android-Viewflow开源项目学习

一直以来，没有写过技术博客。最近想整理一下自己学过的开源项目，通过动手加深对技术的理解

Viewflow项目介绍

Viewflow是github上很好用的横向滑动View的开源项目。他有跟android原生ListView一样的接口，在项目中能够快速运用。

ListView的启发

Viewflow的设计很自然联想到能够上下滑动的ListView，因此大部分人独立去实现这样的View时，比较容易出现的想法是改造ListView。我们通常想到：
将ListView的移动方向改变城左右滑动。
只显示一项数据。

因此Viewflow很自然的是继承AdapterView。

View宽高计算

android View在计算宽高布局时，有onMeasure和onLayout两个方法。

Viewflow中只显示一项ChildView，高度为ChildView的高度与上下padding之和，宽度为屏幕宽度；onLayout为每个childView申请布局。

滑动事件处理

android中不是单一view的滑动处理通常是onInterceptTouchEvent和onTouchEvent两个方法处理触摸事件的响应，Scroller处理视图滑动，并通过VelocityTracker处理滑动速度。Viewflow在滑动中显示什么位置的ChildView可以计算：

private void snapToDestination(){
final int screenWidth = getChildWdith();
final int whichScreen = (getScrollX() + (screenWidth / 2))
/ screenWidth;

snapToScreen(whichScreen);
}

android在绘制View前，都会调用computeScroll，因此在该方法中处理ChildView切换：

@Override
public void computeScroll() {
if(mScroller.computeScrollOffset()){
scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
postInvalidate();
}else if(mNextScreen != INVALID_SCREEN){
mCurrentScreen = Math.max(0,
Math.min(mNextScreen, getChildCount() - 1));
mNextScr
a0db
een = INVALID_SCREEN;
post(new Runnable() {
@Override
public void run() {
postViewSwitched(mLastScrollDirection);
}
});
}
}

android在Touch过程中，设置Scroller参数，获取滑动速率，用于ChildView切换计算

@Override
public boolean onTouchEvent(MotionEvent event) {
if(getChildCount() == 0)
return false;

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

final int action = event.getAction();
final float x = event.getX();
switch(action){
case MotionEvent.ACTION_DOWN:
/**
* If being flinged and user touches, stop the fling. isFinished
* will be false if being flinged.
*/
if(mScroller.isFinished()){
mScroller.abortAnimation();
}

// Remember where the motion event started
mLastMotionX = x;

mTouchState = mScroller.isFinished() ? TOUCH_STATE_REST
: TOUCH_STATE_SCROLLING;

break;
case MotionEvent.ACTION_MOVE:
final int deltaX = (int)(mLastMotionX - x);

boolean xMoved = Math.abs(deltaX) > mTouchSlop;
if(xMoved){

mTouchState = TOUCH_STATE_SCROLLING;

if(mViewInitializeListener != null)
initializeView(deltaX);
}

if(mTouchState == TOUCH_STATE_SCROLLING){
// Scroll to follow the motion event

mLastMotionX = x;

int scrollX = getScrollX();
if(deltaX < 0){
if(scrollX > 0){
scrollBy(Math.max(-scrollX, deltaX), 0);
}
}else if(deltaX > 0){
final int availableToScroll = getChildAt(
getChildCount() - 1).getRight()
- getPaddingRight() - getHorizontalFadingEdgeLength()
- scrollX - getChildWdith();
scrollBy(Math.min(availableToScroll, deltaX), 0);
}

return true;
}
break;
case MotionEvent.ACTION_UP:
if(mTouchState == TOUCH_STATE_SCROLLING){
final VelocityTracker velocityTracker = mVelocityTracker;
velocityTracker.computeCurrentVelocity(1000, mMaximumVelocity);
int veloctiyX = (int) velocityTracker.getXVelocity();

if(veloctiyX > SNAP_VELOCITY && mCurrentScreen > 0){
snapToScreen(mCurrentScreen - 1);
}else if(veloctiyX < - SNAP_VELOCITY &&
mCurrentScreen < getChildCount() - 1){
snapToScreen(mCurrentScreen + 1);
}else{
snapToDestination();
}

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

mTouchState = TOUCH_STATE_REST;
break;
case MotionEvent.ACTION_CANCEL:
snapToDestination();
mTouchState = TOUCH_STATE_REST;
break;
}
return true;
}

ChildView缓存

Viewflow中使用链表处理ChildView的缓存。

private LinkedList<View> mLoadedViews;
private LinkedList<View> mRecycledViews;

Viewflow中加载的ChildView保存到mLoadViews链表中，不再需要加载的ChildView放在mRecycledViews中。在需要生成新的ChildView加载数据时，可以重新利用mRecycleViews中的没在加载的ChildView。

private View obtainView(int position){
View convertView = getRecycledView();
View view = mAdapter.getView(position, convertView, this);

if(view != convertView && convertView != null){
mRecycledViews.add(convertView);
}
mLastObtainedViewWasRecycled = (view == convertView);
LayoutParams p = view.getLayoutParams();
if(p == null){
p = new LayoutParams(LayoutParams.MATCH_PARENT,
LayoutParams.MATCH_PARENT);
view.setLayoutParams(p);
}
return view;
}

在加载时，
mRecycleViews中恢复的ChildView，调用attachViewToParent
新创建的ChildView，调用addViewInLayout

在切换显示View时，处理缓存view：

private void postViewSwitched(int direction){
if(direction == 0)
return;

if(direction > 0){ // to the right
mCurrentAdapterIndex ++;
mCurrentBufferIndex ++;
mLazyInit.remove(LazyInit.LEFT);
mLazyInit.add(LazyInit.RIGHT);

// Recycle view outside buffer range
if(mCurrentAdapterIndex > mSideBuffer){
recycleView(mLoadedViews.removeFirst());
mCurrentBufferIndex --;
}

int newBufferIndex = mCurrentAdapterIndex + mSideBuffer;
if(newBufferIndex < mAdapter.getCount())
mLoadedViews.addLast(makeAndAddView(newBufferIndex, true));

}else { //to the left
mCurrentAdapterIndex --;
mCurrentBufferIndex --;
mLazyInit.add(LazyInit.LEFT);
mLazyInit.remove(LazyInit.RIGHT);

if((mAdapter.getCount() - 1 - mCurrentAdapterIndex) > mSideBuffer){
recycleView(mLoadedViews.removeLast());
}

int newBufferIndex = mCurrentAdapterIndex - mSideBuffer;
if(newBufferIndex > -1){
mLoadedViews.addFirst(makeAndAddView(newBufferIndex, false));
mCurrentBufferIndex ++;
}

}

requestLayout();
setVisibleView(mCurrentBufferIndex, true);
if(mIndicator != null){
mIndicator.onSwitched(mLoadedViews.get(mCurrentBufferIndex),
mCurrentAdapterIndex);
}

if(mViewSwitchListener != null){
mViewSwitchListener.onSwitched(mLoadedViews.get(mCurrentBufferIndex),
mCurrentAdapterIndex);
}
}

 

将切换不需要加载的View加入mRecycleViews中：

protected void recycleView(View v){
if(v == null)
return;
mRecycledViews.addFirst(v);
detachViewFromParent(v);
}

结束语

Viewflow的github链接，本人认识有限，希望大家指正。