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.