RecyclerView浅析

RecyclerView 和 ListView 异同点

作为ListView的升级版，RecyclerView和ListView的相同点包括：

都支持列表的展示，能够滚动（请原谅我的废话）

同样通过Adapter来实现数据与View的绑定

都支持通过ViewType来支持不同的显示样式

他们的区别点更多：

ViewHolder对于ListView来说，是最佳实践，但并非强制要求，而对RecyclerView来说，则是强制要求

ListView支持Item devider属性，而RecyclerView则需要通过设置ItemDecoration来实现

ListView支持Header，而RecyclerView并不支持

ListView支持ItemClickListener, 而RecyclerView则支持ItemTouchListener

RecyclerView可以通过匹配LayoutParamter来支持Grid，StaggeredGrid效果，并且支持横向滚动

AdsListView(ListView的基类).LayoutParameter不支持margin，而RecyclerView可以

RecyclerView的实现

类结构

与ListView相比，RecyclerView胜在分工明确：

Recycler负责实现ViewHolder的回收重用

LayoutManager负责实现itemview的布局，并且处理滚动动画效果

ItemDecoration负责在LayoutManager的基础上，微调itemview的布局

Adapter负责创建ViewHolder，以及ViewHolder和data数据之间的绑定关系

这样的设计，解耦了RecyclerView的内部模块，每个内部模块可以专心的实现自己的功能，而不必担心影响其他的部分，另外，提供了更加丰富有效的定制手段来自定义部分效果。

核心代码分析

以下拉滚动为例分析RecyclerView的实现逻辑：

1.0 onTouchEvent

public void onTouchEvent(MotionEvent e){ ...... final boolean canScrollHorizontally = mLayout.canScrollHorizontally(); final boolean canScrollVertically = mLayout.canScrollVertically(); ...... final MotionEvent vtev = MotionEvent.obtain(e); ...... switch (action) { ...... case MotionEvent.ACTION_MOVE:{ ...... final int x = (int) (MotionEventCompat.getX(e, index) + 0.5f); final int y = (int) (MotionEventCompat.getY(e, index) + 0.5f); int dx = mLastTouchX - x; int dy = mLastTouchY - y; ...... mLastTouchX = x - mScrollOffset[0]; mLastTouchY = y - mScrollOffset[1]; if (scrollByInternal(canScrollHorizontally ? dx : 0, canScrollVertically ? dy : 0, vtev)) { getParent().requestDisallowInterceptTouchEvent(true); } } } }

计算与上一个touch消息相比，Y轴的位移，并调用scrollByInternal函数

1.1 scrollByInternal

boolean scrollByInternal(int x, int y, MotionEvent ev) { int unconsumedX = 0, unconsumedY = 0; int consumedX = 0, consumedY = 0; ...... if (mAdapter != null) { ...... if (x != 0) { consumedX = mLayout.scrollHorizontallyBy(x, mRecycler, mState); unconsumedX = x - consumedX; } if (y != 0) { consumedY = mLayout.scrollVerticallyBy(y, mRecycler, mState); unconsumedY = y - consumedY; } ...... } ...... }

调用LayoutManager.scrollVerticalBy函数

2.0 LinearLayoutManager.scrollVerticalBy

@Overridepublic int scrollVerticallyBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) { if (mOrientation == HORIZONTAL) { return 0; } return scrollBy(dy, recycler, state); }

简单的调用scrollBy函数

2.1 LinearLayoutManager.scrollBy

int scrollBy(int dy, RecyclerView.Recycler recycler, RecyclerView.State state) { if (getChildCount() == 0 || dy == 0) { return 0; } ...... final int layoutDirection = dy > 0 ? LayoutState.LAYOUT_END : LayoutState.LAYOUT_START; final int absDy = Math.abs(dy); updateLayoutState(layoutDirection, absDy, true, state); final int consumed = mLayoutState.mScrollingOffset + fill(recycler, mLayoutState, state, false); if (consumed < 0) { return 0; } final int scrolled = absDy > consumed ? layoutDirection * consumed : dy; mOrientationHelper.offsetChildren(-scrolled); ...... return scrolled; }

