原文链接
下拉刷新是应用中比较常见的功能，最开始下拉刷新主要局限于ListView，网上也有很多成熟的ListView的下拉刷新和上拉加载更多的控件，但如果我要下拉刷新的是WebView这种控件呢？网上也有一些，不过其实谷歌自己也提供了，谷歌提供的这种控件叫SwipeRefreshLayout，它可以实现WebView这类控件的下拉刷新，不过它有一个问题是，它的刷新的样式是固定的，就是在顶部有几个不同颜色的类似进度条的显示（高版本也有类似ProgressBar那样旋转的进度框的样式），如果我想自己定义Header的话，就要改写SwipeRefreshLayout的源码了，先来看看效果图：
image.png
好吧，我承认我这个自定义的Header很丑，不过这不是本文的关键，本文的关键是来描述如何改造SwipeRefreshLayout，只要学会了如何改造，Header想弄成什么样子，价格gif动画什么的，都是很简单的，首先来看看SwipeRefreshLayout的源码，地址在：SDK\sources\android-20\android\support\v4\widget，这里要找到自己的SDK的目录 ，另外我这里用的是20版本进行改造的，其它版本我没看，原理是一样的，把这个目录下的SwipeRefreshLayout.java拷贝出来，发现它还用到了SwipeProgressBar.java和BakedBezierInterpolator.java，这2个文件就是SwipeRefreshLayout默认的下拉刷新的进度条和动画，我们改造后可以把它们删除，不需要。
先来初略看下SwipeRefreshLayout的源码：
public class SwipeRefreshLayout extends ViewGroup {
private static final String LOG_TAG = SwipeRefreshLayout.class.getSimpleName();
private static final long RETURN_TO_ORIGINAL_POSITION_TIMEOUT = 300;
private static final float ACCELERATE_INTERPOLATION_FACTOR = 1.5f;
private static final float DECELERATE_INTERPOLATION_FACTOR = 2f;
private static final float PROGRESS_BAR_HEIGHT = 4;
private static final float MAX_SWIPE_DISTANCE_FACTOR = .6f;
private static final int REFRESH_TRIGGER_DISTANCE = 120;
private static final int INVALID_POINTER = -1;
private SwipeProgressBar mProgressBar; //the thing that shows progress is going
private View mTarget; //the content that gets pulled down
private int mOriginalOffsetTop;
private OnRefreshListener mListener;
private int mFrom;
private boolean mRefreshing = false;
private int mTouchSlop;
private float mDistanceToTriggerSync = -1;
private int mMediumAnimationDuration;
private float mFromPercentage = 0;
private float mCurrPercentage = 0;
private int mProgressBarHeight;
private int mCurrentTargetOffsetTop;
private float mInitialMotionY;
private float mLastMotionY;
private boolean mIsBeingDragged;
private int mActivePointerId = INVALID_POINTER;
// Target is returning to its start offset because it was cancelled or a
// refresh was triggered.
private boolean mReturningToStart;
private final DecelerateInterpolator mDecelerateInterpolator;
private final AccelerateInterpolator mAccelerateInterpolator;
private static final int[] LAYOUT_ATTRS = new int[] {
android.R.attr.enabled
};
private final Animation mAnimateToStartPosition = new Animation() {
@Override
public void applyTransformation(float interpolatedTime, Transformation t) {
int targetTop = 0;
if (mFrom != mOriginalOffsetTop) {
targetTop = (mFrom + (int)((mOriginalOffsetTop - mFrom) * interpolatedTime));
}
int offset = targetTop - mTarget.getTop();
final int currentTop = mTarget.getTop();
if (offset + currentTop < 0) {
offset = 0 - currentTop;
}
setTargetOffsetTopAndBottom(offset);
}
};
private Animation mShrinkTrigger = new Animation() {
@Override
public void applyTransformation(float interpolatedTime, Transformation t) {
float percent = mFromPercentage + ((0 - mFromPercentage) * interpolatedTime);
mProgressBar.setTriggerPercentage(percent);
}
};
private final AnimationListener mReturnToStartPositionListener = new BaseAnimationListener() {
@Override
public void onAnimationEnd(Animation animation) {
// Once the target content has returned to its start position, reset
// the target offset to 0
mCurrentTargetOffsetTop = 0;
}
};
private final AnimationListener mShrinkAnimationListener = new BaseAnimationListener() {
@Override
public void onAnimationEnd(Animation animation) {
mCurrPercentage = 0;
}
};
private final Runnable mReturnToStartPosition = new Runnable() {
@Override
public void run() {
mReturningToStart = true;
animateOffsetToStartPosition(mCurrentTargetOffsetTop + getPaddingTop(),
mReturnToStartPositionListener);
}
};
// Cancel the refresh gesture and animate everything back to its original state.
