ViewPager实现源码分析

1、松手自动计算当前位置，并自动滑动到合适的position的页面

分析：松手属于View的事件分发机制，事件分发的入口在dispatchTouchEvent和onTouchEvent，大概的分发机制如下：

public boolean dispatchTouchEvent(MotionEvent ev) {
boolean consume = false;
if(onInterceptTouchEvent(ev)) {
consume = onTouchEvent(ev);
} else {
consume = child.dispatchTouchEvent(ev);
}

return consume;
}

因此，可以在onTouchEvent中处理松手事件

思路1：查看RecyclerView的源码，看系统源码是否提供ACTION_UP的事件的回调接口，重写该接口，实现松手的相应业务逻辑

思路2：覆盖掉onTouchEvent，ACTION_UP事件自己处理，其他事件交由系统默认处理（本项目采用的方法）
类比ViewPager的实现ACTION_UP时候的实现方式

思路3：查看下RecyclerView的事件处理是否交由其他专门的事件处理类去处理（既在onTouchEvent中是否将事件处理转发到其他专门类处理），查找相应类和回调接口并覆盖掉回调接口方法
分析RecyclerView的onTouchEvent函数，并没发现使用其他事件处理类去接收事件，因此该方法排除。

2.松手自动滑动到指定page

方法一：根据松手的位置距离中心点的位置是否超过一半，确定最终位置，并用smoothScrollToPosition实现滑动到指定位置.

缺点：虽然能实现松手滑动到指定位置，但是滑动效果和ViewPager的松手滑动不大一样

实现代码：

int widthWithMargin = getWidth() + mPageMargin;
// calculate scrollX
int scrollX = mMyScrollX;
int currentPage = scrollX / widthWithMargin;
int offset = scrollX - currentPage*widthWithMargin;
final int dstPage = currentPage + ((offset>widthWithMargin/2) ? 1 : 0);

getHandler().post(new Runnable() {
@Override
public void run() {
LogUtils.i("run to position");
smoothScrollToPosition(dstPage);
}
});

方法二：参考ViewPager的松手实现方法：VelocityTracker+Scroller+Interpolator+回调函数computeScroll,根据用户滑动速度动态改变松手滑动的快慢（本工程采用该方法）

参考源码：ViewPager源码

问题：滑动的自然性和使用ViewPager的滑动自然性不同？

分析：能够准确滑动，但是自然性不足，可能是因为滑动过程的算法参考的代码是4.0的ViewPager的滑动效果

解决：滑动过程的实现算法采用最新版本的ViewPager代码，滑动自然性体验和现有的ViewPager一致。

3.实现接口的回调（类OnPageChangeListener,如：当前滑动到第几页，当前滑动的状态）

源码中查看下该接口的三个函数，分别在哪个点被回调(使用AS的快捷键查看调用层级树)。然后在自定义的RecyclerView中相应的位置回调这三个方法（还未实现。。）

/**
* 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.
*/
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.
*/
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 ViewPager#SCROLL_STATE_IDLE
* @see ViewPager#SCROLL_STATE_DRAGGING
* @see ViewPager#SCROLL_STATE_SETTLING
*/
void onPageScrollStateChanged(int state);
}

3.1 ViewPager中mOnPageChangeListener.onPageScrolled的调用过程

public void computeScroll()     // implement in ViewPagerRecyclerView
private void completeScroll(boolean postEvents)
private boolean performDrag(float x)    // implement in ViewPagerRecyclerView
public void fakeDragBy(float xOffset)
private boolean pageScrolled(int xpos)
-->protected void onPageScrolled(int position, float offset, int offsetPixels)
-->private void dispatchOnPageScrolled(int position, float offset, int offsetPixels)
-->mOnPageChangeListener.onPageScrolled(position, offset, offsetPixels);

/*
dispatchOnPageScrolled是mOnPageChangeListener.onPageScrolled的唯一调用入口
*/
private void dispatchOnPageScrolled(int position, float offset, int offsetPixels) {
if (mOnPageChangeListener != null) {
mOnPageChangeListener.onPageScrolled(position, offset, offsetPixels); // callback onPageScrolled
}
if (mOnPageChangeListeners != null) {
for (int i = 0, z = mOnPageChangeListeners.size(); i < z; i++) {
OnPageChangeListener listener = mOnPageChangeListeners.get(i);
if (listener != null) {
listener.onPageScrolled(position, offset, offsetPixels);
}
}
}
if (mInternalPageChangeListener != null) {
mInternalPageChangeListener.onPageScrolled(position, offset, offsetPixels);
}
}