主要调用了updateLayoutState， fill， mOrientationHelper.offsetChildren三个函数

2.2 LinearLayoutManager.updateLayoutState

private void updateLayoutState(int layoutDirection, int requiredSpace, boolean canUseExistingSpace, RecyclerView.State state) { ...... if (layoutDirection == LayoutState.LAYOUT_END) { mLayoutState.mExtra += mOrientationHelper.getEndPadding(); // get the first child in the direction we are going final View child = getChildClosestToEnd(); // the direction in which we are traversing children mLayoutState.mItemDirection = mShouldReverseLayout ? LayoutState.ITEM_DIRECTION_HEAD : LayoutState.ITEM_DIRECTION_TAIL; mLayoutState.mCurrentPosition = getPosition(child) + mLayoutState.mItemDirection; mLayoutState.mOffset = mOrientationHelper.getDecoratedEnd(child); // calculate how much we can scroll without adding new children (independent of layout) scrollingOffset = mOrientationHelper.getDecoratedEnd(child) - mOrientationHelper.getEndAfterPadding(); } else { ...... } mLayoutState.mAvailable = requiredSpace; if (canUseExistingSpace) { mLayoutState.mAvailable -= scrollingOffset; } mLayoutState.mScrollingOffset = scrollingOffset; }

这个函数主要是计算了几个值，为后面的View重用和滚动做准备，有两个值需要注意一下：

mScrollingOffset是不添加新元素的情况下，能够滚动的具体

mAvailable是需要通过添加新元素来补充的滚动距离

2.3 LinearLayoutManager.fill

int fill(RecyclerView.Recycler recycler, LayoutState layoutState, RecyclerView.State state, boolean stopOnFocusable) { final int start = layoutState.mAvailable; ...... int remainingSpace = layoutState.mAvailable + layoutState.mExtra; LayoutChunkResult layoutChunkResult = new LayoutChunkResult(); while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) { layoutChunkResult.resetInternal(); layoutChunk(recycler, state, layoutState, layoutChunkResult); ...... /** * Consume the available space if: * * layoutChunk did not request to be ignored * * OR we are laying out scrap children * * OR we are not doing pre-layout */ if (!layoutChunkResult.mIgnoreConsumed || mLayoutState.mScrapList != null || !state.isPreLayout()) { layoutState.mAvailable -= layoutChunkResult.mConsumed; // we keep a separate remaining space because mAvailable is important for recycling remainingSpace -= layoutChunkResult.mConsumed; } ...... } ...... return start - layoutState.mAvailable; }

通过循环调用layoutChunk函数来添加足够的View，以满足滚动的需求

2.4 LinearLayoutManager.layoutChunk

void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state, LayoutState layoutState, LayoutChunkResult result) { View view = layoutState.next(recycler); if (view == null) { // if we are laying out views in scrap, this may return null which means there is // no more items to layout. result.mFinished = true; return; } LayoutParams params = (LayoutParams) view.getLayoutParams(); if (layoutState.mScrapList == null) { if (mShouldReverseLayout == (layoutState.mLayoutDirection == LayoutState.LAYOUT_START)) { addView(view); } else { addView(view, 0); } } else { ...... } measureChildWithMargins(view, 0, 0); result.mConsumed = mOrientationHelper.getDecoratedMeasurement(view); int left, top, right, bottom; ...... // We calculate everything with View's bounding box (which includes decor and margins) // To calculate correct layout position, we subtract margins. layoutDecorated(view, left + params.leftMargin, top + params.topMargin, right - params.rightMargin, bottom - params.bottomMargin); ...... }

layoutChunk函数主要做了四件事： - 调用layoutState.next()获得一个View - 把获得的View添加到RecyclerView中 - 调用measureChildWithMargin函数，计算子元素的size - 调用layoutDecorated函数，layout子元素

3.0 LayoutState.next

/** * Gets the view for the next element that we should layout. * Also updates current item index to the next item, based on {@link #mItemDirection} * * @return The next element that we should layout. */ View next(RecyclerView.Recycler recycler) { if (mScrapList != null) { return nextViewFromScrapList(); } final View view = recycler.getViewForPosition(mCurrentPosition); mCurrentPosition += mItemDirection; return view; }

这里先忽略从ScrapList获得View的可能性，直接通过recycler.getViewForPosition来获得新的View

3.1 Recycler.getViewForPosition

/** * Obtain a view initialized for the given position. * * This method should be used by {@link LayoutManager} implementations to obtain * views to represent data from an {@link Adapter}. * <p> * The Recycler may reuse a scrap or detached view from a shared pool if one is * available for the correct view type. If the adapter has not indicated that the * data at the given position has changed, the Recycler will attempt to hand back * a scrap view that was previously initialized for that data without rebinding. * * @param position Position to obtain a view for * @return A view representing the data at <code>position</code> from <code>adapter</code> */ public View getViewForPosition(int position) { return getViewForPosition(position, false); }