private final Runnable mCancel = new Runnable() {
@Override
public void run() {
mReturningToStart = true;
// Timeout fired since the user last moved their finger; animate the
// trigger to 0 and put the target back at its original position
if (mProgressBar != null) {
mFromPercentage = mCurrPercentage;
mShrinkTrigger.setDuration(mMediumAnimationDuration);
mShrinkTrigger.setAnimationListener(mShrinkAnimationListener);
mShrinkTrigger.reset();
mShrinkTrigger.setInterpolator(mDecelerateInterpolator);
startAnimation(mShrinkTrigger);
}
animateOffsetToStartPosition(mCurrentTargetOffsetTop + getPaddingTop(),
mReturnToStartPositionListener);
}
};
/**
* Simple constructor to use when creating a SwipeRefreshLayout from code.
* @param context
*/
public SwipeRefreshLayout(Context context) {
this(context, null);
}
/**
* Constructor that is called when inflating SwipeRefreshLayout from XML.
* @param context
* @param attrs
*/
public SwipeRefreshLayout(Context context, AttributeSet attrs) {
super(context, attrs);
mTouchSlop = ViewConfiguration.get(context).getScaledTouchSlop();
mMediumAnimationDuration = getResources().getInteger(
android.R.integer.config_mediumAnimTime);
setWillNotDraw(false);
mProgressBar = new SwipeProgressBar(this);
final DisplayMetrics metrics = getResources().getDisplayMetrics();
mProgressBarHeight = (int) (metrics.density * PROGRESS_BAR_HEIGHT);
mDecelerateInterpolator = new DecelerateInterpolator(DECELERATE_INTERPOLATION_FACTOR);
mAccelerateInterpolator = new AccelerateInterpolator(ACCELERATE_INTERPOLATION_FACTOR);
final TypedArray a = context.obtainStyledAttributes(attrs, LAYOUT_ATTRS);
setEnabled(a.getBoolean(0, true));
a.recycle();
}
@Override
public void onAttachedToWindow() {
super.onAttachedToWindow();
removeCallbacks(mCancel);
removeCallbacks(mReturnToStartPosition);
}
@Override
public void onDetachedFromWindow() {
super.onDetachedFromWindow();
removeCallbacks(mReturnToStartPosition);
removeCallbacks(mCancel);
}
private void animateOffsetToStartPosition(int from, AnimationListener listener) {
mFrom = from;
mAnimateToStartPosition.reset();
mAnimateToStartPosition.setDuration(mMediumAnimationDuration);
mAnimateToStartPosition.setAnimationListener(listener);
mAnimateToStartPosition.setInterpolator(mDecelerateInterpolator);
mTarget.startAnimation(mAnimateToStartPosition);
}
/**
* Set the listener to be notified when a refresh is triggered via the swipe
* gesture.
*/
public void setOnRefreshListener(OnRefreshListener listener) {
mListener = listener;
}
private void setTriggerPercentage(float percent) {
if (percent == 0f) {
// No-op. A null trigger means it's uninitialized, and setting it to zero-percent
// means we're trying to reset state, so there's nothing to reset in this case.
mCurrPercentage = 0;
return;
}
mCurrPercentage = percent;
mProgressBar.setTriggerPercentage(percent);
}
/**
* Notify the widget that refresh state has changed. Do not call this when
* refresh is triggered by a swipe gesture.
*
* @param refreshing Whether or not the view should show refresh progress.
