item && mItems.size() != 0) {
setScrollingCacheEnabled(false);
return;
}
if (item < 0) {
item = 0;
} else if (item >= mAdapter.getCount()) {
item = mAdapter.getCount() - 1;
}
final int pageLimit = mOffscreenPageLimit;
if (item > (mCurItem + pageLimit) || item < (mCurItem - pageLimit)) {
// We are doing a jump by more than one page. To avoid
// glitches, we want to keep all current pages in the view
// until the scroll ends.
for (int i = 0; i < mItems.size(); i++) {
mItems.get(i).scrolling = true;
}
}

final boolean dispatchSelected = mCurItem != item;
mCurItem = item;
populate();
final int destX = (getWidth() + mPageMargin) * item;
if (smoothScroll) {
smoothScrollTo(destX, 0, velocity);
if (dispatchSelected && mOnPageChangeListener != null) {
mOnPageChangeListener.onPageSelected(item);
}
} else {
if (dispatchSelected && mOnPageChangeListener != null) {
mOnPageChangeListener.onPageSelected(item);
}
completeScroll();
scrollTo(destX, 0);
}
}

public void setOnPageChangeListener(OnPageChangeListener listener) {
mOnPageChangeListener = listener;
}

/**
* Returns the number of pages that will be retained to either side of the
* current page in the view hierarchy in an idle state. Defaults to 1.
*
* @return How many pages will be kept offscreen on either side
* @see #setOffscreenPageLimit(int)
*/
public int getOffscreenPageLimit() {
return mOffscreenPageLimit;
}

/**
* Set the number of pages that should be retained to either side of the
* current page in the view hierarchy in an idle state. Pages beyond this
* limit will be recreated from the adapter when needed.
*
* <p>
* This is offered as an optimization. If you know in advance the number of
* pages you will need to support or have lazy-loading mechanisms in place
* on your pages, tweaking this setting can have benefits in perceived
* smoothness of paging animations and interaction. If you have a small
* number of pages (3-4) that you can keep active all at once, less time
* will be spent in layout for newly created view subtrees as the user pages
* back and forth.
* </p>
*
* <p>
* You should keep this limit low, especially if your pages have complex
* layouts. This setting defaults to 1.
* </p>
*
* @param limit
*            How many pages will be kept offscreen in an idle state.
*/
public void setOffscreenPageLimit(int limit) {
if (limit < DEFAULT_OFFSCREEN_PAGES) {
Log.w(TAG, "Requested offscreen page limit " + limit
+ " too small; defaulting to " + DEFAULT_OFFSCREEN_PAGES);
limit = DEFAULT_OFFSCREEN_PAGES;
}
if (limit != mOffscreenPageLimit) {
mOffscreenPageLimit = limit;
populate();
}
}

/**
* Set the margin between pages.
*
* @param marginPixels
*            Distance between adjacent pages in pixels
* @see #getPageMargin()
* @see #setPageMarginDrawable(Drawable)
* @see #setPageMarginDrawable(int)
*/
public void setPageMargin(int marginPixels) {
final int oldMargin = mPageMargin;
mPageMargin = marginPixels;

final int width = getWidth();
recomputeScrollPosition(width, width, marginPixels, oldMargin);

requestLayout();
}

/**
* Return the margin between pages.
*
* @return The size of the margin in pixels
*/
public int getPageMargin() {
return mPageMargin;
}

/**
* Set a drawable that will be used to fill the margin between pages.
*
* @param d
*            Drawable to display between pages
*/
public void setPageMarginDrawable(Drawable d) {
mMarginDrawable = d;
if (d != null)
refreshDrawableState();
setWillNotDraw(d == null);
invalidate();
}

/**
* Set a drawable that will be used to fill the margin between pages.
*
* @param resId
*            Resource ID of a drawable to display between pages
*/
public void setPageMarginDrawable(int resId) {
setPageMarginDrawable(getContext().getResources().getDrawable(resId));
}

@Override
protected boolean verifyDrawable(Drawable who) {
return super.verifyDrawable(who) || who == mMarginDrawable;
}

@Override
protected void drawableStateChanged() {
super.drawableStateChanged();
final Drawable d = mMarginDrawable;
if (d != null && d.isStateful()) {
d.setState(getDrawableState());
}
}

// We want the duration of the page snap animation to be influenced by the
// distance that
// the screen has to travel, however, we don't want this duration to be
// effected in a
// purely linear fashion. Instead, we use this method to moderate the effect
// that the distance
// of travel has on the overall snap duration.
float distanceInfluenceForSnapDuration(float f) {
f -= 0.5f; // center the values about 0.
f *= 0.3f * Math.PI / 2.0f;
return (float) Math.sin(f);
}

/**
* Like {@link View#scrollBy}, but scroll smoothly instead of
* immediately.
*
* @param x
*            the number of pixels to scroll by on the X axis
* @param y
*            the number of pixels to scroll by on the Y axis
*/
void smoothScrollTo(int x, int y) {
smoothScrollTo(x, y, 0);
}

