一步一步深入理解 CoordinateLayout

public class CoordinatorLayout
extends ViewGroup implements NestedScrollingParent

//定义 V 为泛型，可指定针对哪种类型的View
public static abstract class Behavior<V extends View>
//默认的构造方法
public Behavior() {}
// xml里使用
public Behavior(Context context, AttributeSet attrs) {}

static Behavior parseBehavior(Context context, AttributeSet attrs, String name) {
    //...省略了很多代码，只留下关键的部分
    try {
     //获取构造方法
        Map<String, Constructor<Behavior>> constructors = sConstructors.get();
        //...
        // 注意这一行，这里传递了attrs，所以我们必须要有第二个构造方法！！！
        return c.newInstance(context, attrs);
    } catch (Exception e) {
        //否则会报错 crash
        throw new RuntimeException("Could not inflate Behavior subclass " + fullName, e);
    }
}

LayoutParams getResolvedLayoutParams(View child) {
    final LayoutParams result = (LayoutParams) child.getLayoutParams();
    //如果没有解析过 则去解析
    if (!result.mBehaviorResolved) {
        Class<?> childClass = child.getClass();
        DefaultBehavior defaultBehavior = null;
        while (childClass != null &&
                (defaultBehavior = childClass.getAnnotation(DefaultBehavior.class)) == null) {//如果 有DefaultBehavior这个注解
            childClass = childClass.getSuperclass();
        }
        if (defaultBehavior != null) {
            try {
               //实例化Behavior并把这个Behavior赋值给result
                result.setBehavior(defaultBehavior.value().newInstance());
            } catch (Exception e) {
                Log.e(TAG, "Default behavior class " + defaultBehavior.value().getName() +
                        " could not be instantiated. Did you forget a default constructor?", e);
            }
        }
        result.mBehaviorResolved = true;//标记已经解析过  
    }
    return result;
}

public boolean layoutDependsOn(CoordinatorLayout  parent, V child, View dependency)

boolean dependsOn(CoordinatorLayout parent, View child, View dependency) {
    return dependency == mAnchorDirectChild
            || (mBehavior != null && mBehavior.layoutDependsOn(parent, child, dependency));  
}

@Override
public boolean layoutDependsOn(CoordinatorLayout parent,FloatingActionButton child, View dependency) {
    // We're dependent on all SnackbarLayouts (if enabled)
    return SNACKBAR_BEHAVIOR_ENABLED && dependency instanceof Snackbar.SnackbarLayout;
}

/**
 * Respond to a change in a child's dependent view
 * This method is called whenever a dependent view changes in size or position outside
 * of the standard layout flow. A Behavior may use this method to appropriately update
 * the child view in response.
 */
public boolean onDependentViewChanged(CoordinatorLayout parent, V child, View dependency) {
    return false;
}

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    //之前已经提到过了 解析Behavior，并按依赖顺序重排子View顺序
    prepareChildren();
    //用于addPreDrawListener，OnPreDrawListener里会调用 dispatchOnDependentViewChanged(false)
    ensurePreDrawListener();
    //...
    // 计算 padding width height 处理 fitSystemWindow等
    //...
    final int childCount = mDependencySortedChildren.size();
    for (int i = 0; i < childCount; i++) {
        final View child = mDependencySortedChildren.get(i);
        final LayoutParams lp = (LayoutParams) child.getLayoutParams();
        int keylineWidthUsed = 0;
        //...处理keyline childWidthMeasureSpec等
        final Behavior b = lp.getBehavior();
        //当behavior的onMeasureChild方法返回true的时候，我们就可以拦截Col默认的measure
        if (b == null || !b.onMeasureChild(this, child, childWidthMeasureSpec, keylineWidthUsed,
                childHeightMeasureSpec, 0)) {
            onMeasureChild(child, childWidthMeasureSpec, keylineWidthUsed,
                    childHeightMeasureSpec, 0);
        }
        //...
    }
    //...
    final int width = ViewCompat.resolveSizeAndState(widthUsed, widthMeasureSpec,
            childState & ViewCompat.MEASURED_STATE_MASK);
    final int height = ViewCompat.resolveSizeAndState(heightUsed, heightMeasureSpec,
            childState << ViewCompat.MEASURED_HEIGHT_STATE_SHIFT);
    setMeasuredDimension(width, height);
}

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    final int layoutDirection = ViewCompat.getLayoutDirection(this);
    //mDependencySortedChildren 在 onMeasure里已经排过序了
    final int childCount = mDependencySortedChildren.size();
    for (int i = 0; i < childCount; i++) {
        final View child = mDependencySortedChildren.get(i);
        final LayoutParams lp = (LayoutParams) child.getLayoutParams();
        final Behavior behavior = lp.getBehavior();
        //可以看到，当behavior.onLayoutChild()返回true的时候，就可以拦截掉Col的默认Layout操作！    
        if (behavior == null || !behavior.onLayoutChild(this, child, layoutDirection)) {
            onLayoutChild(child, layoutDirection);
        }
    }
}

