上一篇博客：《连按两次Back键退出应用程序》，我们从源码的角度分析了一下，虽然这个功能实现起来很简单，但是我们还是能通过系统源码学到很多知识。今天我们从源码的角度来分析一下SwipeRefreshLayout这个神奇的控件吧~
SwipeRefreshLayout是当之无愧的下拉刷新全能王，它继承了ViewGroup，因此，我们的分析角度就可以这么来：构造函数->onMeasure->onLayout->onInterceptTouchEvent->onTouchEvent
首先，我们来分析一下构造函数
/**
* 在代码中创建SwipeRefreshLayout的时候，会调用这个方法
* Simple constructor to use when creating a SwipeRefreshLayout from code.
*
* @param context
*/
public SwipeRefreshLayout(Context context) {
this(context, null);
}
/**
* 在布局文件中创建SwipeRefreshLayout的时候，会调用这个方法
* Constructor that is called when inflating SwipeRefreshLayout from XML.
*
* @param context
* @param attrs
*/
public SwipeRefreshLayout(Context context, AttributeSet attrs) {
super(context, attrs);
//ViewConfigurationi:包含了方法和标准的常量用来设置UI的超时、大小和距离
//mTouchSlop:是一个距离，表示滑动的时候，手的移动要大于这个距离才开始移动控件。
mTouchSlop = ViewConfiguration.get(context).getScaledTouchSlop();
mMediumAnimationDuration = getResources().getInteger(
android.R.integer.config_mediumAnimTime);
setWillNotDraw(false);
mDecelerateInterpolator = new DecelerateInterpolator(DECELERATE_INTERPOLATION_FACTOR);
final TypedArray a = context.obtainStyledAttributes(attrs, LAYOUT_ATTRS);
setEnabled(a.getBoolean(0, true));
a.recycle();
final DisplayMetrics metrics = getResources().getDisplayMetrics();
mCircleDiameter = (int) (CIRCLE_DIAMETER * metrics.density);
//创建了下拉刷新中那个会旋转的圈圈，并添加到整个布局里面
createProgressView();
ViewCompat.setChildrenDrawingOrderEnabled(this, true);
// the absolute offset has to take into account that the circle starts at an offset
mSpinnerFinalOffset = DEFAULT_CIRCLE_TARGET * metrics.density;
mTotalDragDistance = mSpinnerFinalOffset;
mNestedScrollingParentHelper = new NestedScrollingParentHelper(this);
mNestedScrollingChildHelper = new NestedScrollingChildHelper(this);
setNestedScrollingEnabled(true);
mOriginalOffsetTop = mCurrentTargetOffsetTop = -mCircleDiameter;
//把那个旋转的小圈圈放到初始位置
moveToStart(1.0f);
}它只实现了两个构造方法，并且在第二个构造方法中对UI进行了一定的初始化
然后我们来看看onMeasure方法
@Override
public void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
if (mTarget == null) {
ensureTarget();
}
if (mTarget == null) {
return;
}
//mTarget的尺寸为match_parent，除去内边距
mTarget.measure(MeasureSpec.makeMeasureSpec(
getMeasuredWidth() - getPaddingLeft() - getPaddingRight(),
MeasureSpec.EXACTLY), MeasureSpec.makeMeasureSpec(
getMeasuredHeight() - getPaddingTop() - getPaddingBottom(), MeasureSpec.EXACTLY));
//设置mCircleView的尺寸
mCircleView.measure(MeasureSpec.makeMeasureSpec(mCircleWidth, MeasureSpec.EXACTLY),
MeasureSpec.makeMeasureSpec(mCircleHeight, MeasureSpec.EXACTLY));
//如果mOriginalOffsetTop未被初始化并且mUsingCustomStart ？，则将下拉小圆的初始位置设置成默认值
if (!mUsingCustomStart && !mOriginalOffsetCalculated) {
mOriginalOffsetCalculated = true;
mCurrentTargetOffsetTop = mOriginalOffsetTop = -mCircleView.getMeasuredHeight();
}
mCircleViewIndex = -1;
// Get the index of the circleview.获取circleview的索引值，主要是为了后面重载getChildDrawingOrder时要用
for (int index = 0; index < getChildCount(); index++) {
if (getChildAt(index) == mCircleView) {
mCircleViewIndex = index;
break;
}
}
}大概就是计算一下各个控件需要的宽高，计算好之后就要考虑如何摆放这些控件了，我们来看看onLayout方法
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
final int width = getMeasuredWidth();
final int height = getMeasuredHeight();
if (getChildCount() == 0) {
return;
}
//再三确认一下目标控件是否正确（可滚动的子控件）
if (mTarget == null) {
ensureTarget();
}
if (mTarget == null) {
return;
}
final View child = mTarget;