/**
* Like {@link View#scrollBy}, but scroll smoothly instead of
* immediately.
*
* @param x
*            the number of pixels to scroll by on the X axis
* @param y
*            the number of pixels to scroll by on the Y axis
* @param velocity
*            the velocity associated with a fling, if applicable. (0
*            otherwise)
*/
void smoothScrollTo(int x, int y, int velocity) {
if (getChildCount() == 0) {
// Nothing to do.
setScrollingCacheEnabled(false);
return;
}
int sx = getScrollX();
int sy = getScrollY();
int dx = x - sx;
int dy = y - sy;
if (dx == 0 && dy == 0) {
completeScroll();
setScrollState(SCROLL_STATE_IDLE);
return;
}

setScrollingCacheEnabled(true);
mScrolling = true;
setScrollState(SCROLL_STATE_SETTLING);

final float pageDelta = (float) Math.abs(dx)
/ (getWidth() + mPageMargin);
int duration = (int) (pageDelta * 100);

velocity = Math.abs(velocity);
if (velocity > 0) {
duration += (duration / (velocity / mBaseLineFlingVelocity))
* mFlingVelocityInfluence;
} else {
duration += 100;
}
duration = Math.min(duration, MAX_SETTLE_DURATION);

mScroller.startScroll(sx, sy, dx, dy, duration);
invalidate();
}

void addNewItem(int position, int index) {
ItemInfo ii = new ItemInfo();
ii.position = position;
ii.object = mAdapter.instantiateItem(this, position);
if (index < 0) {
mItems.add(ii);
} else {
mItems.add(index, ii);
}
}

void dataSetChanged() {
// This method only gets called if our observer is attached, so mAdapter
// is non-null.

boolean needPopulate = mItems.size() < 3
&& mItems.size() < mAdapter.getCount();
int newCurrItem = -1;

for (int i = 0; i < mItems.size(); i++) {
final ItemInfo ii = mItems.get(i);
final int newPos = mAdapter.getItemPosition(ii.object);

if (newPos == PagerAdapter.POSITION_UNCHANGED) {
continue;
}

if (newPos == PagerAdapter.POSITION_NONE) {
mItems.remove(i);
i--;
mAdapter.destroyItem(this, ii.position, ii.object);
needPopulate = true;

if (mCurItem == ii.position) {
// Keep the current item in the valid range
newCurrItem = Math.max(0,
Math.min(mCurItem, mAdapter.getCount() - 1));
}
continue;
}

if (ii.position != newPos) {
if (ii.position == mCurItem) {
// Our current item changed position. Follow it.
newCurrItem = newPos;
}

ii.position = newPos;
needPopulate = true;
}
}

Collections.sort(mItems, COMPARATOR);

if (newCurrItem >= 0) {
// TODO This currently causes a jump.
setCurrentItemInternal(newCurrItem, false, true);
needPopulate = true;
}
if (needPopulate) {
populate();
requestLayout();
}
}

void populate() {
if (mAdapter == null) {
return;
}

// Bail now if we are waiting to populate. This is to hold off
// on creating views from the time the user releases their finger to
// fling to a new position until we have finished the scroll to
// that position, avoiding glitches from happening at that point.
if (mPopulatePending) {
if (DEBUG)
Log.i(TAG, "populate is pending, skipping for now...");
return;
}

// Also, don't populate until we are attached to a window. This is to
// avoid trying to populate before we have restored our view hierarchy
// state and conflicting with what is restored.
if (getWindowToken() == null) {
return;
}

mAdapter.startUpdate(this);

final int pageLimit = mOffscreenPageLimit;
final int startPos = Math.max(0, mCurItem - pageLimit);
final int N = mAdapter.getCount();
final int endPos = Math.min(N - 1, mCurItem + pageLimit);

if (DEBUG)
Log.v(TAG, "populating: startPos=" + startPos + " endPos=" + endPos);

// Add and remove pages in the existing list.
int lastPos = -1;
for (int i = 0; i < mItems.size(); i++) {
ItemInfo ii = mItems.get(i);
if ((ii.position < startPos || ii.position > endPos)
&& !ii.scrolling) {
if (DEBUG)
Log.i(TAG, "removing: " + ii.position + " @ " + i);
mItems.remove(i);
i--;
mAdapter.destroyItem(this, ii.position, ii.object);
} else if (lastPos < endPos && ii.position > startPos) {
// The next item is outside of our range, but we have a gap
// between it and the last item where we want to have a page
// shown. Fill in the gap.
lastPos++;
if (lastPos < startPos) {
lastPos = startPos;
}
while (lastPos <= endPos && lastPos < ii.position) {
if (DEBUG)
Log.i(TAG, "inserting: " + lastPos + " @ " + i);
addNewItem(lastPos, i);
lastPos++;
i++;
}
}
lastPos = ii.position;
}

// Add any new pages we need at the end.
lastPos = mItems.size() > 0 ? mItems.get(mItems.size() - 1).position
: -1;
if (lastPos < endPos) {
lastPos++;
lastPos = lastPos > startPos ? lastPos : startPos;
while (lastPos <= endPos) {
if (DEBUG)
Log.i(TAG, "appending: " + lastPos);
addNewItem(lastPos, -1);
lastPos++;
}
}

if (DEBUG) {
Log.i(TAG, "Current page list:");
for (int i = 0; i < mItems.size(); i++) {
Log.i(TAG, "#" + i + ": page " + mItems.get(i).position);
}
}

ItemInfo curItem = null;
for (int i = 0; i < mItems.size(); i++) {
if (mItems.get(i).position == mCurItem) {
curItem = mItems.get(i);
break;
}
}
mAdapter.setPrimaryItem(this, mCurItem,
curItem != null ? curItem.object : null);

mAdapter.finishUpdate(this);

if (hasFocus()) {
View currentFocused = findFocus();
ItemInfo ii = currentFocused != null ? infoForAnyChild(currentFocused)
: null;
if (ii == null || ii.position != mCurItem) {
for (int i = 0; i < getChildCount(); i++) {
View child = getChildAt(i);
ii = infoForChild(child);
if (ii != null && ii.position == mCurItem) {
if (child.requestFocus(FOCUS_FORWARD)) {
break;
}
}
}
}
}
}

public static class SavedState extends BaseSavedState {
int position;
Parcelable adapterState;
ClassLoader loader;

public SavedState(Parcelable superState) {
super(superState);
}

@Override
public void writeToParcel(Parcel out, int flags) {
super.writeToParcel(out, flags);
out.writeInt(position);
out.writeParcelable(adapterState, flags);
}

