<p>SwipRefreshLayout是google提供的support v4包下面的下拉刷新控件，他继承自ViewGroup,内部可以放几乎所有的滚动控件。This layout should be made the parent of the view that will be refreshed as a result of the gesture and can only support one direct child.</p>    <p>本文不涉及到具体的使用，因为这个控件已经烂大街了，在很多标榜material design设计的app中都标配这个活泼的小球，在这样的情况下，出现了向美团，京东等，下拉出现更有趣动画的实现，比如</p>    <p><img src="https://simg.open-open.com/show/0219a59f6be33090645a55f4b40eeece.gif"></p>    <p>recruit-lifestyle/WaveSwipeRefreshLayout水滴下拉刷新…</p>    <p><img src="https://simg.open-open.com/show/1ddf0b842df8caa94a4a4b54d73a10e0.gif"></p>    <p>WaveRefreshForAndroid这个是基于Android-PullToRefresh修改的而成的水波纹下拉刷新…可能作者主攻ios,所以ios的效果看起来好看点WaveRefresh…</p>    <p><img src="https://simg.open-open.com/show/0fa80af5b8b9d35fdbd0214c54316585.gif"></p>    <p>我们会发现，他们好像各有各的炫酷狂拽吊炸天，但是有好像和亲兄弟一样，都是通过下拉动作，触发一系列的动画和动作。下面就以他们的爹爹SwipRefreshLayout来分析他们是如何实现的，了解了原来，想在SwipRefreshLayout上定制一个自己的下拉库也就易如反掌了。</p>    <h2>SwipeRefreshLayout extends ViewGroup</h2>    <p>再炫的控件也是要继承基础的View，既然是继承ViewGroup，那么他肯定要实现下面俩个方法</p>    <ul>     <li>onMeasure</li>     <li>onLayout</li>    </ul>    <p>找到对应的代码</p>    <h3>onMeasure</h3>    <p>其实SwipRefreshLayout中只有来个子View，一个是类似listview，还有个是它自己添加的circleView，在onMeasure中计算childView的测量值以及模式，以及设置自己的宽和高,</p>    <pre>  <code class="language-java">@Override     public void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {         super.onMeasure(widthMeasureSpec, heightMeasureSpec);        //判断内部控件是否是空，如果是空,就去找到，这里其实就是找mTarget=listview         if (mTarget == null) {             ensureTarget();         }         if (mTarget == null) {             return;         }       //测量listview大小,去掉padding         mTarget.measure(MeasureSpec.makeMeasureSpec(                 getMeasuredWidth() - getPaddingLeft() - getPaddingRight(),                 MeasureSpec.EXACTLY), MeasureSpec.makeMeasureSpec(                 getMeasuredHeight() - getPaddingTop() - getPaddingBottom(), MeasureSpec.EXACTLY));          //测量小圆球大小,绝对大小        //mCircleHeight = mCircleWidth = (int) (CIRCLE_DIAMETER_LARGE * metrics.density);       //mCircleHeight = mCircleWidth = (int) (CIRCLE_DIAMETER * metrics.density);         mCircleView.measure(MeasureSpec.makeMeasureSpec(mCircleWidth, MeasureSpec.EXACTLY),                 MeasureSpec.makeMeasureSpec(mCircleHeight, MeasureSpec.EXACTLY));         //mCurrentTargetOffsetTop 小圆球的上边Y轴坐标         if (!mUsingCustomStart && !mOriginalOffsetCalculated) {             mOriginalOffsetCalculated = true;             mCurrentTargetOffsetTop = mOriginalOffsetTop = -mCircleView.getMeasuredHeight();         }         mCircleViewIndex = -1;         // Get the index of the circleview.         for (int index = 0; index < getChildCount(); index++) {             if (getChildAt(index) == mCircleView) {                 mCircleViewIndex = index;                 break;             }         }     }  </code></pre>    <h3>onLayout</h3>    <p>在onLayout中对子view固定位置，忽然想到，下拉的时候也就是位置在改变，所以每次重新绘制的时候，onlayout中肯定有一个变量，在决定着下拉的高度，仔细看下，果然有个变量mCurrentTargetOffsetTop，如果我们找到了这个变量的变化的触发方法，也就是找到了下拉刷新核心秘密。</p>    <pre>  <code class="language-java">@Override  protected void onLayout(boolean changed, int left, int top, int right, int bottom) {  //获取自己的宽高      final int width = getMeasuredWidth();      final int height = getMeasuredHeight();      if (getChildCount() == 0) {          return;      }      //同样的代码，确定内部的view      if (mTarget == null) {          ensureTarget();      }      if (mTarget == null) {          return;      }     //这里感觉有点多余，childRight不就是width-getPaddingRight()么，老外的想法也真奇怪      final View child = mTarget;      final int childLeft = getPaddingLeft();      final int childTop = getPaddingTop();      final int childWidth = width - getPaddingLeft() - getPaddingRight();      final int childHeight = height - getPaddingTop() - getPaddingBottom();      child.layout(childLeft, childTop, childLeft + childWidth, childTop + childHeight);      int circleWidth = mCircleView.getMeasuredWidth();      int circleHeight = mCircleView.getMeasuredHeight();     //关键的一个参数，mCurrentTargetOffsetTop决定着小圆球下拉过程中的高度      mCircleView.layout((width / 2 - circleWidth / 2), mCurrentTargetOffsetTop,              (width / 2 + circleWidth / 2), mCurrentTargetOffsetTop + circleHeight);  }  </code></pre>    <p>盗图来显示</p>    <p><img src="https://simg.open-open.com/show/5270f060363d7e300d912afa3899e7c8.png"></p>    <h2>View的事件分发</h2>    <p>View的绘制过程中俩个最重要的方法找到了，那么下面就是下拉过程中的事件分发了。