BottomSheets源码解析

2月25日早上，Android官网更新了Support Lirary 23.2版本，其中Design Support Library库新加一个新的东西：Bottom Sheets。然后，第一时间写了篇Teach you how to use Design Support Library: Bottom Sheets，只是简单的讲了它的使用和使用的一些规范。

这篇文章我带大家看看BottomSheetBehavior的源码，能力有限，写的不好的地方，请尽力吐槽。好了，不说废话，直接主题

我们先简单的看下用法

        // The View with the BottomSheetBehavior
        View bottomSheet = coordinatorLayout.findViewById(R.id.bottom_sheet);
        final BottomSheetBehavior behavior = BottomSheetBehavior.from(bottomSheet);
        behavior.setBottomSheetCallback(new BottomSheetBehavior.BottomSheetCallback() {
            @Override
            public void onStateChanged(@NonNull View bottomSheet, int newState) {
                //这里是bottomSheet 状态的改变回调
            }

            @Override
            public void onSlide(@NonNull View bottomSheet, float slideOffset) {
                //这里是拖拽中的回调，根据slideOffset可以做一些动画
            }
        });

对于切换状态,你也可以手动调用behavior.setState(int state); state 的值你可以看我的上一篇戳我

BottomSheetBehavior的定义如下

    public class BottomSheetBehavior<V extends View> extends CoordinatorLayout.Behavior<V>

继承自CoordinatorLayout.Behavior，BottomSheetBehavior.from(V view)方法获得了BootomSheetBehavior的实例，我们进去看看它怎么实现的。

    public static <V extends View> BottomSheetBehavior<V> from(V view) {
        ViewGroup.LayoutParams params = view.getLayoutParams();
        if (!(params instanceof CoordinatorLayout.LayoutParams)) {
            throw new IllegalArgumentException("The view is not a child of CoordinatorLayout");
        }
        CoordinatorLayout.Behavior behavior = ((CoordinatorLayout.LayoutParams) params)
                .getBehavior();
        if (!(behavior instanceof BottomSheetBehavior)) {
            throw new IllegalArgumentException(
                    "The view is not associated with BottomSheetBehavior");
        }
        return (BottomSheetBehavior<V>) behavior;
    }

源码中看出根据传入的参数view的LayoutParams是不是 CoordinatorLayout.LayoutParams，若不是，将抛出"The view is not a child of CoordinatorLayout"的异常，通过 ((CoordinatorLayout.LayoutParams) params).getBehavior()获得一个behavior并判断是不是BottomSheetBehavior，若不是，就抛出异常"The view is not associated with BottomSheetBehavior",都符合就返回了BottomSheetBehavior的实例。这里我们可以知道behavior保存在 CoordinatorLayout.LayoutParams里，那它是 怎么保存的呢，怀着好奇心，我们去看看CoordinatorLayout.LayoutParams中的源码，在LayoutParams的构造函数中，有这么一句：

            if (mBehaviorResolved) {
                mBehavior = parseBehavior(context, attrs, a.getString(
                        R.styleable.CoordinatorLayout_LayoutParams_layout_behavior));
            }

顺藤摸瓜，我们在跟进去看看parseBehavior做了什么

     static final Class<?>[] CONSTRUCTOR_PARAMS = new Class<?>[] {
        Context.class,
        AttributeSet.class
    };

    static Behavior parseBehavior(Context context, AttributeSet attrs, String name) {
       /*
        *省略部分代码
        */
        try {
           /*
            *省略部分代码
            */
            Constructor<Behavior> c = constructors.get(fullName);
            if (c == null) {
                final Class<Behavior> clazz = (Class<Behavior>) Class.forName(fullName, true,
                        context.getClassLoader());
                c = clazz.getConstructor(CONSTRUCTOR_PARAMS);
                c.setAccessible(true);
                constructors.put(fullName, c);
            }
            return c.newInstance(context, attrs);
        } catch (Exception e) {
            throw new RuntimeException("Could not inflate Behavior subclass " + fullName, e);
        }
    }

这里做的事情很简单，就是在实例化CoordinatorLayout.LayoutParams时反射生成Behavior实例，这就是为什么自定义behavior需要重写如下的构造函数

    public class CjjBehavior extends CoordinatorLayout.Behavior{
        public CjjBehavior(Context context, AttributeSet attrs) {
            super(context, attrs);
        }
    }

不然就会看到"Could not inflate Behavior subclass ..."异常 。

目前为止，我们只是了解了CoordinatorLayout.Behavior相关的东西，还是不知道BottomSheetBehavior实现的原理，别急，这就和你说说。

###view布局 当你的View持有Behavior的时候, CoordinatorLayout 在 onLayout 的时候会调用Behavior.onLayoutChild方法进行布局.

