前面只实现了内容随手指滑动而滑动，如何在手指离开屏幕以后还能继续平滑滑动一段距离呢？
要实现平滑滑动，需要借助于Scroller类，它其实是通过起始位置、终止位置或初始速度，加上持续时间，计算出中间时刻的滑动位置，然后不断触发重绘来实现的。即通过不断的移动一个小的距离来实现整体的平滑滑动效果。
提供了两个方法
startScroll()，给它传入滑动的起始位置、终止位置和持续时间。
fling()，给他传入手初始速度和起始位置、终止位置的范围。（不过在我这里的实验结果，在靠近边界的时候，fling()效果不是很好）
具体使用方式如下
1. 在ACTION_UP中根据速度分别调用startScroll()或者fling()，并调用invalidate()触发重绘。
2. 重写computeScroll()方法（），在其中通过Scroller获取到中间时刻的滑动位置，并调用scrollTo()/scrollBy()方法（会设置新的滑动位置并触发重绘），并再次调用invalidate()。
3. 可以在ACTION_DOWN中进行处理，如果正在惯性滑动，则在手指按下时取消当前的惯性滑动。
public class MyFrameLayout extends FrameLayout {
//内容的最大值、最小值
private int minX=0;
private int maxX=0;
//滑动坐标的最大值、最小值
private int minScrollX;
private int maxScrollX;
private int lastX;
VelocityTracker mVelocityTracker;
int scaledMaximumFlingVelocity;
int scaledMinimumFlingVelocity;
private MyScroller mScroller;
public MyFrameLayout(Context context) {
super(context);
init();
}
public MyFrameLayout(Context context, AttributeSet attrs) {
super(context, attrs);
init();
}
public MyFrameLayout(Context context, AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
init();
}
public void init(){
ViewConfiguration configuration = ViewConfiguration.get(getContext());
scaledMinimumFlingVelocity=configuration.getScaledMinimumFlingVelocity();
scaledMaximumFlingVelocity= configuration.getScaledMaximumFlingVelocity();
Log.e("shadowfaxghh", "scaledMinimumFlingVelocity="+scaledMinimumFlingVelocity+" scaledMaximumFlingVelocity="+scaledMaximumFlingVelocity);
mScroller=new MyScroller(getContext());
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
//计算出内容的最大值最小值
for(int i=0; i<getChildCount(); i++){
View childView = getChildAt(i);
if(minX>childView.getLeft())
minX=childView.getLeft();
if(maxX<childView.getRight())
maxX=childView.getRight();
}
//计算出滑动坐标的最大值、最小值//只有内容大小超出自身大小时，才能进行滑动
if(minX<0)
minScrollX=minX;
else
minScrollX=0;
if(maxX>getWidth())
maxScrollX=maxX-getWidth();
else
maxScrollX=0;
}
@Override
public boolean onTouchEvent(MotionEvent event) {
super.onTouchEvent(event);
switch (event.getAction()){
case MotionEvent.ACTION_DOWN:
// Log.e("shadowfaxghh", "ACTION_DOWN");
lastX=(int) event.getRawX();
obtaionVelocityTracker();
mVelocityTracker.addMovement(event);
if(mScroller!=null & !mScroller.isFinished()){
mScroller.abortAnimation();
}
break;
case MotionEvent.ACTION_MOVE:
// Log.e("shadowfaxghh", "ACTION_MOVE");
int newX= (int) event.getRawX();
int deltaX=newX-lastX;
// if (Math.abs(deltaX) >ViewConfiguration.get(getContext()).getScaledTouchSlop()) {
int newScrollX = getScrollX() + (-deltaX);
if (newScrollX > maxScrollX)
newScrollX = maxScrollX;
if (newScrollX < minScrollX)
newScrollX = minScrollX;
scrollTo(newScrollX, getScrollY());
//更新坐标
lastX = newX;
// }
obtaionVelocityTracker();
mVelocityTracker.addMovement(event);
break;
case MotionEvent.ACTION_UP:
Log.e("shadowfaxghh", "ACTION_UP");
lastX=(int) event.getRawX();
if(mVelocityTracker!=null) {
//计算速度
mVelocityTracker.computeCurrentVelocity(1000, scaledMaximumFlingVelocity);
//获取x方向的速度
int xVelocity = (int) mVelocityTracker.getXVelocity();
Log.e("shadowfaxghh", "xVelocity=" + xVelocity);
recycleVelocityTracker();
//使用startScroll()进行滑动
Log.e("shadowfaxghh", "startScroll()");
//假定手指离开以后以当前速度/4继续滑动1000ms
int scrollDuration=1000;
int scrollDistance=Math.abs(scrollDuration/1000 * xVelocity/4);//computeCurrentVelocity(1000//速度是以1000ms为单位的
int startScrollX = getScrollX();
int endScrollX =startScrollX;
if(xVelocity>0){// 手指向右滑动
endScrollX=startScrollX-scrollDistance;
}else if(xVelocity<0){// 手指向左滑动
endScrollX=startScrollX+scrollDistance;
}
if(endScrollX<minScrollX)
endScrollX=minScrollX;
if(endScrollX>maxScrollX)
endScrollX=maxScrollX;
//实际滑动时间ms//当然这是以匀速滑动来计算的，实际上Scroller内部并不是匀速滑动
scrollDuration= (int) (Math.abs((float) (startScrollX-endScrollX)/(float)scrollDistance)*scrollDuration);
mScroller.startScroll(startScrollX, getScrollY(), endScrollX-startScrollX, 0, scrollDuration);
invalidate();
// //使用Fling()进行滑动
// if(Math.abs(xVelocity)>scaledMinimumFlingVelocity) {//快速滑动
// Log.e("shadowfaxghh", "fling()");
