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SwipeRefreshLayout是Android官方的下拉刷新控件，使用简单，界面美观，不熟悉的朋友可以随便搜索了解一下，这里就不废话了，直接进入正题。
这种下拉刷新控件的原理不难，基本就是监听手指的运动，获取手指的坐标，通过计算判断出是哪种操作，然后就是回调相应的接口了。SwipeRefreshLayout是继承自ViewGroup的，根据Android的事件分发机制，触摸事件应该是先传递到ViewGroup，根据onInterceptTouchEvent的返回值决定是否拦截事件的，那么就onInterceptTouchEvent出发：
@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()
|| mRefreshing || mNestedScrollInProgress) {
// Fail fast if we're not in a state where a swipe is possible
return false;
}
switch (action) {
case MotionEvent.ACTION_DOWN:
setTargetOffsetTopAndBottom(mOriginalOffsetTop - mCircleView.getTop(), true);
mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
mIsBeingDragged = false;
final float initialDownY = getMotionEventY(ev, mActivePointerId);
if (initialDownY == -1) {
return false;
}
mInitialDownY = initialDownY;
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 float y = getMotionEventY(ev, mActivePointerId);
if (y == -1) {
return false;
}
final float yDiff = y - mInitialDownY;
if (yDiff > mTouchSlop && !mIsBeingDragged) {
mInitialMotionY = mInitialDownY + mTouchSlop;
mIsBeingDragged = true;
mProgress.setAlpha(STARTING_PROGRESS_ALPHA);
}
break;
case MotionEventCompat.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
case MotionEvent.ACTION_UP:
case MotionEvent.ACTION_CANCEL:
mIsBeingDragged = false;
mActivePointerId = INVALID_POINTER;
break;
}
return mIsBeingDragged;
}
是否拦截的情况有很多种，这里如果满足五个条件之一就直接返回false，使用时触摸事件发生冲突的话就可以从这里出发分析，这里也不具体展开了。简单看一下，在ACTION_DOWN中记录下手指坐标，ACTION_MOVE中计算出移动的距离，并且判断是否大于阈值，是的话就将mIsBeingDragged标志位设为true，ACTION_UP中则将mIsBeingDragged设为false。最后返回的是mIsBeingDragged。
SwipeRefreshLayout一般是嵌套可滚动的View使用的，正常滚动时会满足前面的条件，这时不进行拦截，只有当滚动到顶部才会进入后面action的判断。在手指按下和抬起期间mIsBeingDragged为true，也就是说进行拦截，接下来就是如何处理了，看看onTouchEvent：
@Override
public boolean onTouchEvent(MotionEvent ev) {
....
switch (action) {
case MotionEvent.ACTION_DOWN:
mActivePointerId = MotionEventCompat.getPointerId(ev, 0);
mIsBeingDragged = false;
break;
case MotionEvent.ACTION_MOVE: {
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 overscrollTop = (y - mInitialMotionY) * DRAG_RATE;
if (mIsBeingDragged) {
if (overscrollTop > 0) {
moveSpinner(overscrollTop);
} else {
return false;
}
}
break;
}
....