*/
public void setRefreshing(boolean refreshing) {
if (mRefreshing != refreshing) {
ensureTarget();
mCurrPercentage = 0;
mRefreshing = refreshing;
if (mRefreshing) {
mProgressBar.start();
} else {
mProgressBar.stop();
}
}
}
/**
* @deprecated Use {@link #setColorSchemeResources(int, int, int, int)}
*/
@Deprecated
public void setColorScheme(int colorRes1, int colorRes2, int colorRes3, int colorRes4) {
setColorSchemeResources(colorRes1, colorRes2, colorRes3, colorRes4);
}
/**
* Set the four colors used in the progress animation from color resources.
* The first color will also be the color of the bar that grows in response
* to a user swipe gesture.
*/
public void setColorSchemeResources(int colorRes1, int colorRes2, int colorRes3,
int colorRes4) {
final Resources res = getResources();
setColorSchemeColors(res.getColor(colorRes1), res.getColor(colorRes2),
res.getColor(colorRes3), res.getColor(colorRes4));
}
/**
* Set the four colors used in the progress animation. The first color will
* also be the color of the bar that grows in response to a user swipe
* gesture.
*/
public void setColorSchemeColors(int color1, int color2, int color3, int color4) {
ensureTarget();
mProgressBar.setColorScheme(color1, color2, color3, color4);
}
/**
* @return Whether the SwipeRefreshWidget is actively showing refresh
* progress.
*/
public boolean isRefreshing() {
return mRefreshing;
}
private void ensureTarget() {
// Don't bother getting the parent height if the parent hasn't been laid out yet.
if (mTarget == null) {
if (getChildCount() > 1 && !isInEditMode()) {
throw new IllegalStateException(
"SwipeRefreshLayout can host only one direct child");
}
mTarget = getChildAt(0);
mOriginalOffsetTop = mTarget.getTop() + getPaddingTop();
}
if (mDistanceToTriggerSync == -1) {
if (getParent() != null && ((View)getParent()).getHeight() > 0) {
final DisplayMetrics metrics = getResources().getDisplayMetrics();
mDistanceToTriggerSync = (int) Math.min(
((View) getParent()) .getHeight() * MAX_SWIPE_DISTANCE_FACTOR,
REFRESH_TRIGGER_DISTANCE * metrics.density);
}
}
}
@Override
public void draw(Canvas canvas) {
super.draw(canvas);
mProgressBar.draw(canvas);
}
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
final int width = getMeasuredWidth();
final int height = getMeasuredHeight();
mProgressBar.setBounds(0, 0, width, mProgressBarHeight);
if (getChildCount() == 0) {
return;
}
final View child = getChildAt(0);
final int childLeft = getPaddingLeft();
final int childTop = mCurrentTargetOffsetTop + getPaddingTop();
final int childWidth = width - getPaddingLeft() - getPaddingRight();
final int childHeight = height - getPaddingTop() - getPaddingBottom();
child.layout(childLeft, childTop, childLeft + childWidth, childTop + childHeight);
}
@Override
public void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
if (getChildCount() > 1 && !isInEditMode()) {
throw new IllegalStateException("SwipeRefreshLayout can host only one direct child");
}
if (getChildCount() > 0) {
getChildAt(0).measure(
MeasureSpec.makeMeasureSpec(
getMeasuredWidth() - getPaddingLeft() - getPaddingRight(),
MeasureSpec.EXACTLY),
MeasureSpec.makeMeasureSpec(
getMeasuredHeight() - getPaddingTop() - getPaddingBottom(),
MeasureSpec.EXACTLY));
}
}
/**
* @return Whether it is possible for the child view of this layout to
* scroll up. Override this if the child view is a custom view.
