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。
网友评论