final int childLeft = getPaddingLeft();
final int childTop = getPaddingTop();
final int childWidth = width - getPaddingLeft() - getPaddingRight();
final int childHeight = height - getPaddingTop() - getPaddingBottom();
//将mTarget放在覆盖parent的位置（除去内边距）
child.layout(childLeft, childTop, childLeft + childWidth, childTop + childHeight);
//将mCircleView放在mTarget的平面位置上面居中，初始化时是完全隐藏在屏幕外的
int circleWidth = mCircleView.getMeasuredWidth();
int circleHeight = mCircleView.getMeasuredHeight();
mCircleView.layout((width / 2 - circleWidth / 2), mCurrentTargetOffsetTop,
(width / 2 + circleWidth / 2), mCurrentTargetOffsetTop + circleHeight);
}控件都摆放好了，接下来就要处理用户的下拉动作了
@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
ensureTarget();
final int action = MotionEventCompat.getActionMasked(ev);
//如果当mCircleView正在返回初始位置的同时手指按下了，将标志mReturningToStart复位
if (mReturningToStart && action == MotionEvent.ACTION_DOWN) {
mReturningToStart = false;
}
//如果下拉被禁用、mCircleView正在返回初始位置、mTarget没有到达顶部、
//正在刷新、mNestedScrollInProgress
// 不拦截，不处理点击事件，处理权交还mTarget
if (!isEnabled() || mReturningToStart || canChildScrollUp()
|| mRefreshing || mNestedScrollInProgress) {
// Fail fast if we're not in a state where a swipe is possible
return false;
}
switch (action) {
//手指按下时，记录按下的坐标
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;
//如果是滑动动作，将mIsBeingDragged置为true
if (yDiff > mTouchSlop && !mIsBeingDragged) {
mInitialMotionY = mInitialDownY + mTouchSlop;
mIsBeingDragged = true;
mProgress.setAlpha(STARTING_PROGRESS_ALPHA);
}
break;
//处理多指触控
case MotionEventCompat.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
//手指松开，将标志复位
case MotionEvent.ACTION_UP:
case MotionEvent.ACTION_CANCEL:
mIsBeingDragged = false;
mActivePointerId = INVALID_POINTER;
break;
}
//如果正在被拖拽，拦截该系列的点击事件，并调用自己的onTouchEvent()来处理
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
@Override
public boolean onTouchEvent(MotionEvent ev) {
final int action = MotionEventCompat.getActionMasked(ev);
int pointerIndex = -1;
if (mReturningToStart && action == MotionEvent.ACTION_DOWN) {
mReturningToStart = false;
}
//如果被禁用、CircleView正在复位、没到达顶部、mNestedScrollInProgress，直接返回，不处理该事件
if (!isEnabled() || mReturningToStart || canChildScrollUp() || mNestedScrollInProgress) {
// Fail fast if we're not in a state where a swipe is possible
return false;
}
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);
//下拉的总高度
final float overscrollTop = (y - mInitialMotionY) * DRAG_RATE;
if (mIsBeingDragged) {
if (overscrollTop > 0) {
//spinner可理解为下拉组件，将spinner移到指定的高度
//很关键的方法，进入看看
moveSpinner(overscrollTop);
} else {
return false;
}
}
break;
}
//多指触控的处理
case MotionEventCompat.ACTION_POINTER_DOWN: {
pointerIndex = MotionEventCompat.getActionIndex(ev);
if (pointerIndex < 0) {
Log.e(LOG_TAG, "Got ACTION_POINTER_DOWN event but have an invalid action index.");
return false;
}
mActivePointerId = MotionEventCompat.getPointerId(ev, pointerIndex);
break;
}
case MotionEventCompat.ACTION_POINTER_UP:
onSecondaryPointerUp(ev);
break;
//关键代码！
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);
//计算松开手时下拉的总距离
final float overscrollTop = (y - mInitialMotionY) * DRAG_RATE;
mIsBeingDragged = false;
//关键方法，进去看看
finishSpinner(overscrollTop);
mActivePointerId = INVALID_POINTER;
return false;
}
case MotionEvent.ACTION_CANCEL:
return false;
}
return true;
}这个控件还没有分析完，等待我的更新吧。我的微信公众号：代码也是人。每天都会分享一篇技术干货，赶紧关注吧！觉得我写得好的话，记得收藏一下哈
代码也是人