先介绍一下事件分发最关键的几个点</p>    <ul>     <li>dispatchTouchEvent(MotionEvent ev)</li>     <li>onInterceptTouchEvent(MotionEvent ev)</li>     <li>onTouchEvent(MotionEvent ev)<br> 这三者的关系我之前一直搞不清，最后在任玉刚的《Android开发艺术探索中》在看到下面这段代码才算明白，这是一段伟大的代码,简单几行，事件传递的奥义表达的淋漓尽致 。</li>    </ul>    <pre>  <code class="language-java">public boolean dispatchTouchEvent(MotionEvent ev){   boolean consume=false;          if(onInterceptTouchEvent(ev)){              consume=onTouchEvent(ev);          }else{              consume=child.dispatchTouchEvnet(ev);          }    return consume;  }  </code></pre>    <p>一个滑动事件传过来，首先SwipRefreshLayout在onInterceptTouchEvent中决定要不要拦截当前事件，如果不拦截就分发给listview，如果拦截那么它的onTouchEvent会处理对应的事件.</p>    <h3>onInterceptTouchEvent</h3>    <pre>  <code class="language-java">public boolean onInterceptTouchEvent(MotionEvent ev) {         ensureTarget();         final int action = MotionEventCompat.getActionMasked(ev);       ....      一些边际情况判断      ....            switch (action) {             case MotionEvent.ACTION_DOWN:                 setTargetOffsetTopAndBottom(mOriginalOffsetTop - mCircleView.getTop(), true);                //多指触摸的时候，获取第一个                 mActivePointerId = MotionEventCompat.getPointerId(ev, 0);                 mIsBeingDragged = false;                 final float initialDownY = getMotionEventY(ev, mActivePointerId);                 if (initialDownY == -1) {                     return false;                 }     //记录下按下的位置，老套路了                 mInitialDownY = initialDownY;                 break;               case MotionEvent.ACTION_MOVE:                 final float y = getMotionEventY(ev, mActivePointerId);                 if (y == -1) {                     return false;                 }     //最后的位置，和按下的位置获取滑动距离yDiff                 final float yDiff = y - mInitialDownY;     //滑动距离大于最小滑动距离时候拦截这个事件                 if (yDiff > mTouchSlop && !mIsBeingDragged) {                     mInitialMotionY = mInitialDownY + mTouchSlop;                     mIsBeingDragged = true;                     mProgress.setAlpha(STARTING_PROGRESS_ALPHA);                 }                 break;           }           return mIsBeingDragged;     }  </code></pre>    <h3>onTouchEvent</h3>    <pre>  <code class="language-java">public boolean onTouchEvent(MotionEvent ev) {          。。。         各种情况判断         。。。         switch (action) {             case MotionEvent.ACTION_MOVE: {                 final float y = MotionEventCompat.getY(ev, pointerIndex);                //关键的来了，move事件传递到这里，当前Y-初始位置，再乘以阻尼系数.5f，得到一个距离overscrollTop，传到了moveSpinner中，那moveSpinner(overscrollTop)肯定就是触发滑动的关键方法了                 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;                //手指抬起来的时候，在finishSpinner中判断要触发刷新onRefresh还是显示个动画就弹回去                 finishSpinner(overscrollTop);                 mActivePointerId = INVALID_POINTER;                 return false;             }         }           return true;     }  </code></pre>    <h3>moveSpinner</h3>    <p>这个方法，在各种计算之后，设置mCircleView的scale和alpha，然后又设置了圆球中间的mProgress的角度，并没有更新mCurrentTargetOffsetTop ,最后调用了setTargetOffsetTopAndBottom方法，接着看</p>    <pre>  <code class="language-java">private void moveSpinner(float overscrollTop) {             // 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);       }             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 */);   }  </code></pre>    <h3>setTargetOffsetTopAndBottom</h3>    <p>将mCircleView 显示出来，设置offset，也就是会触发mCircleView的ondraw，然后mCurrentTargetOffsetTop变量再次被赋值，如果api<11的时候手动触发刷新,这样下一次swiprefreshlayout执行 onmeasure和onlayout的时候，就知道circleview在哪里，多大。=""</p>    <pre>  <code class="language-java">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();         }     }  </code></pre>    <p>MotionEvent重复下去，mCurrentTargetOffsetTop不断更新位置，SwipRefreshLayout不断的draw，小圆球跟着手指移动的动画就完成了。</p>    <p>那么如果要自己定制这样的动画，怎么做？</p>    <p>首先流程不变，circleView要换成自己要的View，moveSpinner方法要大改，子view根据overscrollTop，计算出百分百，阻尼，进度来等等数据一步一步的设置，ACTION_UP的时候，还有改造finishSpinner设置手指抬起的时候，View的显示逻辑.这样简单的定制就完成了。</p>    <p> </p>    <p>来自： <a href="/misc/goto?guid=4959674732373210165" rel="nofollow">http://xujinyang.github.io/2016/06/19/SwipeRefreshLayout要点详解/</a></p>    <p> </p>