NestedScrollView

1. ParentView

1. NestedScrollingParent

2. NestedScrollingParentHelper

NestedScrollingParent 是一个嵌套滑动父View的相关方法的接口对象。NestedScrollingParentHelper 是一个嵌套滑动父View的事件辅助类，主要是暴露出来与子view交互。

Method

• onStartNestedScroll
  子View要开始滑动时，会寻找要跟它嵌套滑动的父view。当返回 true 时，表示这个父View将跟他一起执行嵌套滑动。

   /*
    * @param child Direct child of this ViewParent containing target
    * @param target View that initiated the nested scroll
    * @param nestedScrollAxes Flags consisting of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL},
    *                         {@link ViewCompat#SCROLL_AXIS_VERTICAL} or both
    * @return true if this ViewParent accepts the nested scroll opera tion
    */
    public boolean onStartNestedScroll(View child, View target, int nestedScrollAxes);

• onNestedScrollAccepted
  当父View要跟随嵌套滑动后，此方法就提供一个机会，让父View和子View初始化嵌套滑动的配置，比如 横向滑动，纵向滑动，还是都有。

/**
     * @param child Direct child of this ViewParent containing target
     * @param target View that initiated the nested scroll
     * @param nestedScrollAxes Flags consisting of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL},
     *                         {@link ViewCompat#SCROLL_AXIS_VERTICAL} or both
     * @see #onStartNestedScroll(View, View, int)
     * @see #onStopNestedScroll(View)
     */
    public void onNestedScrollAccepted(View child, View target, int nestedScrollAxes);

• onStopNestedScroll
  当嵌套滑动结束（结束就是 touch 的ACTION_UP,ACTION_CANCEL）时就会调用。

/**
     * 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
     */
    public void onStopNestedScroll(View target);

• onNestedScroll
  当正在执行嵌套滑动的的子View调度了嵌套滑动事件时，就会调用此方法。将已消耗的滑动距离和剩余的滑动距离都告诉父View。

    /**
     * @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
     */
    public void onNestedScroll(View target, int dxConsumed, int dyConsumed,
            int dxUnconsumed, int dyUnconsumed);

• onNestedPreScroll
  当子View执行 dispatchNestedPreScroll(子view中的方法接口)时当用此方法。当正在执行嵌套滑动时，在子view滑动前，父view可以在方法中做些操作。

/**
    * @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
    public void onNestedPreScroll(View target, int dx, int dy, int[] consumed);
    */

• getNestedScrollAxes
  返回父view当前的坐标轴

/**
    * 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
    */
   public int getNestedScrollAxes();

• 还有些 Fling（快速滑动）的接口方法，这里就不讲了，大家可以自己看下源码。

2. ChildView

1. NestedScrollingChild

2. NestedScrollingChildHelper

NestedScrollingChild 是一个嵌套滑动子View的相关方法的接口对象。NestedScrollingChildHelper 是一个嵌套滑动子View的事件辅助类，主要是暴露出来与父view交互。

Method

• setNestedScrollingEnabled
  设置子view是否可以滑动，如果 true 就是可以，否则不可以。
  当子view正在嵌套滑动时，设置为false，效果将与 stopNestedScroll() 一样。

/**
     * Enable or disable nested scrolling for this view.
     *
     * @param enabled true to enable nested scrolling, false to disable
     *
     * @see #isNestedScrollingEnabled()
     */
    public void setNestedScrollingEnabled(boolean enabled);

• isNestedScrollingEnabled
  返回子view 是否可以嵌套滑动

/**
     * Returns true if nested scrolling is enabled for this view.
     *
     * @return true if nested scrolling is enabled
     *
     * @see #setNestedScrollingEnabled(boolean)
     */
    public boolean isNestedScrollingEnabled();

• startNestedScroll
  1.开始一个嵌套滑动（横向嵌套滑动、纵向、横纵结合）。
  2.startNestedScroll 将会初始化滑动操作。当触发ACTION_DOWN事件时，就会触发 startNestedScroll。当执行ViewParent#requestDisallowInterceptTouchEvent(boolean) 时，会自动打断嵌套滑动操作。当结束嵌套滑动时，必须调用 stopNestedScroll 方法。
  3.如果 此方法返回true，表示找到了一个配合一起滑动的父View。如果返回 false,表明父View不关心此次的滑动事件。
  当正在滑动时，调用此方法，将会默认返回 true。
  4.每次滑动的步骤中一旦 startNestedScroll 计算出了滑动的距离，都会调用 dispatchNestedPreScroll 方法。如果此时有一起嵌套滑动的父View，父View就至少会消耗一部分滑动距离，这时子View的滑动距离就要相应的调整。
  5.在子View对剩余的滚动距离做了响应后，就要调用 dispatchNestedScroll 方法，将 已消耗的距离和剩余的距离告诉父View，父View可能会有不同的操作