@Override
public String toString() {
return "FragmentPager.SavedState{"
+ Integer.toHexString(System.identityHashCode(this))
+ " position=" + position + "}";
}

public static final Creator<SavedState> CREATOR = ParcelableCompat
.newCreator(new ParcelableCompatCreatorCallbacks<SavedState>() {
@Override
public SavedState createFromParcel(Parcel in,
ClassLoader loader) {
return new SavedState(in, loader);
}

@Override
public SavedState[] newArray(int size) {
return new SavedState[size];
}
});

SavedState(Parcel in, ClassLoader loader) {
super(in);
if (loader == null) {
loader = getClass().getClassLoader();
}
position = in.readInt();
adapterState = in.readParcelable(loader);
this.loader = loader;
}
}

@Override
public Parcelable onSaveInstanceState() {
Parcelable superState = super.onSaveInstanceState();
SavedState ss = new SavedState(superState);
ss.position = mCurItem;
if (mAdapter != null) {
ss.adapterState = mAdapter.saveState();
}
return ss;
}

@Override
public void onRestoreInstanceState(Parcelable state) {
if (!(state instanceof SavedState)) {
super.onRestoreInstanceState(state);
return;
}

SavedState ss = (SavedState) state;
super.onRestoreInstanceState(ss.getSuperState());

if (mAdapter != null) {
mAdapter.restoreState(ss.adapterState, ss.loader);
setCurrentItemInternal(ss.position, false, true);
} else {
mRestoredCurItem = ss.position;
mRestoredAdapterState = ss.adapterState;
mRestoredClassLoader = ss.loader;
}
}

@Override
public void addView(View child, int index, LayoutParams params) {
if (mInLayout) {
addViewInLayout(child, index, params);
child.measure(mChildWidthMeasureSpec, mChildHeightMeasureSpec);
} else {
super.addView(child, index, params);
}

if (USE_CACHE) {
if (child.getVisibility() != GONE) {
child.setDrawingCacheEnabled(mScrollingCacheEnabled);
} else {
child.setDrawingCacheEnabled(false);
}
}
}

ItemInfo infoForChild(View child) {
for (int i = 0; i < mItems.size(); i++) {
ItemInfo ii = mItems.get(i);
if (mAdapter.isViewFromObject(child, ii.object)) {
return ii;
}
}
return null;
}

ItemInfo infoForAnyChild(View child) {
ViewParent parent;
while ((parent = child.getParent()) != this) {
if (parent == null || !(parent instanceof View)) {
return null;
}
child = (View) parent;
}
return infoForChild(child);
}

@Override
protected void onAttachedToWindow() {
super.onAttachedToWindow();
mFirstLayout = true;
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// For simple implementation, or internal size is always 0.
// We depend on the container to specify the layout size of
// our view. We can't really know what it is since we will be
// adding and removing different arbitrary views and do not
// want the layout to change as this happens.
setMeasuredDimension(getDefaultSize(0, widthMeasureSpec),
getDefaultSize(0, heightMeasureSpec));

// Children are just made to fill our space.
mChildWidthMeasureSpec = MeasureSpec.makeMeasureSpec(getMeasuredWidth()
- getPaddingLeft() - getPaddingRight(), MeasureSpec.EXACTLY);
mChildHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
getMeasuredHeight() - getPaddingTop() - getPaddingBottom(),
MeasureSpec.EXACTLY);

// Make sure we have created all fragments that we need to have shown.
mInLayout = true;
populate();
mInLayout = false;

// Make sure all children have been properly measured.
final int size = getChildCount();
for (int i = 0; i < size; ++i) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
if (DEBUG)
Log.v(TAG, "Measuring #" + i + " " + child + ": "
+ mChildWidthMeasureSpec);
child.measure(mChildWidthMeasureSpec, mChildHeightMeasureSpec);
}
}
}

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
super.onSizeChanged(w, h, oldw, oldh);

// Make sure scroll position is set correctly.
if (w != oldw) {
recomputeScrollPosition(w, oldw, mPageMargin, mPageMargin);
}
}

private void recomputeScrollPosition(int width, int oldWidth, int margin,
int oldMargin) {
final int widthWithMargin = width + margin;
if (oldWidth > 0) {
final int oldScrollPos = getScrollX();
final int oldwwm = oldWidth + oldMargin;
final int oldScrollItem = oldScrollPos / oldwwm;
final float scrollOffset = (float) (oldScrollPos % oldwwm) / oldwwm;
final int scrollPos = (int) ((oldScrollItem + scrollOffset) * widthWithMargin);
scrollTo(scrollPos, getScrollY());
if (!mScroller.isFinished()) {
// We now return to your regularly scheduled scroll, already in
// progress.
final int newDuration = mScroller.getDuration()
- mScroller.timePassed();
mScroller.startScroll(scrollPos, 0, mCurItem * widthWithMargin,
0, newDuration);
}
} else {
int scrollPos = mCurItem * widthWithMargin;
if (scrollPos != getScrollX()) {
completeScroll();
scrollTo(scrollPos, getScrollY());
}
}
}