3.2 ViewPager中mOnPageChangeListener.onPageSelected的调用过程

public void onRestoreInstanceState(Parcelable state)
public boolean onTouchEvent(MotionEvent ev) case MotionEvent.ACTION_UP:// implement in ViewPagerRecyclerView
public void endFakeDrag()
public void setAdapter(PagerAdapter adapter)
public void setCurrentItem(int item)    // implement in ViewPagerRecyclerView
void dataSetChanged()
-->void setCurrentItemInternal(int item, boolean smoothScroll, boolean always, int velocity)
-->private void dispatchOnPageSelected(int position)

/*
dispatchOnPageSelected是mOnPageChangeListener.onPageSelected的唯一调用入口
*/
private void dispatchOnPageSelected(int position) {
if (mOnPageChangeListener != null) {
mOnPageChangeListener.onPageSelected(position);
}
if (mOnPageChangeListeners != null) {
for (int i = 0, z = mOnPageChangeListeners.size(); i < z; i++) {
OnPageChangeListener listener = mOnPageChangeListeners.get(i);
if (listener != null) {
listener.onPageSelected(position);
}
}
}
if (mInternalPageChangeListener != null) {
mInternalPageChangeListener.onPageSelected(position);  // callback onPageSelected
}
}

3.3 ViewPager中mOnPageChangeListener.onPageScrollStateChanged的调用过程

private final Runnable mEndScrollRunnable
void smoothScrollTo(int x, int y, int velocity) // implement in ViewPagerRecyclerView
public boolean onInterceptTouchEvent(MotionEvent ev)
public boolean onTouchEvent(MotionEvent ev) // implement in ViewPagerRecyclerView
public boolean beginFakeDrag()
private void setScrollState(int newState)
-->private void dispatchOnScrollStateChanged(int state)

/*
dispatchOnPageSelected是mOnPageChangeListener.onPageSelected的唯一调用入口
*/
private void dispatchOnScrollStateChanged(int state) {
if (mOnPageChangeListener != null) {
mOnPageChangeListener.onPageScrollStateChanged(state);
}
if (mOnPageChangeListeners != null) {
for (int i = 0, z = mOnPageChangeListeners.size(); i < z; i++) {
OnPageChangeListener listener = mOnPageChangeListeners.get(i);
if (listener != null) {
listener.onPageScrollStateChanged(state);
}
}
}
if (mInternalPageChangeListener != null) {
mInternalPageChangeListener.onPageScrollStateChanged(state);
}
}

问题：测试的时候，状态为IDLE的时候会有问题

问题：如何测试onPageScrolled

4.设置当前选中Item

ViewPager：如果当前是第一次设置的话，则启动requestLayout，并且设置当前选中的item，在布局中重新布局

requestLayout(); ===》使用标志位isFirstLayout

5.setPagerMargin的实现

5.1.ViewPager的setPagerMargin实现

/*
Set the margin between pages.
Parameters:
marginPixels Distance between adjacent pages in pixels
*/
429     public void More ...setPageMargin(int marginPixels) {
430         final int oldMargin = mPageMargin;
// record PageMargin for Layout
431         mPageMargin = marginPixels;
432
433         final int width = getWidth();
434         recomputeScrollPosition(width, width, marginPixels, oldMargin);
435
436         requestLayout();
437     }

899     @Override
900     protected void More ...onLayout(boolean changed, int l, int t, int r, int b) {
901         mInLayout = true;
902         populate();
903         mInLayout = false;
904
905         final int count = getChildCount();
906         final int width = r-l;
907
908         for (int i = 0; i < count; i++) {
909             View child = getChildAt(i);
910             ItemInfo ii;
// Use the mPageMargin parameter in the layout
911             if (child.getVisibility() != GONE && (ii=infoForChild(child)) != null) {
912                 int loff = (width + mPageMargin) * ii.position;
913                 int childLeft = getPaddingLeft() + loff;
914                 int childTop = getPaddingTop();
...
918                 child.layout(childLeft, childTop,
919                         childLeft + child.getMeasuredWidth(),
920                         childTop + child.getMeasuredHeight());
921             }
922         }
923         mFirstLayout = false;
924     }