####注意:我们将持有的Behavior 的View 叫做BehaviorView

我们查看onLayoutChild 的源码

    @Override
    public boolean onLayoutChild(CoordinatorLayout parent, V child, int layoutDirection) {
        // First let the parent lay it out
        if (mState != STATE_DRAGGING && mState != STATE_SETTLING) {
            parent.onLayoutChild(child, layoutDirection);
        }
        // Offset the bottom sheet
        mParentHeight = parent.getHeight();
        mMinOffset = Math.max(0, mParentHeight - child.getHeight());
        mMaxOffset = mParentHeight - mPeekHeight;
        if (mState == STATE_EXPANDED) {
            ViewCompat.offsetTopAndBottom(child, mMinOffset);
        } else if (mHideable && mState == STATE_HIDDEN) {
            ViewCompat.offsetTopAndBottom(child, mParentHeight);
        } else if (mState == STATE_COLLAPSED) {
            ViewCompat.offsetTopAndBottom(child, mMaxOffset);
        }
        if (mViewDragHelper == null) {
            mViewDragHelper = ViewDragHelper.create(parent, mDragCallback);
        }
        mViewRef = new WeakReference<>(child);
        mNestedScrollingChildRef = new WeakReference<>(findScrollingChild(child));
        return true;
    }

这里主要做了几件事情:

1. 对BehaviorView 的摆放:先调用父类 对 BehaviorView 进行布局,根据 PeekHeight 和 State 对 BehaviorView 位置的进行偏移,偏移到合适的位置.

2. 对mMinOffset,mMaxOffset的计算,根据mMinOffset 和mMaxOffset 可以确定BehaviorView 的偏移范围.即 距离CoordinatorLayout 原点 Y轴mMinOffset 到mMaxOffset;

3. 始化了ViewDragHelper 类.ViewDragHelper是一个非常厉害的组件.我们这边使用它处理进行拖拽和滑动事件.

4. 存储BehaviorView 的软引用和递归找到第一个NestedScrollingChild组件,当然NestedScrollingChild也可以为空.下面的逻辑对于NestedScrollingChild为空的情况做了处理的.

onLayoutChild做的事情还是挺少的.算是一些初始化的东西

因为State 默认为STATE_COLLAPSED,偏移量为ParentHeight - PeekHeight, 这时候BehaviorView 被往下调整了,露出屏幕的高度为PeekHeight 的大小.

在Android 5.0上可能是因为优化的原因还是别的因素. 当一开始的 PeekHeight为0的时候 整个BehaviorView 被移到屏幕外, 它就不会被绘制上去.导致你看不到BehaviorView的画面,但是它是存在的.实实在在存在着

我的好基友dim给出了解决方案Android support 23.2 使用BottomSheetBehavior 的坑

###事件拦截 ####touch 事件会先被onInterceptTouchEvent()捕获,进行判断是否拦截.

@Override
public boolean onInterceptTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) {
    if (!child.isShown()) {
        return false;
    }
    int action = MotionEventCompat.getActionMasked(event);
    // Record the velocity
    if (action == MotionEvent.ACTION_DOWN) {
        reset();
    }
    if (mVelocityTracker == null) {
        mVelocityTracker = VelocityTracker.obtain();
    }
    mVelocityTracker.addMovement(event);
    switch (action) {
        case MotionEvent.ACTION_UP:
        case MotionEvent.ACTION_CANCEL:
            mTouchingScrollingChild = false;
            mActivePointerId = MotionEvent.INVALID_POINTER_ID;
            // Reset the ignore flag
            if (mIgnoreEvents) {
                mIgnoreEvents = false;
                return false;
            }
            break;
        case MotionEvent.ACTION_DOWN:
            int initialX = (int) event.getX();
            mInitialY = (int) event.getY();
            View scroll = mNestedScrollingChildRef.get();
            if (scroll != null && parent.isPointInChildBounds(scroll, initialX, mInitialY)) {
                mActivePointerId = event.getPointerId(event.getActionIndex());
                mTouchingScrollingChild = true;
            }
            mIgnoreEvents = mActivePointerId == MotionEvent.INVALID_POINTER_ID &&
                    !parent.isPointInChildBounds(child, initialX, mInitialY);
            break;
    }
    if (!mIgnoreEvents && mViewDragHelper.shouldInterceptTouchEvent(event)) {
        return true;
    }
    // We have to handle cases that the ViewDragHelper does not capture the bottom sheet because
    // it is not the top most view of its parent. This is not necessary when the touch event is
    // happening over the scrolling content as nested scrolling logic handles that case.
    View scroll = mNestedScrollingChildRef.get();
    return action == MotionEvent.ACTION_MOVE && scroll != null &&
            !mIgnoreEvents && mState != STATE_DRAGGING &&
            !parent.isPointInChildBounds(scroll, (int) event.getX(), (int) event.getY()) &&
            Math.abs(mInitialY - event.getY()) > mViewDragHelper.getTouchSlop();
}

