一、关键词Behavior

先来介绍一下Behavior，我们在开发过程中可能会遇到这样一个小需求，就是在Button点击滑动的过程中，还额外的需要一个TextView一起跟着滑动，倒是手指点的是Button上面的，那么怎么去控制TextView的联动效果呢？这里就用到了CoordinatorLayout +Behavior。

二、CoordinatorLayout如何获取到子控件的Behavior？

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Behavior作为一个中间者，目的是协调子控件之间的触摸事件。
CoordinatorLayout有两个主要的用法：1.作为顶层应用的装饰或者chrome的布局；2.作为一个能响应特定的一个或多个子视图交互的容器。

三、CoordinatorLayout + NestedScrollingParent 完成补全事件分发所不能完成的事情

总所周知，Android事件纷发机制主要是有三个函数：dispatchTouchEvent/onInterceptTouchEvent和onTouchEvent；
这种纷发机制本身有个漏洞：如果子View获得处理Touch事件的时候，父View就再也没有机会去处理touch事件能力，也就是当前的触摸事件跟父容器没有半点关系了。
但是NestedScrolling机制可以解决这一漏洞：
内部View在滚动的时候，首先将dx，dy交给NestedScrollingParent，NestedScrollingParent可对其进行部分消耗，剩余的部分还给内部子View。
NestedScrolling机制主要和NestedScrollingParent有关，来看NestedScrollingParent的源码：

public interface NestedScrollingParent {
    /**
     * 当子View调用NestedScrollingChild的方法startNestedScroll时，会调用该方法，一定要按照
     *自己的需求返回true，该方法决定了当前控件是否能接收到其内部View滑动的参数。
     * @param child Direct child of this ViewParent containing target
     * @param target View that initiated the nested scroll
     * @param axes Flags consisting of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL},
     *                         {@link ViewCompat#SCROLL_AXIS_VERTICAL} or both
     * @return true if this ViewParent accepts the nested scroll operation
     */
    boolean onStartNestedScroll(@NonNull View child, @NonNull View target, @ScrollAxis int axes);

    /**
     * React to the successful claiming of a nested scroll operation.
     *
     * <p>This method will be called after
     * {@link #onStartNestedScroll(View, View, int) onStartNestedScroll} returns true. It offers
     * an opportunity for the view and its superclasses to perform initial configuration
     * for the nested scroll. Implementations of this method should always call their superclass's
     * implementation of this method if one is present.</p>
     *
     * @param child Direct child of this ViewParent containing target
     * @param target View that initiated the nested scroll
     * @param axes Flags consisting of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL},
     *                         {@link ViewCompat#SCROLL_AXIS_VERTICAL} or both
     * @see #onStartNestedScroll(View, View, int)
     * @see #onStopNestedScroll(View)
     */
    void onNestedScrollAccepted(@NonNull View child, @NonNull View target, @ScrollAxis int axes);

    /**
     * React to a nested scroll operation ending.
     *
     * <p>Perform cleanup after a nested scrolling operation.
     * This method will be called when a nested scroll stops, for example when a nested touch
     * scroll ends with a {@link MotionEvent#ACTION_UP} or {@link MotionEvent#ACTION_CANCEL} event.
     * Implementations of this method should always call their superclass's implementation of this
     * method if one is present.</p>
     *
     * @param target View that initiated the nested scroll
     */
    void onStopNestedScroll(@NonNull View target);

    /**
    * 
     * React to a nested scroll in progress.
     * 当子View调用dispatchNestedPreScroll方法时，会调用该方法，该方法会传入内部View移        
     * 动的dx，dy；如果你需要消耗一定的dx,dy就通过最后一个参数consumed进行指定。
     * 例如我需要消耗一半的dy，就可以写consumed[1] = dy/2
     * @param target The descendent view controlling the nested scroll
     * @param dxConsumed Horizontal scroll distance in pixels already consumed by target
     * @param dyConsumed Vertical scroll distance in pixels already consumed by target
     * @param dxUnconsumed Horizontal scroll distance in pixels not consumed by target
     * @param dyUnconsumed Vertical scroll distance in pixels not consumed by target
     */
    void onNestedScroll(@NonNull View target, int dxConsumed, int dyConsumed,
            int dxUnconsumed, int dyUnconsumed);

