NestedScrollView

1. ParentView

1. NestedScrollingParent

2. NestedScrollingParentHelper

NestedScrollingParent 是一個嵌套滑動父View的相關(guān)方法的接口對象。NestedScrollingParentHelper 是一個嵌套滑動父View的事件輔助類，主要是暴露出來與子view交互。

Method

• onStartNestedScroll
  子View要開始滑動時，會尋找要跟它嵌套滑動的父view。當(dāng)返回 true 時，表示這個父View將跟他一起執(zhí)行嵌套滑動。

   /*
    * @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
  當(dāng)父View要跟隨嵌套滑動后，此方法就提供一個機會，讓父View和子View初始化嵌套滑動的配置，比如 橫向滑動，縱向滑動，還是都有。

/**
     * @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
  當(dāng)嵌套滑動結(jié)束（結(jié)束就是 touch 的ACTION_UP,ACTION_CANCEL）時就會調(diào)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
  當(dāng)正在執(zhí)行嵌套滑動的的子View調(diào)度了嵌套滑動事件時，就會調(diào)用此方法。將已消耗的滑動距離和剩余的滑動距離都告訴父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
  當(dāng)子View執(zhí)行 dispatchNestedPreScroll(子view中的方法接口)時當(dāng)用此方法。當(dāng)正在執(zhí)行嵌套滑動時，在子view滑動前，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當(dāng)前的坐標(biāo)軸

/**
    * 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();

• 還有些 Fling（快速滑動）的接口方法，這里就不講了，大家可以自己看下源碼。

2. ChildView

1. NestedScrollingChild

2. NestedScrollingChildHelper

NestedScrollingChild 是一個嵌套滑動子View的相關(guān)方法的接口對象。NestedScrollingChildHelper 是一個嵌套滑動子View的事件輔助類，主要是暴露出來與父view交互。

Method

• setNestedScrollingEnabled
  設(shè)置子view是否可以滑動，如果 true 就是可以，否則不可以。
  當(dāng)子view正在嵌套滑動時，設(shè)置為false，效果將與 stopNestedScroll() 一樣。

/**
     * 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 是否可以嵌套滑動

/**
     * 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.開始一個嵌套滑動（橫向嵌套滑動、縱向、橫縱結(jié)合）。
  2.startNestedScroll 將會初始化滑動操作。當(dāng)觸發(fā)ACTION_DOWN事件時，就會觸發(fā) startNestedScroll。當(dāng)執(zhí)行ViewParent#requestDisallowInterceptTouchEvent(boolean) 時，會自動打斷嵌套滑動操作。當(dāng)結(jié)束嵌套滑動時，必須調(diào)用 stopNestedScroll 方法。
  3.如果 此方法返回true，表示找到了一個配合一起滑動的父View。如果返回 false,表明父View不關(guān)心此次的滑動事件。
  當(dāng)正在滑動時，調(diào)用此方法，將會默認返回 true。
  4.每次滑動的步驟中一旦 startNestedScroll 計算出了滑動的距離，都會調(diào)用 dispatchNestedPreScroll 方法。如果此時有一起嵌套滑動的父View，父View就至少會消耗一部分滑動距離，這時子View的滑動距離就要相應(yīng)的調(diào)整。
  5.在子View對剩余的滾動距離做了響應(yīng)后，就要調(diào)用 dispatchNestedScroll 方法，將 已消耗的距離和剩余的距離告訴父View，父View可能會有不同的操作

/**
     * 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
  結(jié)束嵌套滑動

/**
     * 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
  是否有配合滑動的父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化凍后，將嵌套滑動的相關(guān)信息傳遞給父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消費滑動距離之前，先告訴父View。在子View的onInterceptTouchEvent或者onTouch中(一般在MontionEvent.ACTION_MOVE事件里)，調(diào)用該方法通知父View滑動的距離。該方法的第三第四個參數(shù)返回父view消費掉的scroll長度和子View的窗體偏移量。如果這個scroll沒有被消費完，則子view進行處理剩下的一些距離，由于窗體進行了移動，如果你記錄了手指最后的位置，需要根據(jù)第四個參數(shù)offsetInWindow計算偏移量，才能保證下一次的touch事件的計算是正確的。
  如果父view接受了它的滾動參數(shù)，進行了部分消費，則這個函數(shù)返回true，否則為false。
  這個函數(shù)一般在子view處理scroll前調(diào)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);

• 還有些 Fling 方法，就不一一介紹了，大家可以自己查看源碼。

交互對應(yīng)流程

NestedScrollView-流程圖.png

查看源碼：在 NestedScrollView 的一些touch 監(jiān)聽中就可以看到對以上方法的調(diào)用。同時，可以看到在ChildHelper中主要調(diào)用了ViewParentCompat類的方法。ViewParentCompat是一個和父view交互的兼容類，它會判斷api version，如果在Lollipop以上，就是用view自帶的方法，否則判斷是否實現(xiàn)了NestedScrollingParent接口，去調(diào)用接口的方法。