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
mInLayout = true;
populate();
mInLayout = false;

final int count = getChildCount();
final int width = r - l;

for (int i = 0; i < count; i++) {
View child = getChildAt(i);
ItemInfo ii;
if (child.getVisibility() != GONE
&& (ii = infoForChild(child)) != null) {
int loff = (width + mPageMargin) * ii.position;
int childLeft = getPaddingLeft() + loff;
int childTop = getPaddingTop();
if (DEBUG)
Log.v(TAG,
"Positioning #" + i + " " + child + " f="
+ ii.object + ":" + childLeft + ","
+ childTop + " " + child.getMeasuredWidth()
+ "x" + child.getMeasuredHeight());
child.layout(childLeft, childTop,
childLeft + child.getMeasuredWidth(),
childTop + child.getMeasuredHeight());
}
}
mFirstLayout = false;
}

@Override
public void computeScroll() {
if (DEBUG)
Log.i(TAG, "computeScroll: finished=" + mScroller.isFinished());
if (!mScroller.isFinished()) {
if (mScroller.computeScrollOffset()) {
if (DEBUG)
Log.i(TAG, "computeScroll: still scrolling");
int oldX = getScrollX();
int oldY = getScrollY();
int x = mScroller.getCurrX();
int y = mScroller.getCurrY();

if (oldX != x || oldY != y) {
scrollTo(x, y);
}

if (mOnPageChangeListener != null) {
final int widthWithMargin = getWidth() + mPageMargin;
final int position = x / widthWithMargin;
final int offsetPixels = x % widthWithMargin;
final float offset = (float) offsetPixels / widthWithMargin;
mOnPageChangeListener.onPageScrolled(position, offset,
offsetPixels);
}

// Keep on drawing until the animation has finished.
invalidate();
return;
}
}

// Done with scroll, clean up state.
completeScroll();
}

private void completeScroll() {
boolean needPopulate = mScrolling;
if (needPopulate) {
// Done with scroll, no longer want to cache view drawing.
setScrollingCacheEnabled(false);
mScroller.abortAnimation();
int oldX = getScrollX();
int oldY = getScrollY();
int x = mScroller.getCurrX();
int y = mScroller.getCurrY();
if (oldX != x || oldY != y) {
scrollTo(x, y);
}
setScrollState(SCROLL_STATE_IDLE);
}
mPopulatePending = false;
mScrolling = false;
for (int i = 0; i < mItems.size(); i++) {
ItemInfo ii = mItems.get(i);
if (ii.scrolling) {
needPopulate = true;
ii.scrolling = false;
}
}
if (needPopulate) {
populate();
}
}

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
/*
* This method JUST determines whether we want to intercept the motion.
* If we return true, onMotionEvent will be called and we do the actual
* scrolling there.
*/

final int action = ev.getAction() & MotionEventCompat.ACTION_MASK;

// Always take care of the touch gesture being complete.
if (action == MotionEvent.ACTION_CANCEL
|| action == MotionEvent.ACTION_UP) {
// Release the drag.
if (DEBUG)
Log.v(TAG, "Intercept done!");
mIsBeingDragged = false;
mIsUnableToDrag = false;
mActivePointerId = INVALID_POINTER;
return false;
}

// Nothing more to do here if we have decided whether or not we
// are dragging.
if (action != MotionEvent.ACTION_DOWN) {
if (mIsBeingDragged) {
if (DEBUG)
Log.v(TAG, "Intercept returning true!");
return true;
}
if (mIsUnableToDrag) {
if (DEBUG)
Log.v(TAG, "Intercept returning false!");
return false;
}
}

switch (action) {
case MotionEvent.ACTION_MOVE: {
/*
* mIsBeingDragged == false, otherwise the shortcut would have
* caught it. Check whether the user has moved far enough from his
* original down touch.
*/

/*
* Locally do absolute value. mLastMotionY is set to the y value of
* the down event.
*/
final int activePointerId = mActivePointerId;
if (activePointerId == INVALID_POINTER) {
// If we don't have a valid id, the touch down wasn't on
// content.
break;
}

final int pointerIndex = MotionEventCompat.findPointerIndex(ev,
activePointerId);
final float x = MotionEventCompat.getX(ev, pointerIndex);
final float dx = x - mLastMotionX;
final float xDiff = Math.abs(dx);
final float y = MotionEventCompat.getY(ev, pointerIndex);
final float yDiff = Math.abs(y - mLastMotionY);
final int scrollX = getScrollX();
final boolean atEdge = (dx > 0 && scrollX == 0)
|| (dx < 0 && mAdapter != null && scrollX >= (mAdapter
.getCount() - 1) * getWidth() - 1);
if (DEBUG)
Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff + ","
+ yDiff);

if (canScroll(this, false, (int) dx, (int) x, (int) y)) {
// Nested view has scrollable area under this point. Let it be
// handled there.
mInitialMotionX = mLastMotionX = x;
mLastMotionY = y;
return false;
}
if (xDiff > mTouchSlop && xDiff > yDiff) {
if (DEBUG)
Log.v(TAG, "Starting drag!");
mIsBeingDragged = true;
setScrollState(SCROLL_STATE_DRAGGING);
mLastMotionX = x;
setScrollingCacheEnabled(true);
} else {
if (yDiff > mTouchSlop) {
// The finger has moved enough in the vertical
// direction to be counted as a drag... abort
// any attempt to drag horizontally, to work correctly
// with children that have scrolling containers.
if (DEBUG)
Log.v(TAG, "Starting unable to drag!");
mIsUnableToDrag = true;
}
}
break;
}

case MotionEvent.ACTION_DOWN: {
/*
* Remember location of down touch. ACTION_DOWN always refers to
* pointer index 0.
*/
mLastMotionX = mInitialMotionX = ev.getX();
mLastMotionY = ev.getY();
mActivePointerId = MotionEventCompat.getPointerId(ev, 0);

