NestedScrollView嵌套RecyclerView
天气渐寒,然学习不可懈怠,记录一下使用NestedScrollView嵌套RecyclerView的两个问题,以后遇到可以来这里温故.
应该说在MD中，RecyclerView代替了ListView，而NestedScrollView代替了ScrollView，他们两个都可以用来跟ToolBar交互，实现上拉下滑中ToolBar的变化。在NestedScrollView的名字中其实就可以看出他的作用了，Nested是嵌套的意思，而ToolBar基本需要嵌套使用.
- 问题一,使用NestedScrollView嵌套RecyclerView时,滑动lRecyclerView列表会出现强烈的卡顿感.
体验极其不流畅,这不是我们希望的.于是,百度了一下轻松找到解决办法.
mRecyclerView.setNestedScrollingEnabled(false);
加上这句之后,整个世界都平静了,非常流畅啊!
这里面做了啥?
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild { ... }
点击RecyclerView源码,发现其实现了ScrollingView和NestedScrollingChild两个接口.
ScrollingView点进源码查看.
public interface ScrollingView { /** * <p>Compute the horizontal range that the horizontal scrollbar * represents.</p> * * @return the total horizontal range represented by the horizontal * scrollbar * * @see #computeHorizontalScrollExtent() * @see #computeHorizontalScrollOffset() * @see android.widget.ScrollBarDrawable */ int computeHorizontalScrollRange(); /** * <p>Compute the horizontal offset of the horizontal scrollbar's thumb * within the horizontal range. This value is used to compute the position * of the thumb within the scrollbar's track.</p> * @return the horizontal offset of the scrollbar's thumb * * @see #computeHorizontalScrollRange() * @see #computeHorizontalScrollExtent() * @see android.widget.ScrollBarDrawable */ int computeHorizontalScrollOffset(); /** * <p>Compute the horizontal extent of the horizontal scrollbar's thumb * within the horizontal range. This value is used to compute the length * of the thumb within the scrollbar's track.</p> * * @return the horizontal extent of the scrollbar's thumb * * @see #computeHorizontalScrollRange() * @see #computeHorizontalScrollOffset() * @see android.widget.ScrollBarDrawable */ int computeHorizontalScrollExtent(); /** * <p>Compute the vertical range that the vertical scrollbar represents.</p> * * * @return the total vertical range represented by the vertical scrollbar * * <p>The default range is the drawing height of this view.</p> * * @see #computeVerticalScrollExtent() * @see #computeVerticalScrollOffset() * @see android.widget.ScrollBarDrawable */ int computeVerticalScrollRange(); /** * <p>Compute the vertical offset of the vertical scrollbar's thumb * within the horizontal range. This value is used to compute the position * of the thumb within the scrollbar's track.</p> * * @return the vertical offset of the scrollbar's thumb * * @see #computeVerticalScrollRange() * @see #computeVerticalScrollExtent() * @see android.widget.ScrollBarDrawable */ int computeVerticalScrollOffset(); /** * <p>Compute the vertical extent of the vertical scrollbar's thumb * within the vertical range. This value is used to compute the length * of the thumb within the scrollbar's track.</p> * * * @return the vertical extent of the scrollbar's thumb * * @see #computeVerticalScrollRange() * @see #computeVerticalScrollOffset() * @see android.widget.ScrollBarDrawable */ int computeVerticalScrollExtent(); }
可以看出该接口主要关联横向和纵向滑动距离的计算.那NestedScrollingChild呢?这是个什么?