case MotionEvent.ACTION_UP: {
pointerIndex = MotionEventCompat.findPointerIndex(ev, mActivePointerId);
if (pointerIndex < 0) {
Log.e(LOG_TAG, "Got ACTION_UP event but don't have an active pointer id.");
return false;
}
final float y = MotionEventCompat.getY(ev, pointerIndex);
final float overscrollTop = (y - mInitialMotionY) * DRAG_RATE;
mIsBeingDragged = false;
finishSpinner(overscrollTop);
mActivePointerId = INVALID_POINTER;
return false;
}
case MotionEvent.ACTION_CANCEL:
return false;
}
return true;
}
这里省略了一些代码，前面还有几行跟上面的类似，也是在满足其中一个条件时直接返回；switch中也还有几行处理多指触控的，这些都略过了。看一下ACTION_MOVE中计算了手指移动的距离，这时的mIsBeingDragged正常情况下应为true，当距离大于零就会执行moveSpinner。在ACTION_UP中则会执行finishSpinner，到这里就可以猜出，执行刷新的逻辑主要就在这两个方法中。
看这两个方法前，要知道两个重要的成员变量：一个是mCircleView，是CircleImageView的实例，继承了ImageView，主要绘制进度圈的背景；另一个是mProgress，是MaterialProgressDrawable的实例，继承自Drawable且实现Animatable接口，主要绘制进度圈，SwipeRefreshLayout正是通过调用其方法来绘制动画。接下来就先看一下moveSpinner：
<span style="font-size:18px;">
private void moveSpinner(float overscrollTop) {
mProgress.showArrow(true);
float originalDragPercent = overscrollTop / mTotalDragDistance;
float dragPercent = Math.min(1f, Math.abs(originalDragPercent));
float adjustedPercent = (float) Math.max(dragPercent - .4, 0) * 5 / 3;
float extraOS = Math.abs(overscrollTop) - mTotalDragDistance;
float slingshotDist = mUsingCustomStart ? mSpinnerFinalOffset - mOriginalOffsetTop
: mSpinnerFinalOffset;
float tensionSlingshotPercent = Math.max(0, Math.min(extraOS, slingshotDist * 2)
/ slingshotDist);
float tensionPercent = (float) ((tensionSlingshotPercent / 4) - Math.pow(
(tensionSlingshotPercent / 4), 2)) * 2f;
float extraMove = (slingshotDist) * tensionPercent * 2;
int targetY = mOriginalOffsetTop + (int) ((slingshotDist * dragPercent) + extraMove);
// where 1.0f is a full circle
if (mCircleView.getVisibility() != View.VISIBLE) {
mCircleView.setVisibility(View.VISIBLE);
}
if (!mScale) {
ViewCompat.setScaleX(mCircleView, 1f);
ViewCompat.setScaleY(mCircleView, 1f);
}
if (mScale) {
setAnimationProgress(Math.min(1f, overscrollTop / mTotalDragDistance));
}
if (overscrollTop < mTotalDragDistance) {
if (mProgress.getAlpha() > STARTING_PROGRESS_ALPHA
&& !isAnimationRunning(mAlphaStartAnimation)) {
// Animate the alpha
startProgressAlphaStartAnimation();
}
} else {
if (mProgress.getAlpha() < MAX_ALPHA && !isAnimationRunning(mAlphaMaxAnimation)) {
// Animate the alpha
startProgressAlphaMaxAnimation();
}
}
float strokeStart = adjustedPercent * .8f;
mProgress.setStartEndTrim(0f, Math.min(MAX_PROGRESS_ANGLE, strokeStart));
mProgress.setArrowScale(Math.min(1f, adjustedPercent));
float rotation = (-0.25f + .4f * adjustedPercent + tensionPercent * 2) * .5f;
mProgress.setProgressRotation(rotation);
setTargetOffsetTopAndBottom(targetY - mCurrentTargetOffsetTop, true /* requires update */);
}</span>showArrow是显示箭头，中间那一坨主要也是一些math和设置进度圈的样式，倒数第二行执行了setProgressRotation，传入的是经过一堆计算后的rotation，这堆计算主要是优化效果，比如在刚开始移动时增长比较快，超过刷新的距离后就增长比较慢。传入该方法后，mProgress就根据它来绘制进度圈，因此主要的动画就应该在这个方法内。最后一行执行setTargetOffsetTopAndBottom，我们来看一下：<span style="font-size:18px;">private void setTargetOffsetTopAndBottom(int offset, boolean requiresUpdate) {
mCircleView.bringToFront();
mCircleView.offsetTopAndBottom(offset);
mCurrentTargetOffsetTop = mCircleView.getTop();
if (requiresUpdate && android.os.Build.VERSION.SDK_INT < 11) {
invalidate();
}