####onInterceptTouchEvent 做了几件事情:

1. 判断是否拦截事件.先使用mViewDragHelper.shouldInterceptTouchEvent(event)拦截.

2. 使用mVelocityTracker 记录手指动作,用于后期计算Y 轴速率.

3. 判断点击事件是否在NestedChildView 上,将 boolean 存到mTouchingScrollingChild 标记位中,这个主要是用于ViewDragHelper.Callback 中的判断.

4. ACTION_UP 和ACTION_CANCEL 对标记位进行复位,好在下一轮 Touch 事件中使用.

####onTouchEvent处理

 @Override
    public boolean onTouchEvent(CoordinatorLayout parent, V child, MotionEvent event) {
        if (!child.isShown()) {
            return false;
        }
        int action = MotionEventCompat.getActionMasked(event);
        if (mState == STATE_DRAGGING && action == MotionEvent.ACTION_DOWN) {
            return true;
        }
        mViewDragHelper.processTouchEvent(event);
        // Record the velocity
        if (action == MotionEvent.ACTION_DOWN) {
            reset();
        }
        if (mVelocityTracker == null) {
            mVelocityTracker = VelocityTracker.obtain();
        }
        mVelocityTracker.addMovement(event);
        // The ViewDragHelper tries to capture only the top-most View. We have to explicitly tell it
        // to capture the bottom sheet in case it is not captured and the touch slop is passed.
        if (action == MotionEvent.ACTION_MOVE) {
            if (Math.abs(mInitialY - event.getY()) > mViewDragHelper.getTouchSlop()) {
                mViewDragHelper.captureChildView(child, event.getPointerId(event.getActionIndex()));
            }
        }
        return true;
    }

####onTouchEvent 主要做了几件事情:

1. 使用mVelocityTracker 记录手指动作.用于后期计算Y 轴速率.

2. 使用mViewDragHelper 处理Touch 事件.可能会产生拖动效果.

3. mViewDragHelper 在滑动的时候对BehaviorView 的再一次捕获.再一次明确告诉ViewDragHelper 我要移动的是BehaviorView 组件.什么情况需要主动告诉ViewDragHelper ?比如:当你点击在BehaviorView 的区域,但是BehaviorView 的视图的层级不是最高的,或者你点击的区域不在 BehaviorView 上,ViewDragHelper 在做处理滑动的时候找不到BehaviorView, 这个时候你要手动告知它现在要移动的是BehaviorView,情景类似ViewDragHelper处理EdgeDrag 的样子.

####注意 即使你的onInterceptTouchEvent 返回false,也可能因为下面的View 没有处理这个Touch事件,而导致Touch 事件上发被Behavior的onTouchEvent 被截取.

NestedScrolling事件处理

当 CoordinatorLayout 的子控件有 NestedScrollingChild 产生 Nested 事件的时候.会调用onStartNestedScroll 这个方法

    @Override
    public boolean onStartNestedScroll(CoordinatorLayout coordinatorLayout, V child,
            View directTargetChild, View target, int nestedScrollAxes) {
            return (nestedScrollAxes & ViewCompat.SCROLL_AXIS_VERTICAL) != 0;//滑动Y轴方向的判断
    }

返回值 true :表示 BehaviorView 要和NestedScrollingChild 配合消耗这个 NestedScrolling 事件,这里可以看出只要是纵向的滑动都会返回true.

####onNestedPreScroll NestedScrollingChild的在滑动的时候会触发onNestedPreScroll 方法,询问BehaviorView消耗多少Y轴上面的滑动.