if (mScrollState == SCROLL_STATE_SETTLING) {
// Let the user 'catch' the pager as it animates.
mIsBeingDragged = true;
mIsUnableToDrag = false;
setScrollState(SCROLL_STATE_DRAGGING);
} else {
completeScroll();
mIsBeingDragged = false;
mIsUnableToDrag = false;
}

if (DEBUG)
Log.v(TAG, "Down at " + mLastMotionX + "," + mLastMotionY
+ " mIsBeingDragged=" + mIsBeingDragged
+ "mIsUnableToDrag=" + mIsUnableToDrag);
break;
}

case MotionEventCompat.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
}

/*
* The only time we want to intercept motion events is if we are in the
* drag mode.
*/
return mIsBeingDragged;
}

@Override
public boolean onTouchEvent(MotionEvent ev) {
if (mFakeDragging) {
// A fake drag is in progress already, ignore this real one
// but still eat the touch events.
// (It is likely that the user is multi-touching the screen.)
return true;
}

if (ev.getAction() == MotionEvent.ACTION_DOWN && ev.getEdgeFlags() != 0) {
// Don't handle edge touches immediately -- they may actually belong
// to one of our
// descendants.
return false;
}

if (mAdapter == null || mAdapter.getCount() == 0) {
// Nothing to present or scroll; nothing to touch.
return false;
}

if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
}
mVelocityTracker.addMovement(ev);

final int action = ev.getAction();
boolean needsInvalidate = false;

switch (action & MotionEventCompat.ACTION_MASK) {
case MotionEvent.ACTION_DOWN: {
/*
* If being flinged and user touches, stop the fling. isFinished
* will be false if being flinged.
*/
completeScroll();

// Remember where the motion event started
mLastMotionX = mInitialMotionX = ev.getX();
mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
break;
}
case MotionEvent.ACTION_MOVE:
if (!mIsBeingDragged) {
final int pointerIndex = MotionEventCompat.findPointerIndex(ev,
mActivePointerId);
final float x = MotionEventCompat.getX(ev, pointerIndex);
final float xDiff = Math.abs(x - mLastMotionX);
final float y = MotionEventCompat.getY(ev, pointerIndex);
final float yDiff = Math.abs(y - mLastMotionY);
if (DEBUG)
Log.v(TAG, "Moved x to " + x + "," + y + " diff=" + xDiff
+ "," + yDiff);
if (xDiff > mTouchSlop && xDiff > yDiff) {
if (DEBUG)
Log.v(TAG, "Starting drag!");
mIsBeingDragged = true;
mLastMotionX = x;
setScrollState(SCROLL_STATE_DRAGGING);
setScrollingCacheEnabled(true);
}
}
if (mIsBeingDragged) {
// Scroll to follow the motion event
final int activePointerIndex = MotionEventCompat
.findPointerIndex(ev, mActivePointerId);
final float x = MotionEventCompat.getX(ev, activePointerIndex);
final float deltaX = mLastMotionX - x;
mLastMotionX = x;
float oldScrollX = getScrollX();
float scrollX = oldScrollX + deltaX;
final int width = getWidth();
final int widthWithMargin = width + mPageMargin;

final int lastItemIndex = mAdapter.getCount() - 1;
final float leftBound = Math.max(0, (mCurItem - 1)
* widthWithMargin);
final float rightBound = Math.min(mCurItem + 1, lastItemIndex)
* widthWithMargin;
if (scrollX < leftBound) {
if (leftBound == 0) {
float over = -scrollX;
needsInvalidate = mLeftEdge.onPull(over / width);
}
scrollX = leftBound;
} else if (scrollX > rightBound) {
if (rightBound == lastItemIndex * widthWithMargin) {
float over = scrollX - rightBound;
needsInvalidate = mRightEdge.onPull(over / width);
}
scrollX = rightBound;
}
// Don't lose the rounded component
mLastMotionX += scrollX - (int) scrollX;
scrollTo((int) scrollX, getScrollY());
if (mOnPageChangeListener != null) {
final int position = (int) scrollX / widthWithMargin;
final int positionOffsetPixels = (int) scrollX
% widthWithMargin;
final float positionOffset = (float) positionOffsetPixels
/ widthWithMargin;
mOnPageChangeListener.onPageScrolled(position,
positionOffset, positionOffsetPixels);
}
}
break;
case MotionEvent.ACTION_UP:
if (mIsBeingDragged) {
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
int initialVelocity = (int) VelocityTrackerCompat.getXVelocity(
velocityTracker, mActivePointerId);
mPopulatePending = true;
final int widthWithMargin = getWidth() + mPageMargin;
final int scrollX = getScrollX();
final int currentPage = scrollX / widthWithMargin;
int nextPage = initialVelocity > 0 ? currentPage
: currentPage + 1;
setCurrentItemInternal(nextPage, true, true, initialVelocity);

mActivePointerId = INVALID_POINTER;
endDrag();
needsInvalidate = mLeftEdge.onRelease()
| mRightEdge.onRelease();
}
break;
case MotionEvent.ACTION_CANCEL:
if (mIsBeingDragged) {
setCurrentItemInternal(mCurItem, true, true);
mActivePointerId = INVALID_POINTER;
endDrag();
needsInvalidate = mLeftEdge.onRelease()
| mRightEdge.onRelease();
}
break;
case MotionEventCompat.ACTION_POINTER_DOWN: {
final int index = MotionEventCompat.getActionIndex(ev);
final float x = MotionEventCompat.getX(ev, index);
mLastMotionX = x;
mActivePointerId = MotionEventCompat.getPointerId(ev, index);
break;
}
case MotionEventCompat.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
mLastMotionX = MotionEventCompat.getX(ev,
MotionEventCompat.findPointerIndex(ev, mActivePointerId));
break;
}
if (needsInvalidate) {
invalidate();
}
return true;
}