*/
public boolean canChildScrollUp() {
if (android.os.Build.VERSION.SDK_INT < 14) {
if (mTarget instanceof AbsListView) {
final AbsListView absListView = (AbsListView) mTarget;
return absListView.getChildCount() > 0
&& (absListView.getFirstVisiblePosition() > 0 || absListView.getChildAt(0)
.getTop() < absListView.getPaddingTop());
} else {
return mTarget.getScrollY() > 0;
}
} else {
return ViewCompat.canScrollVertically(mTarget, -1);
}
}
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
ensureTarget();
final int action = MotionEventCompat.getActionMasked(ev);
if (mReturningToStart && action == MotionEvent.ACTION_DOWN) {
mReturningToStart = false;
}
if (!isEnabled() || mReturningToStart || canChildScrollUp()) {
// Fail fast if we're not in a state where a swipe is possible
return false;
}
switch (action) {
case MotionEvent.ACTION_DOWN:
mLastMotionY = mInitialMotionY = ev.getY();
mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
mIsBeingDragged = false;
mCurrPercentage = 0;
break;
case MotionEvent.ACTION_MOVE:
if (mActivePointerId == INVALID_POINTER) {
Log.e(LOG_TAG, "Got ACTION_MOVE event but don't have an active pointer id.");
return false;
}
final int pointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId);
if (pointerIndex < 0) {
Log.e(LOG_TAG, "Got ACTION_MOVE event but have an invalid active pointer id.");
return false;
}
final float y = MotionEventCompat.getY(ev, pointerIndex);
final float yDiff = y - mInitialMotionY;
if (yDiff > mTouchSlop) {
mLastMotionY = y;
mIsBeingDragged = true;
}
break;
case MotionEventCompat.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
case MotionEvent.ACTION_UP:
case MotionEvent.ACTION_CANCEL:
mIsBeingDragged = false;
mCurrPercentage = 0;
mActivePointerId = INVALID_POINTER;
break;
}
return mIsBeingDragged;
}
@Override
public void requestDisallowInterceptTouchEvent(boolean b) {
// Nope.
}
@Override
public boolean onTouchEvent(MotionEvent ev) {
final int action = MotionEventCompat.getActionMasked(ev);
if (mReturningToStart && action == MotionEvent.ACTION_DOWN) {
mReturningToStart = false;
}
if (!isEnabled() || mReturningToStart || canChildScrollUp()) {
// Fail fast if we're not in a state where a swipe is possible
return false;
}
switch (action) {
case MotionEvent.ACTION_DOWN:
mLastMotionY = mInitialMotionY = ev.getY();
mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
mIsBeingDragged = false;
mCurrPercentage = 0;
break;
case MotionEvent.ACTION_MOVE:
final int pointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId);
if (pointerIndex < 0) {
Log.e(LOG_TAG, "Got ACTION_MOVE event but have an invalid active pointer id.");
return false;
}
final float y = MotionEventCompat.getY(ev, pointerIndex);
final float yDiff = y - mInitialMotionY;
if (!mIsBeingDragged && yDiff > mTouchSlop) {
mIsBeingDragged = true;
}
if (mIsBeingDragged) {
// User velocity passed min velocity; trigger a refresh
if (yDiff > mDistanceToTriggerSync) {
// User movement passed distance; trigger a refresh
startRefresh();
} else {
// Just track the user's movement
setTriggerPercentage(
mAccelerateInterpolator.getInterpolation(
yDiff / mDistanceToTriggerSync));
updateContentOffsetTop((int) (yDiff));
if (mLastMotionY > y && mTarget.getTop() == getPaddingTop()) {
// If the user puts the view back at the top, we
// don't need to. This shouldn't be considered
// cancelling the gesture as the user can restart from the top.