    /**
     * React to a nested scroll in progress before the target view consumes a portion of the scroll.
     *
     * <p>When working with nested scrolling often the parent view may want an opportunity
     * to consume the scroll before the nested scrolling child does. An example of this is a
     * drawer that contains a scrollable list. The user will want to be able to scroll the list
     * fully into view before the list itself begins scrolling.</p>
     *
     * <p><code>onNestedPreScroll</code> is called when a nested scrolling child invokes
     * {@link View#dispatchNestedPreScroll(int, int, int[], int[])}. The implementation should
     * report how any pixels of the scroll reported by dx, dy were consumed in the
     * <code>consumed</code> array. Index 0 corresponds to dx and index 1 corresponds to dy.
     * This parameter will never be null. Initial values for consumed[0] and consumed[1]
     * will always be 0.</p>
     *
     * @param target View that initiated the nested scroll
     * @param dx Horizontal scroll distance in pixels
     * @param dy Vertical scroll distance in pixels
     * @param consumed Output. The horizontal and vertical scroll distance consumed by this parent
     */
    void onNestedPreScroll(@NonNull View target, int dx, int dy, @NonNull int[] consumed);

    /**
     * Request a fling from a nested scroll.
     *
     * <p>This method signifies that a nested scrolling child has detected suitable conditions
     * for a fling. Generally this means that a touch scroll has ended with a
     * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds
     * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity}
     * along a scrollable axis.</p>
     *
     * <p>If a nested scrolling child view would normally fling but it is at the edge of
     * its own content, it can use this method to delegate the fling to its nested scrolling
     * parent instead. The parent may optionally consume the fling or observe a child fling.</p>
     *
     * @param target View that initiated the nested scroll
     * @param velocityX Horizontal velocity in pixels per second
     * @param velocityY Vertical velocity in pixels per second
     * @param consumed true if the child consumed the fling, false otherwise
     * @return true if this parent consumed or otherwise reacted to the fling
     */
    boolean onNestedFling(@NonNull View target, float velocityX, float velocityY, boolean consumed);

    /**
     * React to a nested fling before the target view consumes it.
     *
     * <p>This method siginfies that a nested scrolling child has detected a fling with the given
     * velocity along each axis. Generally this means that a touch scroll has ended with a
     * {@link VelocityTracker velocity} in the direction of scrolling that meets or exceeds
     * the {@link ViewConfiguration#getScaledMinimumFlingVelocity() minimum fling velocity}
     * along a scrollable axis.</p>
     *
     * <p>If a nested scrolling parent is consuming motion as part of a
     * {@link #onNestedPreScroll(View, int, int, int[]) pre-scroll}, it may be appropriate for
     * it to also consume the pre-fling to complete that same motion. By returning
     * <code>true</code> from this method, the parent indicates that the child should not
     * fling its own internal content as well.</p>
     *
     * @param target View that initiated the nested scroll
     * @param velocityX Horizontal velocity in pixels per second
     * @param velocityY Vertical velocity in pixels per second
     * @return true if this parent consumed the fling ahead of the target view
     */
    boolean onNestedPreFling(@NonNull View target, float velocityX, float velocityY);

    /**
     * Return the current axes of nested scrolling for this NestedScrollingParent.
     *
     * <p>A NestedScrollingParent returning something other than {@link ViewCompat#SCROLL_AXIS_NONE}
     * is currently acting as a nested scrolling parent for one or more descendant views in
     * the hierarchy.</p>
     *
     * @return Flags indicating the current axes of nested scrolling
     * @see ViewCompat#SCROLL_AXIS_HORIZONTAL
     * @see ViewCompat#SCROLL_AXIS_VERTICAL
     * @see ViewCompat#SCROLL_AXIS_NONE
     */
    @ScrollAxis
    int getNestedScrollAxes();
}