@Override
public void draw(Canvas canvas) {
super.draw(canvas);
boolean needsInvalidate = false;

final int overScrollMode = ViewCompat.getOverScrollMode(this);
if (overScrollMode == ViewCompat.OVER_SCROLL_ALWAYS
|| (overScrollMode == ViewCompat.OVER_SCROLL_IF_CONTENT_SCROLLS
&& mAdapter != null && mAdapter.getCount() > 1)) {
if (!mLeftEdge.isFinished()) {
final int restoreCount = canvas.save();
final int height = getHeight() - getPaddingTop()
- getPaddingBottom();

canvas.rotate(270);
canvas.translate(-height + getPaddingTop(), 0);
mLeftEdge.setSize(height, getWidth());
needsInvalidate |= mLeftEdge.draw(canvas);
canvas.restoreToCount(restoreCount);
}
if (!mRightEdge.isFinished()) {
final int restoreCount = canvas.save();
final int width = getWidth();
final int height = getHeight() - getPaddingTop()
- getPaddingBottom();
final int itemCount = mAdapter != null ? mAdapter.getCount()
: 1;

canvas.rotate(90);
canvas.translate(-getPaddingTop(), -itemCount
* (width + mPageMargin) + mPageMargin);
mRightEdge.setSize(height, width);
needsInvalidate |= mRightEdge.draw(canvas);
canvas.restoreToCount(restoreCount);
}
} else {
mLeftEdge.finish();
mRightEdge.finish();
}

if (needsInvalidate) {
// Keep animating
invalidate();
}
}

@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);

// Draw the margin drawable if needed.
if (mPageMargin > 0 && mMarginDrawable != null) {
final int scrollX = getScrollX();
final int width = getWidth();
final int offset = scrollX % (width + mPageMargin);
if (offset != 0) {
// Pages fit completely when settled; we only need to draw when
// in between
final int left = scrollX - offset + width;
mMarginDrawable.setBounds(left, 0, left + mPageMargin,
getHeight());
mMarginDrawable.draw(canvas);
}
}
}

/**
* Start a fake drag of the pager.
*
* <p>
* A fake drag can be useful if you want to synchronize the motion of the
* ViewPager with the touch scrolling of another view, while still letting
* the ViewPager control the snapping motion and fling behavior. (e.g.
* parallax-scrolling tabs.) Call {@link #fakeDragBy(float)} to simulate the
* actual drag motion. Call {@link #endFakeDrag()} to complete the fake drag
* and fling as necessary.
*
* <p>
* During a fake drag the ViewPager will ignore all touch events. If a real
* drag is already in progress, this method will return false.
*
* @return true if the fake drag began successfully, false if it could not
*         be started.
*
* @see #fakeDragBy(float)
* @see #endFakeDrag()
*/
public boolean beginFakeDrag() {
if (mIsBeingDragged) {
return false;
}
mFakeDragging = true;
setScrollState(SCROLL_STATE_DRAGGING);
mInitialMotionX = mLastMotionX = 0;
if (mVelocityTracker == null) {
mVelocityTracker = VelocityTracker.obtain();
} else {
mVelocityTracker.clear();
}
final long time = SystemClock.uptimeMillis();
final MotionEvent ev = MotionEvent.obtain(time, time,
MotionEvent.ACTION_DOWN, 0, 0, 0);
mVelocityTracker.addMovement(ev);
ev.recycle();
mFakeDragBeginTime = time;
return true;
}

/**
* End a fake drag of the pager.
*
* @see #beginFakeDrag()
* @see #fakeDragBy(float)
*/
public void endFakeDrag() {
if (!mFakeDragging) {
throw new IllegalStateException(
"No fake drag in progress. Call beginFakeDrag first.");
}

final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
int initialVelocity = (int) VelocityTrackerCompat.getYVelocity(
velocityTracker, mActivePointerId);
mPopulatePending = true;
if ((Math.abs(initialVelocity) > mMinimumVelocity)
|| Math.abs(mInitialMotionX - mLastMotionX) >= (getWidth() / 3)) {
if (mLastMotionX > mInitialMotionX) {
setCurrentItemInternal(mCurItem - 1, true, true);
} else {
setCurrentItemInternal(mCurItem + 1, true, true);
}
} else {
setCurrentItemInternal(mCurItem, true, true);
}
endDrag();

mFakeDragging = false;
}

/**
* Fake drag by an offset in pixels. You must have called
* {@link #beginFakeDrag()} first.
*
* @param xOffset
*            Offset in pixels to drag by.
* @see #beginFakeDrag()
* @see #endFakeDrag()
*/
public void fakeDragBy(float xOffset) {
if (!mFakeDragging) {
throw new IllegalStateException(
"No fake drag in progress. Call beginFakeDrag first.");
}

mLastMotionX += xOffset;
float scrollX = getScrollX() - xOffset;
final int width = getWidth();
final int widthWithMargin = width + mPageMargin;

final float leftBound = Math.max(0, (mCurItem - 1) * widthWithMargin);
final float rightBound = Math
.min(mCurItem + 1, mAdapter.getCount() - 1) * widthWithMargin;
if (scrollX < leftBound) {
scrollX = leftBound;
} else if (scrollX > rightBound) {
scrollX = rightBound;
}
// Don't lose the rounded component
mLastMotionX += scrollX - (int) scrollX;
scrollTo((int) scrollX, getScrollY());
if (mOnPageChangeListener != null) {
final int position = (int) scrollX / widthWithMargin;
final int positionOffsetPixels = (int) scrollX % widthWithMargin;
final float positionOffset = (float) positionOffsetPixels
/ widthWithMargin;
mOnPageChangeListener.onPageScrolled(position, positionOffset,
positionOffsetPixels);
}