//
// //注意xVelocity要取反，因为velocity正负值跟屏幕坐标系一致，但是scroll参数跟屏幕坐标系相反
// mScroller.fling(getScrollX(), getScrollY(), -xVelocity, 0, minScrollX,maxScrollX, 0, 0);
// invalidate();
// }
}
break;
case MotionEvent.ACTION_CANCEL:
Log.e("shadowfaxghh", "ACTION_CANCEL");
lastX=(int) event.getRawX();
recycleVelocityTracker();
break;
default:
break;
}
return true;
}
@Override
public void computeScroll() {
// super.computeScroll();
if(mScroller!=null && mScroller.computeScrollOffset()){
int currX = mScroller.getCurrX();
int currY = mScroller.getCurrY();
scrollTo(currX, currY);
invalidate();//某些情况下scrollTo()无法触发重绘，需要自己invalidate()触发
}
}
private void obtaionVelocityTracker(){
if(mVelocityTracker==null)
mVelocityTracker=VelocityTracker.obtain();
}
private void recycleVelocityTracker(){
if(mVelocityTracker!=null){
mVelocityTracker.clear();
mVelocityTracker.recycle();
mVelocityTracker=null;
}
}
@Override
protected void onScrollChanged(int l, int t, int oldl, int oldt) {
super.onScrollChanged(l, t, oldl, oldt);
}
}可以看一下Scroller中相关方法的源码，如下代码基于android-2.3.3_r1
startScroll()
public void startScroll(int startX, int startY, int dx, int dy, int duration) {
mMode = SCROLL_MODE;
mFinished = false;
mDuration = duration;
mStartTime = AnimationUtils.currentAnimationTimeMillis();
mStartX = startX;
mStartY = startY;
mFinalX = startX + dx;
mFinalY = startY + dy;
mDeltaX = dx;
mDeltaY = dy;
mDurationReciprocal = 1.0f / (float) mDuration;
Log.e("shadowfaxghh", "Scroller startScroll()mStartX="+mStartX+" mFinalX="+mFinalX+" mDuration="+mDuration);
}
fling()
public void fling(int startX, int startY, int velocityX, int velocityY,
int minX, int maxX, int minY, int maxY) {
mMode = FLING_MODE;
mFinished = false;
float velocity = (float)Math.hypot(velocityX, velocityY);
mVelocity = velocity;
//计算出滑动时间
mDuration = (int) (1000 * velocity / mDeceleration); // Duration is in
// milliseconds
mStartTime = AnimationUtils.currentAnimationTimeMillis();
mStartX = startX;
mStartY = startY;
mCoeffX = velocity == 0 ? 1.0f : velocityX / velocity;
mCoeffY = velocity == 0 ? 1.0f : velocityY / velocity;
//计算出滑动距离
int totalDistance = (int) ((velocity * velocity) / (2 * mDeceleration));
mMinX = minX;
mMaxX = maxX;
mMinY = minY;
mMaxY = maxY; //跟传入的参数进行比较
mFinalX = startX + Math.round(totalDistance* mCoeffX);
// Pin to mMinX <= mFinalX <=mMaxX
mFinalX = Math.min(mFinalX, mMaxX);
mFinalX = Math.max(mFinalX, mMinX);
mFinalY = startY + Math.round(totalDistance* mCoeffY);
// Pin to mMinY <= mFinalY <=mMaxY
mFinalY = Math.min(mFinalY, mMaxY);
mFinalY = Math.max(mFinalY, mMinY);
Log.e("shadowfaxghh", "Scroller fling() mStartX="+mStartX+" mFinalX="+mFinalX+" mDuration="+mDuration);
}
computeScrollOffset()
public boolean computeScrollOffset() {//返回滑动是否已经结束
if (mFinished) {
return false;
}
int timePassed = (int)(AnimationUtils.currentAnimationTimeMillis()- mStartTime);
//从开始滑动到现在还没有结束
if (timePassed < mDuration) {
switch (mMode) {//分两种模式计算中间时刻的速度
case SCROLL_MODE:
float x = (float)timePassed * mDurationReciprocal;
if (mInterpolator == null)
x = viscousFluid(x);
else
x = mInterpolator.getInterpolation(x);
mCurrX = mStartX + Math.round(x * mDeltaX);
mCurrY = mStartY + Math.round(x * mDeltaY);
break;
case FLING_MODE:
float timePassedSeconds = timePassed / 1000.0f;
float distance = (mVelocity * timePassedSeconds)
- (mDeceleration * timePassedSeconds * timePassedSeconds/ 2.0f);
mCurrX = mStartX + Math.round(distance * mCoeffX);
// Pin to mMinX<= mCurrX <= mMaxX
mCurrX = Math.min(mCurrX, mMaxX);
mCurrX = Math.max(mCurrX, mMinX);
mCurrY = mStartY + Math.round(distance * mCoeffY);
// Pin to mMinY<= mCurrY <= mMaxY
mCurrY = Math.min(mCurrY, mMaxY);
mCurrY = Math.max(mCurrY, mMinY);
if (mCurrX == mFinalX && mCurrY == mFinalY) {
mFinished = true;
}
break;
}
}
else {
mCurrX = mFinalX;
mCurrY = mFinalY;
mFinished = true;
}
return true;
}
startScroll() vs fling()
参考ScrollView的源码，startScroll()和fling()并不是同时使用的
当手指离开屏幕的时候，如果速度大于minFlingVelocity，则进行fling()。否则停止（或回弹）。
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