removeCallbacks(mCancel);
} else {
updatePositionTimeout();
}
}
mLastMotionY = y;
}
break;
case MotionEventCompat.ACTION_POINTER_DOWN: {
final int index = MotionEventCompat.getActionIndex(ev);
mLastMotionY = MotionEventCompat.getY(ev, index);
mActivePointerId = MotionEventCompat.getPointerId(ev, index);
break;
}
case MotionEventCompat.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
case MotionEvent.ACTION_UP:
case MotionEvent.ACTION_CANCEL:
mIsBeingDragged = false;
mCurrPercentage = 0;
mActivePointerId = INVALID_POINTER;
return false;
}
return true;
}
private void startRefresh() {
removeCallbacks(mCancel);
mReturnToStartPosition.run();
setRefreshing(true);
mListener.onRefresh();
}
private void updateContentOffsetTop(int targetTop) {
final int currentTop = mTarget.getTop();
if (targetTop > mDistanceToTriggerSync) {
targetTop = (int) mDistanceToTriggerSync;
} else if (targetTop < 0) {
targetTop = 0;
}
setTargetOffsetTopAndBottom(targetTop - currentTop);
}
private void setTargetOffsetTopAndBottom(int offset) {
mTarget.offsetTopAndBottom(offset);
mCurrentTargetOffsetTop = mTarget.getTop();
}
private void updatePositionTimeout() {
removeCallbacks(mCancel);
postDelayed(mCancel, RETURN_TO_ORIGINAL_POSITION_TIMEOUT);
}
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;
mLastMotionY = MotionEventCompat.getY(ev, newPointerIndex);
mActivePointerId = MotionEventCompat.getPointerId(ev, newPointerIndex);
}
}
/**
* Classes that wish to be notified when the swipe gesture correctly
* triggers a refresh should implement this interface.
*/
public interface OnRefreshListener {
public void onRefresh();
}
/**
* Simple AnimationListener to avoid having to implement unneeded methods in
* AnimationListeners.
*/
private class BaseAnimationListener implements AnimationListener {
@Override
public void onAnimationStart(Animation animation) {
}
@Override
public void onAnimationEnd(Animation animation) {
}
@Override
public void onAnimationRepeat(Animation animation) {
}
}
}
洋洋洒洒几百行，还是有点多的，不过里面有不少是默认的刷新动画相关的东西，我们都可以忽略掉，只看核心的部分：
- 测量
@Override
public void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
if (getChildCount() > 1 && !isInEditMode()) {
throw new IllegalStateException("SwipeRefreshLayout can host only one direct child");
}
if (getChildCount() > 0) {
getChildAt(0).measure(
MeasureSpec.makeMeasureSpec(
getMeasuredWidth() - getPaddingLeft() - getPaddingRight(),
MeasureSpec.EXACTLY),
MeasureSpec.makeMeasureSpec(
getMeasuredHeight() - getPaddingTop() - getPaddingBottom(),
MeasureSpec.EXACTLY));
}
}
这是测量控件的大小，首先，判断了SwipeRefreshLayout的child的个数，可以看到如果多于1个，就直接抛出异常了，默认只允许有一个child，完后对child的长宽进行了测量，这里没什么好说的，就是用长宽值减去了padding的值，最后得到控件实际的长宽
- 摆放
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
final int width = getMeasuredWidth();
final int height = getMeasuredHeight();
mProgressBar.setBounds(0, 0, width, mProgressBarHeight);
if (getChildCount() == 0) {
return;
}
final View child = getChildAt(0);
final int childLeft = getPaddingLeft();
final int childTop = mCurrentTargetOffsetTop + getPaddingTop();
final int childWidth = width - getPaddingLeft() - getPaddingRight();
final int childHeight = height - getPaddingTop() - getPaddingBottom();
child.layout(childLeft, childTop, childLeft + childWidth, childTop + childHeight);
}
量完了控件实际的长宽，自然要把它摆放在正确的位置了，mProgressBar我们不管，这是默认的进度，我们会删掉，完后计算左下右上的位置，并调用layout方法将其放置在相应的位置上，这应该都看得懂
- 初始化滑动控件
private void ensureTarget() {
// Don't bother getting the parent height if the parent hasn't been laid out yet.