}</span><span style="font-size:18px;">private void finishSpinner(float overscrollTop) {
if (overscrollTop > mTotalDragDistance) {
setRefreshing(true, true /* notify */);
} else {
// cancel refresh
mRefreshing = false;
mProgress.setStartEndTrim(0f, 0f);
Animation.AnimationListener listener = null;
if (!mScale) {
listener = new Animation.AnimationListener() {
@Override
public void onAnimationStart(Animation animation) {
}
@Override
public void onAnimationEnd(Animation animation) {
if (!mScale) {
startScaleDownAnimation(null);
}
}
@Override
public void onAnimationRepeat(Animation animation) {
}
};
}
animateOffsetToStartPosition(mCurrentTargetOffsetTop, listener);
mProgress.showArrow(false);
}
}</span>
逻辑也很简单，当移动的距离超过设定值时就执行setRefreshing（true，true），在该方法里更新一些成员变量的值后会执行animateOffsetToCorrectPosition，由名字就知道是执行动画将进度圈移动到正确位置的（也就是头部）。如果移动的距离没有超过设定值，就会执行animateOffsetToStartPosition。一起看一下animateOffsetToCorrectPosition和animateOffsetToStartPosition这两个方法：
<span style="font-size:18px;">private void animateOffsetToCorrectPosition(int from, AnimationListener listener) {
mFrom = from;
mAnimateToCorrectPosition.reset();
mAnimateToCorrectPosition.setDuration(ANIMATE_TO_TRIGGER_DURATION);
mAnimateToCorrectPosition.setInterpolator(mDecelerateInterpolator);
if (listener != null) {
mCircleView.setAnimationListener(listener);
}
mCircleView.clearAnimation();
mCircleView.startAnimation(mAnimateToCorrectPosition);
}
private void animateOffsetToStartPosition(int from, AnimationListener listener) {
if (mScale) {
// Scale the item back down
startScaleDownReturnToStartAnimation(from, listener);
} else {
mFrom = from;
mAnimateToStartPosition.reset();
mAnimateToStartPosition.setDuration(ANIMATE_TO_START_DURATION);
mAnimateToStartPosition.setInterpolator(mDecelerateInterpolator);
if (listener != null) {
mCircleView.setAnimationListener(listener);
}
mCircleView.clearAnimation();
mCircleView.startAnimation(mAnimateToStartPosition);
}
}</span>逻辑基本相同，进行一些设置后，最后都会执行mCircleView的startAnimation，只是传入的值以及监听器不同。如果是要执行刷新的操作，传入的值是头部高度，监听器为：
<span style="font-size:18px;">private Animation.AnimationListener mRefreshListener = new Animation.AnimationListener() {
@Override
public void onAnimationStart(Animation animation) {
}
@Override
public void onAnimationRepeat(Animation animation) {
}
@Override
public void onAnimationEnd(Animation animation) {
if (mRefreshing) {
// Make sure the progress view is fully visible
mProgress.setAlpha(MAX_ALPHA);
mProgress.start();
if (mNotify) {
if (mListener != null) {
mListener.onRefresh();
}
}
mCurrentTargetOffsetTop = mCircleView.getTop();
} else {
reset();
}
}
};</span>动画完成后，也就是进度圈移动到头部后，会执行mProgress.start();这里执行的就是在刷新时进度圈转啊转的动画。接下来注意到如果mListener不为空就会执行onRefresh方法，这个mListener其实就是执行setOnRefreshListener所设置的监听器，因此在这里完成刷新。如果是执行回到初始位置的操作，传入的值为初始高度（也就是顶部之上），监听器为<span style="font-size:18px;">listener = new Animation.AnimationListener() {
@Override
public void onAnimationStart(Animation animation) {
}
@Override
public void onAnimationEnd(Animation animation) {
if (!mScale) {
startScaleDownAnimation(null);
}
}
@Override
public void onAnimationRepeat(Animation animation) {
}
};</span>移动到初始位置后会执行startScaleDownAnimation，也就是消失的动画了，到这里整个刷新流程就结束了。Android下拉刷新控件SwipeRefreshLayout源码浅析
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原文地址：http://blog.csdn.net/u011443509/article/details/52017355
