高级UI9-CooridinatorLayout原理应用+nestedscrolling组合完成下拉特效

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.
     *
     * 

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.

* * @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. * *

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.

* * @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. * *

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.

* *

onNestedPreScroll 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 * consumed 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.

* * @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. * *

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.

* *

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.

* * @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. * *

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.

* *

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 * true from this method, the parent indicates that the child should not * fling its own internal content as well.

* * @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. * *

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.

* * @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(); }

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