if (mTarget == null) {
if (getChildCount() > 1 && !isInEditMode()) {
throw new IllegalStateException(
"SwipeRefreshLayout can host only one direct child");
}
mTarget = getChildAt(0);
mOriginalOffsetTop = mTarget.getTop() + getPaddingTop();
}
if (mDistanceToTriggerSync == -1) {
if (getParent() != null && ((View)getParent()).getHeight() > 0) {
final DisplayMetrics metrics = getResources().getDisplayMetrics();
mDistanceToTriggerSync = (int) Math.min(
((View) getParent()) .getHeight() * MAX_SWIPE_DISTANCE_FACTOR,
REFRESH_TRIGGER_DISTANCE * metrics.density);
}
}
}
这个方法用来确认要滑动的控件，将滑动控件赋值给mTarget，并定义了mOriginalOffsetTop变量，它是控件当前顶部位置加上SwipeRefreshLayout的paddingTop的值，也就是控件的顶部坐标了，下面的mDistanceToTriggerSync这个变量是滑动多少距离后触发刷新，可以看到它是通过父控件的高度乘以一个因子，或者从默认的最大滑动距离两个值取小
- 事件拦截
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
ensureTarget();
final int action = MotionEventCompat.getActionMasked(ev);
if (mReturningToStart && action == MotionEvent.ACTION_DOWN) {
mReturningToStart = false;
}
if (!isEnabled() || mReturningToStart || canChildScrollUp()) {
// Fail fast if we're not in a state where a swipe is possible
return false;
}
switch (action) {
case MotionEvent.ACTION_DOWN:
mLastMotionY = mInitialMotionY = ev.getY();
mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
mIsBeingDragged = false;
mCurrPercentage = 0;
break;
case MotionEvent.ACTION_MOVE:
if (mActivePointerId == INVALID_POINTER) {
Log.e(LOG_TAG, "Got ACTION_MOVE event but don't have an active pointer id.");
return false;
}
final int pointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId);
if (pointerIndex < 0) {
Log.e(LOG_TAG, "Got ACTION_MOVE event but have an invalid active pointer id.");
return false;
}
final float y = MotionEventCompat.getY(ev, pointerIndex);
final float yDiff = y - mInitialMotionY;
if (yDiff > mTouchSlop) {
mLastMotionY = y;
mIsBeingDragged = true;
}
break;
case MotionEventCompat.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
case MotionEvent.ACTION_UP:
case MotionEvent.ACTION_CANCEL:
mIsBeingDragged = false;
mCurrPercentage = 0;
mActivePointerId = INVALID_POINTER;
break;
}
return mIsBeingDragged;
}
这里是对用户的操作进行拦截，首先调用了第3步中的ensureTarget来确保滑动控件被初始化了，完后可以看到获取了ACTION，这里跟平时直接用getAction方法来获取还是有些区别的，这里用到的getActionMasked方法可以捕获到多个手指的事件，这个后面会说，下面的mReturningToStart这个是布尔变量，初始为false，用来判断滑动控件是否在执行滑动回顶点的动画，在动画开始的时候，会将这个值赋值为true，完后这里判断如果值为true，并且为按下事件，则赋值为false，表示是一次新的滑动了。
if (!isEnabled() || mReturningToStart || canChildScrollUp())
这3个分别是判断是否可用，是否正在执行动画，以及是否能滚动，如果不满足条件，直接返回false，就不往下执行了。
ACTION_DOWN，mLastMotionY是最后滑动的y坐标，mInitialMotionY是按下时初始的y坐标，mActivePointerId是第一个按下手指的id，mIsBeingDragged是一个布尔变量，表示是否正在拖动，mCurrPercentage是默认动画相关的百分比，改造后用不到
ACTION_MOVE，首先判断了id是否为-1，是的话直接报错返回（UP或CANCEL时会赋值为-1），完后获取现在的y坐标，并计算与初始y坐标的差是否大于mTouchSlop，这个值是ViewConfiguration类中定义的一个常量，它定义了最小滑动距离，小于这个值表示不是正常的滑动，一般你随便滑动一下肯定超过这个值，ViewConfiguration中定义了很多这种标准值，有兴趣的可以自己去看看，如果大于mTouchSlop，则将mLastMotionY赋值为新值，并且将mIsBeingDragged赋值为true，表示控件开始随手指滑动了
ACTION_UP/ACTION_CANCEL，这里很简单，不说了
最后还有一个ACTION_POINTER_UP，这里调用了onSecondaryPointerUp方法，这个具体到onTouchEvent中再说
（由于字数太多不能发在同一篇里，下接这里）
作者：gesanri
来源：CSDN
原文：https://blog.csdn.net/gesanri/article/details/50149059
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