@Override
public boolean onInterceptTouchEvent(MotionEvent ev) {
    MotionEvent cancelEvent = null;
    final int action = MotionEventCompat.getActionMasked(ev);
    // Make sure we reset in case we had missed a previous important event.
    if (action == MotionEvent.ACTION_DOWN) {
        //down的时候，跟大部分ViewGroup一样，需要重置一些状态以及变量，比如 mBehaviorTouchView
        resetTouchBehaviors();
    }
    //这里看performIntercept TYPE_ON_INTERCEPT标记是 onInterceptTouchEvent
    final boolean intercepted = performIntercept(ev, TYPE_ON_INTERCEPT);
    if (cancelEvent != null) {
        cancelEvent.recycle();
    }
    //当事件为UP和Cancel的时候去重置（同down）
    if (action == MotionEvent.ACTION_UP || action == MotionEvent.ACTION_CANCEL) {
        resetTouchBehaviors();
    }
    return intercepted;
}

@Override
public boolean onTouchEvent(MotionEvent ev) {
    boolean handled = false;
    boolean cancelSuper = false;
    MotionEvent cancelEvent = null;
    final int action = MotionEventCompat.getActionMasked(ev);
    // mBehaviorTouchView不为null（代表之前有behavior处理了down事件） 或者 performIntercept返回true 那么事件就交给mBehaviorTouchView
    if (mBehaviorTouchView != null || (cancelSuper = performIntercept(ev, TYPE_ON_TOUCH))) {
        // Safe since performIntercept guarantees that
        // mBehaviorTouchView != null if it returns true
        final LayoutParams lp = (LayoutParams) mBehaviorTouchView.getLayoutParams();
        final Behavior b = lp.getBehavior();
        if (b != null) {
            // 交给 behavior去处理事件  
            handled = b.onTouchEvent(this, mBehaviorTouchView, ev);
        }
    }
    // Keep the super implementation correct
    // 省略调用默认实现 up&cancel的时候重置状态
    //...
    return handled;
}

// type 标记是intercept还是touch
private boolean performIntercept(MotionEvent ev, final int type) {
    boolean intercepted = false;
    boolean newBlock = false;
    MotionEvent cancelEvent = null;
    final int action = MotionEventCompat.getActionMasked(ev);
    final List<View> topmostChildList = mTempList1;
    //按Z轴排序 原因很简单 让最上面的View先处理事件  
    getTopSortedChildren(topmostChildList);
    // Let topmost child views inspect first
    final int childCount = topmostChildList.size();
    for (int i = 0; i < childCount; i++) {
        final View child = topmostChildList.get(i);
        final LayoutParams lp = (LayoutParams) child.getLayoutParams();
        final Behavior b = lp.getBehavior();
        //当前事件已经被某个behavior拦截了（or newBlock），并且事件不为down，那么就发送一个 取消事件 给所有在拦截的behavior之后的behavior
        if ((intercepted || newBlock) && action != MotionEvent.ACTION_DOWN) {
            // Cancel all behaviors beneath the one that intercepted.
            // If the event is "down" then we don't have anything to cancel yet.
            if (b != null) {
                if (cancelEvent == null) {
                    final long now = SystemClock.uptimeMillis();
                    cancelEvent = MotionEvent.obtain(now, now,
                            MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0);
                }
                switch (type) {
                    case TYPE_ON_INTERCEPT:
                        b.onInterceptTouchEvent(this, child, cancelEvent);
                        break;
                    case TYPE_ON_TOUCH:
                        b.onTouchEvent(this, child, cancelEvent);
                        break;
                }
            }
            continue;
        }
        // 如果还没有被拦截，那么继续询问每个Behavior 是否要处理该事件
        if (!intercepted && b != null) {
            switch (type) {
                case TYPE_ON_INTERCEPT:
                    intercepted = b.onInterceptTouchEvent(this, child, ev);
                    break;
                case TYPE_ON_TOUCH:
                    intercepted = b.onTouchEvent(this, child, ev);
                    break;
            }
            //如果有behavior处理了当前的事件，那么把它赋值给mBehaviorTouchView,它其实跟ViewGroup源码中的mFirstTouchTarget作用是一样的
            if (intercepted) {
                mBehaviorTouchView = child;
            }
        }
        // Don't keep going if we're not allowing interaction below this.
        // Setting newBlock will make sure we cancel the rest of the behaviors.
        // 是否拦截一切在它之后的交互 好暴力-0-
        final boolean wasBlocking = lp.didBlockInteraction();
        final boolean isBlocking = lp.isBlockingInteractionBelow(this, child);
        newBlock = isBlocking && !wasBlocking;
        if (isBlocking && !newBlock) {
            // Stop here since we don't have anything more to cancel - we already did
            // when the behavior first started blocking things below this point.
            break;
        }
    }
    topmostChildList.clear();
    return intercepted;
}