/**
     * Begin a nestable scroll operation along the given axes.
     *
     * <p>A view starting a nested scroll promises to abide by the following contract:</p>
     *
     * <p>The view will call startNestedScroll upon initiating a scroll operation. In the case
     * of a touch scroll this corresponds to the initial {@link MotionEvent#ACTION_DOWN}.
     * In the case of touch scrolling the nested scroll will be terminated automatically in
     * the same manner as {@link ViewParent#requestDisallowInterceptTouchEvent(boolean)}.
     * In the event of programmatic scrolling the caller must explicitly call
     * {@link #stopNestedScroll()} to indicate the end of the nested scroll.</p>
     *
     * <p>If <code>startNestedScroll</code> returns true, a cooperative parent was found.
     * If it returns false the caller may ignore the rest of this contract until the next scroll.
     * Calling startNestedScroll while a nested scroll is already in progress will return true.</p>
     *
     * <p>At each incremental step of the scroll the caller should invoke
     * {@link #dispatchNestedPreScroll(int, int, int[], int[]) dispatchNestedPreScroll}
     * once it has calculated the requested scrolling delta. If it returns true the nested scrolling
     * parent at least partially consumed the scroll and the caller should adjust the amount it
     * scrolls by.</p>
     *
     * <p>After applying the remainder of the scroll delta the caller should invoke
     * {@link #dispatchNestedScroll(int, int, int, int, int[]) dispatchNestedScroll}, passing
     * both the delta consumed and the delta unconsumed. A nested scrolling parent may treat
     * these values differently. See
     * {@link NestedScrollingParent#onNestedScroll(View, int, int, int, int)}.
     * </p>
     *
     * @param axes Flags consisting of a combination of {@link ViewCompat#SCROLL_AXIS_HORIZONTAL}
     *             and/or {@link ViewCompat#SCROLL_AXIS_VERTICAL}.
     * @return true if a cooperative parent was found and nested scrolling has been enabled for
     *         the current gesture.
     *
     * @see #stopNestedScroll()
     * @see #dispatchNestedPreScroll(int, int, int[], int[])
     * @see #dispatchNestedScroll(int, int, int, int, int[])
     */
    public boolean startNestedScroll(int axes);

• stopNestedScroll
  结束嵌套滑动

/**
     * Stop a nested scroll in progress.
     *
     * <p>Calling this method when a nested scroll is not currently in progress is harmless.</p>
     *
     * @see #startNestedScroll(int)
     */
    public void stopNestedScroll();

• hasNestedScrollingParent
  是否有配合滑动的父View

/**
     * Returns true if this view has a nested scrolling parent.
     *
     * <p>The presence of a nested scrolling parent indicates that this view has initiated
     * a nested scroll and it was accepted by an ancestor view further up the view hierarchy.</p>
     *
     * @return whether this view has a nested scrolling parent
     */
    public boolean hasNestedScrollingParent();

• dispatchNestedScroll
  子View化冻后，将嵌套滑动的相关信息传递给父View

/**
     * Dispatch one step of a nested scroll in progress.
     *
     * @param dxConsumed Horizontal distance in pixels consumed by this view during this scroll step
     * @param dyConsumed Vertical distance in pixels consumed by this view during this scroll step
     * @param dxUnconsumed Horizontal scroll distance in pixels not consumed by this view
     * @param dyUnconsumed Horizontal scroll distance in pixels not consumed by this view
     * @param offsetInWindow Optional. If not null, on return this will contain the offset
     *                       in local view coordinates of this view from before this operation
     *                       to after it completes. View implementations may use this to adjust
     *                       expected input coordinate tracking.
     * @return true if the event was dispatched, false if it could not be dispatched.
     * @see #dispatchNestedPreScroll(int, int, int[], int[])
     */
    public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed,
            int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow);

• dispatchNestedPreScroll
  在子View消费滑动距离之前，先告诉父View。在子View的onInterceptTouchEvent或者onTouch中(一般在MontionEvent.ACTION_MOVE事件里)，调用该方法通知父View滑动的距离。该方法的第三第四个参数返回父view消费掉的scroll长度和子View的窗体偏移量。如果这个scroll没有被消费完，则子view进行处理剩下的一些距离，由于窗体进行了移动，如果你记录了手指最后的位置，需要根据第四个参数offsetInWindow计算偏移量，才能保证下一次的touch事件的计算是正确的。
  如果父view接受了它的滚动参数，进行了部分消费，则这个函数返回true，否则为false。
  这个函数一般在子view处理scroll前调用。

/**
     * Dispatch one step of a nested scroll in progress before this view consumes any portion of it.
     *
     * @param dx Horizontal scroll distance in pixels
     * @param dy Vertical scroll distance in pixels
     * @param consumed Output. If not null, consumed[0] will contain the consumed component of dx
     *                 and consumed[1] the consumed dy.
     * @param offsetInWindow Optional. If not null, on return this will contain the offset
     *                       in local view coordinates of this view from before this operation
     *                       to after it completes. View implementations may use this to adjust
     *                       expected input coordinate tracking.
     * @return true if the parent consumed some or all of the scroll delta
     * @see #dispatchNestedScroll(int, int, int, int, int[])
     */
    public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow);

• 还有些 Fling 方法，就不一一介绍了，大家可以自己查看源码。

交互对应流程

NestedScrollView-流程图.png

查看源码：在 NestedScrollView 的一些touch 监听中就可以看到对以上方法的调用。同时，可以看到在ChildHelper中主要调用了ViewParentCompat类的方法。ViewParentCompat是一个和父view交互的兼容类，它会判断api version，如果在Lollipop以上，就是用view自带的方法，否则判断是否实现了NestedScrollingParent接口，去调用接口的方法。