// Synthesize an event for the VelocityTracker.
final long time = SystemClock.uptimeMillis();
final MotionEvent ev = MotionEvent.obtain(mFakeDragBeginTime, time,
MotionEvent.ACTION_MOVE, mLastMotionX, 0, 0);
mVelocityTracker.addMovement(ev);
ev.recycle();
}

/**
* Returns true if a fake drag is in progress.
*
* @return true if currently in a fake drag, false otherwise.
*
* @see #beginFakeDrag()
* @see #fakeDragBy(float)
* @see #endFakeDrag()
*/
public boolean isFakeDragging() {
return mFakeDragging;
}

private void onSecondaryPointerUp(MotionEvent ev) {
final int pointerIndex = MotionEventCompat.getActionIndex(ev);
final int pointerId = MotionEventCompat.getPointerId(ev, pointerIndex);
if (pointerId == mActivePointerId) {
// This was our active pointer going up. Choose a new
// active pointer and adjust accordingly.
final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
mLastMotionX = MotionEventCompat.getX(ev, newPointerIndex);
mActivePointerId = MotionEventCompat.getPointerId(ev,
newPointerIndex);
if (mVelocityTracker != null) {
mVelocityTracker.clear();
}
}
}

private void endDrag() {
mIsBeingDragged = false;
mIsUnableToDrag = false;

if (mVelocityTracker != null) {
mVelocityTracker.recycle();
mVelocityTracker = null;
}
}

private void setScrollingCacheEnabled(boolean enabled) {
if (mScrollingCacheEnabled != enabled) {
mScrollingCacheEnabled = enabled;
if (USE_CACHE) {
final int size = getChildCount();
for (int i = 0; i < size; ++i) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
child.setDrawingCacheEnabled(enabled);
}
}
}
}
}

/**
* Tests scrollability within child views of v given a delta of dx.
*
* @param v
*            View to test for horizontal scrollability
* @param checkV
*            Whether the view v passed should itself be checked for
*            scrollability (true), or just its children (false).
* @param dx
*            Delta scrolled in pixels
* @param x
*            X coordinate of the active touch point
* @param y
*            Y coordinate of the active touch point
* @return true if child views of v can be scrolled by delta of dx.
*/
protected boolean canScroll(View v, boolean checkV, int dx, int x, int y) {
if (v instanceof ViewGroup) {
final ViewGroup group = (ViewGroup) v;
final int scrollX = v.getScrollX();
final int scrollY = v.getScrollY();
final int count = group.getChildCount();
// Count backwards - let topmost views consume scroll distance
// first.
for (int i = count - 1; i >= 0; i--) {
// TODO: Add versioned support here for transformed views.
// This will not work for transformed views in Honeycomb+
final View child = group.getChildAt(i);
if (x + scrollX >= child.getLeft()
&& x + scrollX < child.getRight()
&& y + scrollY >= child.getTop()
&& y + scrollY < child.getBottom()
&& canScroll(child, true, dx,
x + scrollX - child.getLeft(), y + scrollY
- child.getTop())) {
return true;
}
}
}

return checkV && ViewCompat.canScrollHorizontally(v, -dx);
}

@Override
public boolean dispatchKeyEvent(KeyEvent event) {
// Let the focused view and/or our descendants get the key first
return super.dispatchKeyEvent(event) || executeKeyEvent(event);
}

/**
* You can call this function yourself to have the scroll view perform
* scrolling from a key event, just as if the event had been dispatched to
* it by the view hierarchy.
*
* @param event
*            The key event to execute.
* @return Return true if the event was handled, else false.
*/
public boolean executeKeyEvent(KeyEvent event) {
boolean handled = false;
if (event.getAction() == KeyEvent.ACTION_DOWN) {
switch (event.getKeyCode()) {
case KeyEvent.KEYCODE_DPAD_LEFT:
handled = arrowScroll(FOCUS_LEFT);
break;
case KeyEvent.KEYCODE_DPAD_RIGHT:
handled = arrowScroll(FOCUS_RIGHT);
break;
case KeyEvent.KEYCODE_TAB:
if (KeyEventCompat.hasNoModifiers(event)) {
handled = arrowScroll(FOCUS_FORWARD);
} else if (KeyEventCompat.hasModifiers(event,
KeyEvent.META_SHIFT_ON)) {
handled = arrowScroll(FOCUS_BACKWARD);
}
break;
}
}
return handled;
}

public boolean arrowScroll(int direction) {
View currentFocused = findFocus();
if (currentFocused == this)
currentFocused = null;

boolean handled = false;