5.2 RecyclerView的setPagerMargin的实现

思路：查看RecyclerView中如何布局子View。
问题：查看onLayout后发现布局过程有点复杂，代码中调用dispatchLayoutStep1();，dispatchLayoutStep2();，dispatchLayoutStep3();
分析：既然从入口函数到目标（View的布局过程）有点复杂（代码量有点多），那么就从后往前推。首先，先查看RecyclerView中哪个地方调用了子View的布局，
既：RecyclerView中应该有调用child.layout或者view.layout,然后顺藤摸瓜，往上找出调用过程栈。经过查找，子view的布局过程如下：

onFocusSearchFailed
onLayoutChildren（LayoutManager中必须要覆盖的方法）
LinearLayoutManager.scrollBy
LinearLayoutManager.fill
LinearLayoutManager.layoutChunk
LayoutManager.layoutDecoratedWithMargins 该方法为public方法，可以覆盖掉
child.layout

想重新布局，但是RecyclerView的布局方式和ViewPager的布局方式不同，滚动的时候布局会乱。
解决：
当用户设置pageMargin的时候，自定义PageMarginItemDecoration，让pageMargin的大小和ItemDecoration的mPageMarginWidth大小相等
然后整个RecyclerView控件的高度 + 上下分割线的总和
让控件高度超出屏幕的高度
在当前屏幕就看不到分割线了
滑动的时候，又可以看到分割线

每个子View的测量和布局过程

void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
LayoutState layoutState, LayoutChunkResult result) {
View view = layoutState.next(recycler);
if (view == null) {
if (DEBUG && layoutState.mScrapList == null) {
throw new RuntimeException("received null view when unexpected");
}
// if we are laying out views in scrap, this may return null which means there is
// no more items to layout.
result.mFinished = true;
return;
}
LayoutParams params = (LayoutParams) view.getLayoutParams();
if (layoutState.mScrapList == null) {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
addView(view);
} else {
addView(view, 0);
}
} else {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
addDisappearingView(view);
} else {
addDisappearingView(view, 0);
}
}

// 1.measure child view
measureChildWithMargins(view, 0, 0);
result.mConsumed = mOrientationHelper.getDecoratedMeasurement(view);
int left, top, right, bottom;
if (mOrientation == VERTICAL) {
if (isLayoutRTL()) {
right = getWidth() - getPaddingRight();
left = right - mOrientationHelper.getDecoratedMeasurementInOther(view);
} else {
left = getPaddingLeft();
right = left + mOrientationHelper.getDecoratedMeasurementInOther(view);
}
if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
bottom = layoutState.mOffset;
top = layoutState.mOffset - result.mConsumed;
} else {
top = layoutState.mOffset;
bottom = layoutState.mOffset + result.mConsumed;
}
} else {
top = getPaddingTop();
bottom = top + mOrientationHelper.getDecoratedMeasurementInOther(view);

if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
right = layoutState.mOffset;
left = layoutState.mOffset - result.mConsumed;
} else {
left = layoutState.mOffset;
right = layoutState.mOffset + result.mConsumed;
}
}

// 2.layout child view
// We calculate everything with View's bounding box (which includes decor and margins)
// To calculate correct layout position, we subtract margins.
layoutDecoratedWithMargins(view, left, top, right, bottom);

// Consume the available space if the view is not removed OR changed
if (params.isItemRemoved() || params.isItemChanged()) {
result.mIgnoreConsumed = true;
}
result.mFocusable = view.isFocusable();
}

public void measureChildWithMargins(View child, int widthUsed, int heightUsed) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();

final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);//use ItemDecoration
widthUsed += insets.left + insets.right;
heightUsed += insets.top + insets.bottom;

final int widthSpec = getChildMeasureSpec(getWidth(), getWidthMode(),
getPaddingLeft() + getPaddingRight() +
lp.leftMargin + lp.rightMargin + widthUsed, lp.width,
canScrollHorizontally());
final int heightSpec = getChildMeasureSpec(getHeight(), getHeightMode(),
getPaddingTop() + getPaddingBottom() +
lp.topMargin + lp.bottomMargin + heightUsed, lp.height,
canScrollVertically());
if (shouldMeasureChild(child, widthSpec, heightSpec, lp)) {
child.measure(widthSpec, heightSpec);
}
}

setMargin/setPadding均失败，查询代码，设置padding

转换思路：让RecyclerView控件的宽度大于屏幕宽度，而不是去考虑更改ItemDecoration的位置（源码不可动）

参考文献：

如何获取Android RecyclerView滑动的距离

如何获取 RecyclerView 的滑动距离？

RecyclerView

ViewPager

ViewPager源码

演示视频