NestedScrollView
1. ParentView
1. NestedScrollingParent
2. NestedScrollingParentHelper
NestedScrollingParent 是一個嵌套滑動父View的相關方法的接口對象。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。當返回 true 時，表示這個父View將跟他一起執行嵌套滑動。
/*
* @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
當嵌套滑動結束（結束就是 touch 的ACTION_UP,ACTION_CANCEL）時就會調用。
/**
* 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
當正在執行嵌套滑動的的子View調度了嵌套滑動事件時，就會調用此方法。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
當子View執行 dispatchNestedPreScroll(子view中的方法接口)時當用此方法。當正在執行嵌套滑動時，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當前的坐標軸
/**
* 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
設置子view是否可以滑動，如果 true 就是可以，否則不可以。
當子view正在嵌套滑動時，設置為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、橫縱結合）。
2.startNestedScroll 將會初始化滑動操作。當觸發ACTION_DOWN事件時，就會觸發 startNestedScroll。當執行ViewParent#requestDisallowInterceptTouchEvent(boolean) 時，會自動打斷嵌套滑動操作。當結束嵌套滑動時，必須調用 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不關心此次的滑動事件。
當正在滑動時，調用此方法，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的調整。
5.在子View對剩余的滾動距離做了響應后，就要調用 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)，調用該方法通知父View滑動的距離。該方法的第三第四個參數返回父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，需要根據第四個參數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，進行了部分消費，則這個函數返回true，否則為false。
這個函數一般在子view處理scroll前調用。
/**
* 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。
交互對應流程
NestedScrollView-流程圖.png
查看源碼：在 NestedScrollView 的一些touch 監聽中就可以看到對以上方法的調用。同時，可以看到在ChildHelper中主要調用了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，否則判斷是否實現了NestedScrollingParent接口，去調用接口的方法。