View nextFocused = FocusFinder.getInstance().findNextFocus(this,
currentFocused, direction);
if (nextFocused != null && nextFocused != currentFocused) {
if (direction == View.FOCUS_LEFT) {
// If there is nothing to the left, or this is causing us to
// jump to the right, then what we really want to do is page
// left.
if (currentFocused != null
&& nextFocused.getLeft() >= currentFocused.getLeft()) {
handled = pageLeft();
} else {
handled = nextFocused.requestFocus();
}
} else if (direction == View.FOCUS_RIGHT) {
// If there is nothing to the right, or this is causing us to
// jump to the left, then what we really want to do is page
// right.
if (currentFocused != null
&& nextFocused.getLeft() <= currentFocused.getLeft()) {
handled = pageRight();
} else {
handled = nextFocused.requestFocus();
}
}
} else if (direction == FOCUS_LEFT || direction == FOCUS_BACKWARD) {
// Trying to move left and nothing there; try to page.
handled = pageLeft();
} else if (direction == FOCUS_RIGHT || direction == FOCUS_FORWARD) {
// Trying to move right and nothing there; try to page.
handled = pageRight();
}
if (handled) {
playSoundEffect(SoundEffectConstants
.getContantForFocusDirection(direction));
}
return handled;
}

boolean pageLeft() {
if (mCurItem > 0) {
setCurrentItem(mCurItem - 1, true);
return true;
}
return false;
}

boolean pageRight() {
if (mAdapter != null && mCurItem < (mAdapter.getCount() - 1)) {
setCurrentItem(mCurItem + 1, true);
return true;
}
return false;
}

/**
* We only want the current page that is being shown to be focusable.
*/
@Override
public void addFocusables(ArrayList<View> views, int direction,
int focusableMode) {
final int focusableCount = views.size();

final int descendantFocusability = getDescendantFocusability();

if (descendantFocusability != FOCUS_BLOCK_DESCENDANTS) {
for (int i = 0; i < getChildCount(); i++) {
final View child = getChildAt(i);
if (child.getVisibility() == VISIBLE) {
ItemInfo ii = infoForChild(child);
if (ii != null && ii.position == mCurItem) {
child.addFocusables(views, direction, focusableMode);
}
}
}
}

// we add ourselves (if focusable) in all cases except for when we are
// FOCUS_AFTER_DESCENDANTS and there are some descendants focusable.
// this is
// to avoid the focus search finding layouts when a more precise search
// among the focusable children would be more interesting.
if (descendantFocusability != FOCUS_AFTER_DESCENDANTS ||
// No focusable descendants
(focusableCount == views.size())) {
// Note that we can't call the superclass here, because it will
// add all views in. So we need to do the same thing View does.
if (!isFocusable()) {
return;
}
if ((focusableMode & FOCUSABLES_TOUCH_MODE) == FOCUSABLES_TOUCH_MODE
&& isInTouchMode() && !isFocusableInTouchMode()) {
return;
}
if (views != null) {
views.add(this);
}
}
}

/**
* We only want the current page that is being shown to be touchable.
*/
@Override
public void addTouchables(ArrayList<View> views) {
// Note that we don't call super.addTouchables(), which means that
// we don't call View.addTouchables(). This is okay because a ViewPager
// is itself not touchable.
for (int i = 0; i < getChildCount(); i++) {
final View child = getChildAt(i);
if (child.getVisibility() == VISIBLE) {
ItemInfo ii = infoForChild(child);
if (ii != null && ii.position == mCurItem) {
child.addTouchables(views);
}
}
}
}

/**
* We only want the current page that is being shown to be focusable.
*/
@Override
protected boolean onRequestFocusInDescendants(int direction,
Rect previouslyFocusedRect) {
int index;
int increment;
int end;
int count = getChildCount();
if ((direction & FOCUS_FORWARD) != 0) {
index = 0;
increment = 1;
end = count;
} else {
index = count - 1;
increment = -1;
end = -1;
}
for (int i = index; i != end; i += increment) {
View child = getChildAt(i);
if (child.getVisibility() == VISIBLE) {
ItemInfo ii = infoForChild(child);
if (ii != null && ii.position == mCurItem) {
if (child.requestFocus(direction, previouslyFocusedRect)) {
return true;
}
}
}
}
return false;
}

@Override
public boolean dispatchPopulateAccessibilityEvent(AccessibilityEvent event) {
// ViewPagers should only report accessibility info for the current
// page,
// otherwise things get very confusing.

// TODO: Should this note something about the paging container?

final int childCount = getChildCount();
for (int i = 0; i < childCount; i++) {
final View child = getChildAt(i);
if (child.getVisibility() == VISIBLE) {
final ItemInfo ii = infoForChild(child);
if (ii != null && ii.position == mCurItem
&& child.dispatchPopulateAccessibilityEvent(event)) {
return true;
}
}
}

return false;
}

private class PagerObserver extends DataSetObserver {

@Override
public void onChanged() {
dataSetChanged();
}

@Override
public void onInvalidated() {
dataSetChanged();
}
}
}

设置setOffscreenPageLimit(0),ViewPager就不会缓存了，PS:无法和ToolBar一起使用

第二种方式

使用Fragment的setUserVisibleHint方法（相当于Activity中的onResume方法）

public class HomeFragment extends Fragment {

private View contentView;

/**
* 是否创建
*/
protected boolean isCreate = false;

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
contentView = inflater.inflate(R.layout.fragment_home, container, false);

return contentView;
}

@Override
public void onCreate(Bundle savedInstanceState) {
isCreate=true;
}
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if (isVisibleToUser && isCreate) {
//相当于Fragment的onResume
//在这里处理加载数据等操作
} else {
//相当于Fragment的onPause
}
}

其它

如果不希望切换ViewPager的时候每次都重新创建Fragment的话，也很简单只需要将适配器的两个方法注释掉就好：

@Override
public void destroyItem(ViewGroup container, int position, Object object) {
//super.destroyItem(container, position, object);
}

@Override
public void destroyItem(View container, int position, Object object) {
//super.destroyItem(container, position, object);

}
//不让他们调用父类方法就不会销毁Fagment了

最后

附上自己git

https://github.com/aohanyao/NoCacheViewPager