3.2 Recycler.getViewForPosition

View getViewForPosition(int position, boolean dryRun) { if (position < 0 || position >= mState.getItemCount()) { throw new IndexOutOfBoundsException("Invalid item position " + position + "(" + position + "). Item count:" + mState.getItemCount()); } boolean fromScrap = false; ViewHolder holder = null; // 0) If there is a changed scrap, try to find from there if (mState.isPreLayout()) { holder = getChangedScrapViewForPosition(position); fromScrap = holder != null; } // 1) Find from scrap by position if (holder == null) { holder = getScrapViewForPosition(position, INVALID_TYPE, dryRun); ...... } if (holder == null) { final int offsetPosition = mAdapterHelper.findPositionOffset(position); ...... final int type = mAdapter.getItemViewType(offsetPosition); // 2) Find from scrap via stable ids, if exists if (mAdapter.hasStableIds()) { holder = getScrapViewForId(mAdapter.getItemId(offsetPosition), type, dryRun); ...... } if (holder == null && mViewCacheExtension != null) { // We are NOT sending the offsetPosition because LayoutManager does not // know it. final View view = mViewCacheExtension .getViewForPositionAndType(this, position, type); if (view != null) { holder = getChildViewHolder(view); ...... } } if (holder == null) { // fallback to recycler // try recycler. // Head to the shared pool. holder = getRecycledViewPool().getRecycledView(type); ...... } if (holder == null) { holder = mAdapter.createViewHolder(RecyclerView.this, type); } } ...... boolean bound = false; if (mState.isPreLayout() && holder.isBound()) { // do not update unless we absolutely have to. holder.mPreLayoutPosition = position; } else if (!holder.isBound() || holder.needsUpdate() || holder.isInvalid()) { if (DEBUG && holder.isRemoved()) { throw new IllegalStateException("Removed holder should be bound and it should" + " come here only in pre-layout. Holder: " + holder); } final int offsetPosition = mAdapterHelper.findPositionOffset(position); holder.mOwnerRecyclerView = RecyclerView.this; mAdapter.bindViewHolder(holder, offsetPosition); attachAccessibilityDelegate(holder.itemView); bound = true; if (mState.isPreLayout()) { holder.mPreLayoutPosition = position; } } final ViewGroup.LayoutParams lp = holder.itemView.getLayoutParams(); final LayoutParams rvLayoutParams; if (lp == null) { rvLayoutParams = (LayoutParams) generateDefaultLayoutParams(); holder.itemView.setLayoutParams(rvLayoutParams); } else if (!checkLayoutParams(lp)) { rvLayoutParams = (LayoutParams) generateLayoutParams(lp); holder.itemView.setLayoutParams(rvLayoutParams); } else { rvLayoutParams = (LayoutParams) lp; } rvLayoutParams.mViewHolder = holder; rvLayoutParams.mPendingInvalidate = fromScrap && bound; return holder.itemView;

这个函数是Recycler的核心函数，主要完成了两件工作： 1.根据缓存的优先级，获取ViewHolder实例 2. rebind view holder & 设置layoutParameter

具体来说，Recycler中的缓存优先级为： 1. changed scrap 2. Scrap view 3. ViewCacheExtension 4. RecycledViewPool 5. adapter(adater不是缓存，而是通过adater.createViewHolder函数构造ViewHolder实例)

4.0 LinearLayoutManager.addView

public void addView(View child) { addView(child, -1); } public void addView(View child, int index) { addViewInt(child, index, false); }

4.1 LinearLayoutManager.addViewInt

private void addViewInt(View child, int index, boolean disappearing) { final ViewHolder holder = getChildViewHolderInt(child); ...... final LayoutParams lp = (LayoutParams) child.getLayoutParams(); if (holder.wasReturnedFromScrap() || holder.isScrap()) { if (holder.isScrap()) { holder.unScrap(); } else { holder.clearReturnedFromScrapFlag(); } mChildHelper.attachViewToParent(child, index, child.getLayoutParams(), false); if (DISPATCH_TEMP_DETACH) { ViewCompat.dispatchFinishTemporaryDetach(child); } } else if (child.getParent() == mRecyclerView) { // it was not a scrap but a valid child // ensure in correct position int currentIndex = mChildHelper.indexOfChild(child); if (index == -1) { index = mChildHelper.getChildCount(); } ...... if (currentIndex != index) { mRecyclerView.mLayout.moveView(currentIndex, index); } } else { mChildHelper.addView(child, index, false); lp.mInsetsDirty = true; if (mSmoothScroller != null && mSmoothScroller.isRunning()) { mSmoothScroller.onChildAttachedToWindow(child); } } ...... }