点进其源码,经过一顿翻译和分析,发现其定义的方法并不多:
public interface NestedScrollingChild { /** * 设置嵌套滑动是否能用 * * @param enabled true to enable nested scrolling, false to disable */ public void setNestedScrollingEnabled(boolean enabled); /** * 判断嵌套滑动是否可用 * * @return true if nested scrolling is enabled */ public boolean isNestedScrollingEnabled(); /** * 开始嵌套滑动 * * @param axes 表示方向轴，有横向和竖向 */ public boolean startNestedScroll(int axes); /** * 停止嵌套滑动 */ public void stopNestedScroll(); /** * 判断是否有父View 支持嵌套滑动 * @return whether this view has a nested scrolling parent */ public boolean hasNestedScrollingParent(); /** * 在子View的onInterceptTouchEvent或者onTouch中，调用该方法通知父View滑动的距离 * * @param dx x轴上滑动的距离 * @param dy y轴上滑动的距离 * @param consumed 父view消费掉的scroll长度 * @param offsetInWindow 子View的窗体偏移量 * @return 支持的嵌套的父View 是否处理了 滑动事件 */ public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow); /** * 子view处理scroll后调用 * * @param dxConsumed x轴上被消费的距离（横向） * @param dyConsumed y轴上被消费的距离（竖向） * @param dxUnconsumed x轴上未被消费的距离 * @param dyUnconsumed y轴上未被消费的距离 * @param offsetInWindow 子View的窗体偏移量 * @return true if the event was dispatched, false if it could not be dispatched. */ public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow); /** * 滑行时调用 * * @param velocityX x 轴上的滑动速率 * @param velocityY y 轴上的滑动速率 * @param consumed 是否被消费 * @return true if the nested scrolling parent consumed or otherwise reacted to the fling */ public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed); /** * 进行滑行前调用 * * @param velocityX x 轴上的滑动速率 * @param velocityY y 轴上的滑动速率 * @return true if a nested scrolling parent consumed the fling */ public boolean dispatchNestedPreFling(float velocityX, float velocityY); }
再回头看下RecyclerView中
public class RecyclerView extends ViewGroup implements ScrollingView, NestedScrollingChild { public RecyclerView(Context context, @Nullable AttributeSet attrs, int defStyle) { super(context, attrs, defStyle); if (attrs != null) { TypedArray a = context.obtainStyledAttributes(attrs, CLIP_TO_PADDING_ATTR, defStyle, 0); mClipToPadding = a.getBoolean(0, true); a.recycle(); } else { mClipToPadding = true; } setScrollContainer(true);
//默认获取焦点 setFocusableInTouchMode(true); final ViewConfiguration vc = ViewConfiguration.get(context); mTouchSlop = vc.getScaledTouchSlop(); mMinFlingVelocity = vc.getScaledMinimumFlingVelocity(); mMaxFlingVelocity = vc.getScaledMaximumFlingVelocity(); setWillNotDraw(getOverScrollMode() == View.OVER_SCROLL_NEVER); mItemAnimator.setListener(mItemAnimatorListener); initAdapterManager(); initChildrenHelper(); // If not explicitly specified this view is important for accessibility. if (ViewCompat.getImportantForAccessibility(this) == ViewCompat.IMPORTANT_FOR_ACCESSIBILITY_AUTO) { ViewCompat.setImportantForAccessibility(this, ViewCompat.IMPORTANT_FOR_ACCESSIBILITY_YES); } mAccessibilityManager = (AccessibilityManager) getContext() .getSystemService(Context.ACCESSIBILITY_SERVICE); setAccessibilityDelegateCompat(new RecyclerViewAccessibilityDelegate(this)); // Create the layoutManager if specified. boolean nestedScrollingEnabled = true; if (attrs != null) { int defStyleRes = 0; TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.RecyclerView, defStyle, defStyleRes); String layoutManagerName = a.getString(R.styleable.RecyclerView_layoutManager); int descendantFocusability = a.getInt( R.styleable.RecyclerView_android_descendantFocusability, -1); if (descendantFocusability == -1) { setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS); } a.recycle(); createLayoutManager(context, layoutManagerName, attrs, defStyle, defStyleRes); if (Build.VERSION.SDK_INT >= 21) { a = context.obtainStyledAttributes(attrs, NESTED_SCROLLING_ATTRS, defStyle, defStyleRes); nestedScrollingEnabled = a.getBoolean(0, true); a.recycle(); } } else { setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS); } //默认支持嵌套滚动 // Re-set whether nested scrolling is enabled so that it is set on all API levels setNestedScrollingEnabled(nestedScrollingEnabled); } @Override public void setNestedScrollingEnabled(boolean enabled) { getScrollingChildHelper().setNestedScrollingEnabled(enabled); } @Override public boolean isNestedScrollingEnabled() { return getScrollingChildHelper().isNestedScrollingEnabled(); } @Override public boolean startNestedScroll(int axes) { return getScrollingChildHelper().startNestedScroll(axes); } @Override public void stopNestedScroll() { getScrollingChildHelper().stopNestedScroll(); } @Override public boolean hasNestedScrollingParent() { return getScrollingChildHelper().hasNestedScrollingParent(); } @Override public boolean dispatchNestedScroll(int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed, int[] offsetInWindow) { return getScrollingChildHelper().dispatchNestedScroll(dxConsumed, dyConsumed, dxUnconsumed, dyUnconsumed, offsetInWindow); } @Override public boolean dispatchNestedPreScroll(int dx, int dy, int[] consumed, int[] offsetInWindow) { return getScrollingChildHelper().dispatchNestedPreScroll(dx, dy, consumed, offsetInWindow); } @Override public boolean dispatchNestedFling(float velocityX, float velocityY, boolean consumed) { return getScrollingChildHelper().dispatchNestedFling(velocityX, velocityY, consumed); } @Override public boolean dispatchNestedPreFling(float velocityX, float velocityY) { return getScrollingChildHelper().dispatchNestedPreFling(velocityX, velocityY); }
这是全部都交给getScrollingChildHelper()这个方法的返回对象处理了啊，看看这个方法是怎么实现的。
private NestedScrollingChildHelper getScrollingChildHelper() { if (mScrollingChildHelper == null) { mScrollingChildHelper = new NestedScrollingChildHelper(this); } return mScrollingChildHelper; }
这样的话.NestedScrollingChild 接口的方法都交给NestedScrollingChildHelper这个代理对象处理了.
那么到这个类的源码中定位到我们关注方法的实现;
public class NestedScrollingChildHelper {
private final View mView;
private ViewParent mNestedScrollingParent;
private boolean mIsNestedScrollingEnabled;
private int[] mTempNestedScrollConsumed;
/**
* Construct a new helper for a given view.
*/
public NestedScrollingChildHelper(View view) {
mView = view;
}
/**
* Enable nested scrolling.
*
* <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
* method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
* signature to implement the standard policy.</p>
*
* @param enabled true to enable nested scrolling dispatch from this view, false otherwise
*/
public void setNestedScrollingEnabled(boolean enabled) {
if (mIsNestedScrollingEnabled) {
ViewCompat.stopNestedScroll(mView);
}
mIsNestedScrollingEnabled = enabled;
}
/**
* Check if nested scrolling is enabled for this view.
*
* <p>This is a delegate method. Call it from your {@link android.view.View View} subclass
* method/{@link android.support.v4.view.NestedScrollingChild} interface method with the same
* signature to implement the standard policy.</p>
*
* @return true if nested scrolling is enabled for this view
*/
public boolean isNestedScrollingEnabled() {
return mIsNestedScrollingEnabled;
}
.
.
.
}
可以看出RecyclerView默认是setNestedScrollingEnabled(true),是支持嵌套滚动的,也就是说当它嵌套在NestedScrollView中时,默认会随着NestedScrollView滚动而滚动,放弃了自己的滚动.
所以给我们的感觉就是滞留、卡顿.主动将该值置false可以有效解决该问题.
- 问题二,使用NestedScrollView嵌套RecyclerView时,每次打开界面都是定位在RecyclerView在屏幕顶端,列表上面的布局都被顶上去了.
这个问题花了我不少时间去查原因,最终定位到是RecyclerView抢占了焦点,自动滚动导致的.
查看RecyclerView的源码发现，它会在构造方法中调用setFocusableInTouchMode(true),所以抢到焦点后一定会定位到第一行的位置突出RecyclerView的显示
解决方法就是NestScrollView节点添加
android:focusableInTouchMode="true"然后在NestScrollView的子节点view添加:
android:descendantFocusability="blocksDescendants"
或者 直接mRecyclerVIew.setFocusableInTouchMode(false)
- 总结
不明白的地方还是要多看源码 , 看源码有几个好处.第一点,有些问题只能从源码里找出究竟 , 一点一点剖析才能看出问题的根本, 弄透一个接口的作用 , 那么有类似需求的时候,可以自定义控件实现该接口;再一个,就是在看的过程中, 我们会熟悉源码的风格,命令的方式和逻辑的先后 .总之 , 看一个好轮子的源码 , 是为了写轮子做准备.