  @Override
    public void onNestedPreScroll(CoordinatorLayout coordinatorLayout, V child, View target, int dx,
            int dy, int[] consumed) {
        View scrollingChild = mNestedScrollingChildRef.get();
        if (target != scrollingChild) {
            return;
        }
        int currentTop = child.getTop();
        int newTop = currentTop - dy;
        if (dy > 0) { // Upward
            if (newTop < mMinOffset) {
                consumed[1] = currentTop - mMinOffset;
                ViewCompat.offsetTopAndBottom(child, -consumed[1]);
                setStateInternal(STATE_EXPANDED);
            } else {
                consumed[1] = dy;
                ViewCompat.offsetTopAndBottom(child, -dy);
                setStateInternal(STATE_DRAGGING);
            }
        } else if (dy < 0) { // Downward
            if (!ViewCompat.canScrollVertically(target, -1)) {
                if (newTop <= mMaxOffset || mHideable) {
                    consumed[1] = dy;
                    ViewCompat.offsetTopAndBottom(child, -dy);
                    setStateInternal(STATE_DRAGGING);
                } else {
                    consumed[1] = currentTop - mMaxOffset;
                    ViewCompat.offsetTopAndBottom(child, -consumed[1]);
                    setStateInternal(STATE_COLLAPSED);
                }
            }
        }
        dispatchOnSlide(child.getTop());
        mLastNestedScrollDy = dy;
        mNestedScrolled = true;
    }

####onNestedPreScroll 方法主要做几件事情:

1. 判断发起NestedScrolling 的 View 是否是我们在onLayoutChild 找到的那个控件.不是的话,不做处理.不处理就是不消耗y 轴,把所有的Scroll 交给发起的 View 自己消耗.

2. 根据dy 判断方向,根据之前的偏移范围算出偏移量.使用ViewCompat.offsetTopAndBottom 对BehaviorView 进行偏移操作.

3. 消耗Y轴的偏移量.发起 NestedScrollingChild 会自动响应剩下的部分

其中comsume[]是个数组,consumed[1]表示 Parent 在 Y 轴消耗的值, NestedScrollingChild 会消耗除BehaviorView消耗剩下的那部分( 比如: NestedScrollingChild 要滑动20像素,因为BehaviorView消耗了10像素,最后NestedScrollingChild 只滑动了10像素);

onStopNestedScroll在Nestd事件结束触发. 主要做的事情: 根据BehaviorView当前的状态对它的最终位置的确定,有必要的话调用ViewDragHelper.smoothSlideViewTo 进行滑动. ####注意 当你是往下滑动且Hideable 为 true ,他会 使用上面计算的Y轴的速率的判断.是否应该切换到Hideable 的状态.

####onNestedPreFling 这个是 NestedScrollingChild 要滑行时候触发的,询问 BehaviorView是否消耗这个滑行.

@Override
public boolean onNestedPreFling(CoordinatorLayout coordinatorLayout, V child, View target,
                                float velocityX, float velocityY) {
    return target == mNestedScrollingChildRef.get() &&
            (mState != STATE_EXPANDED ||
                    super.onNestedPreFling(coordinatorLayout, child, target,
                            velocityX, velocityY));
}

处理逻辑是:发起Nested事件要与onLayoutChild 找到的那个控件一致且当前状态是一个STATE_EXPANDED状态.

返回值: true表示BehaviorView 消耗滑行事件,那么NestedScrollingChild就不会有滑行了

####ViewDragHelper.Callback ViewDragHelper网上教程挺多的,就不多讲了,他主要是处理滑动拖拽的.

####小技巧 在说说一个小技巧，Android官网中有这样一句话：Enums often require more than twice as much memory as static constants. You should strictly avoid using enums on Android,就是说枚举比静态常量更加耗费内存，我们应该避免使用，然后我看BottomSheetBehavior源码中 mState 是这样定义的：

    public static final int STATE_DRAGGING = 1;
    public static final int STATE_SETTLING = 2;
    public static final int STATE_EXPANDED = 3;
    public static final int STATE_COLLAPSED = 4;
    public static final int STATE_HIDDEN = 5;

    @IntDef({STATE_EXPANDED, STATE_COLLAPSED, STATE_DRAGGING, STATE_SETTLING, STATE_HIDDEN})
    @Retention(RetentionPolicy.SOURCE)
    public @interface State {}

    @State
    private int mState = STATE_COLLAPSED;

弥补了Android不建议使用枚举的缺陷。

###Have a nice weekend ! Bye bye.

转载请注明出处，不然我咬你哦！

###Thanks: dim
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