根据View的状态不同,添加到RecyclerView的方式也有差异： 1. 如果View当前为detached，attach 到 RecyclerView 2. 如果View当前还在RecyclerView中，则是移动View的位置 3. 初次之外，添加到RecyclerView中

5.0 LinearLayoutManager.measureChildWithMargins

public void measureChildWithMargins(View child, int widthUsed, int heightUsed) { final LayoutParams lp = (LayoutParams) child.getLayoutParams(); final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child); widthUsed += insets.left + insets.right; heightUsed += insets.top + insets.bottom; final int widthSpec = getChildMeasureSpec(getWidth(), getPaddingLeft() + getPaddingRight() + lp.leftMargin + lp.rightMargin + widthUsed, lp.width, canScrollHorizontally()); final int heightSpec = getChildMeasureSpec(getHeight(), getPaddingTop() + getPaddingBottom() + lp.topMargin + lp.bottomMargin + heightUsed, lp.height, canScrollVertically()); child.measure(widthSpec, heightSpec); }

调用measure函数，重新计算child的长和宽。 值得关注的是mRecyclerView.getItemDecorInsetsForChild

5.1 RecyclerView.getItemDecoinsetsForChild

Rect getItemDecorInsetsForChild(View child) { final LayoutParams lp = (LayoutParams) child.getLayoutParams(); if (!lp.mInsetsDirty) { return lp.mDecorInsets; } final Rect insets = lp.mDecorInsets; insets.set(0, 0, 0, 0); final int decorCount = mItemDecorations.size(); for (int i = 0; i < decorCount; i++) { mTempRect.set(0, 0, 0, 0); mItemDecorations.get(i).getItemOffsets(mTempRect, child, this, mState); insets.left += mTempRect.left; insets.top += mTempRect.top; insets.right += mTempRect.right; insets.bottom += mTempRect.bottom; } lp.mInsetsDirty = false; return insets; }

这里就是ItemDecoration起作用的地方了

6.0 LinearLayoutManager.layoutDecorated

public void layoutDecorated(View child, int left, int top, int right, int bottom) { final Rect insets = ((LayoutParams) child.getLayoutParams()).mDecorInsets; child.layout(left + insets.left, top + insets.top, right - insets.right, bottom - insets.bottom); }

调用layout函数，重新layout child

回到2.1 scrollBy函数，接下来调用的是OrientationHelper.offsetChildren函数

7.0 OrientationHelper.offsetChildren

public static OrientationHelper createVerticalHelper(RecyclerView.LayoutManager layoutManager) { return new OrientationHelper(layoutManager) { @Override public void offsetChildren(int amount) { mLayoutManager.offsetChildrenVertical(amount); } ...... }

直接看VerticalHelper的实现，调用LayoutManager自身的函数

7.1 LayoutManager.offsetChildrenVertical

/** * Offset all child views attached to the parent RecyclerView by dy pixels along * the vertical axis. * * @param dy Pixels to offset by */ public void offsetChildrenVertical(int dy) { if (mRecyclerView != null) { mRecyclerView.offsetChildrenVertical(dy); } }

7.2 RecyclerView.offsetChildrenVertical

public void offsetChildrenVertical(int dy) { final int childCount = mChildHelper.getChildCount(); for (int i = 0; i < childCount; i++) { mChildHelper.getChildAt(i).offsetTopAndBottom(dy); } }

最终通过offsetTopAndBottom函数来设置每一个child的top和bottom，这个实现和ListView如出一辙。

7.3 View.offsetTopAndBottom

public void offsetTopAndBottom(int offset) { if (offset != 0) { final boolean matrixIsIdentity = hasIdentityMatrix(); if (matrixIsIdentity) { if (isHardwareAccelerated()) { invalidateViewProperty(false, false); } else { final ViewParent p = mParent; if (p != null && mAttachInfo != null) { final Rect r = mAttachInfo.mTmpInvalRect; int minTop; int maxBottom; int yLoc; if (offset < 0) { minTop = mTop + offset; maxBottom = mBottom; yLoc = offset; } else { minTop = mTop; maxBottom = mBottom + offset; yLoc = 0; } r.set(0, yLoc, mRight - mLeft, maxBottom - minTop); p.invalidateChild(this, r); } } } else { invalidateViewProperty(false, false); } mTop += offset; mBottom += offset; mRenderNode.offsetTopAndBottom(offset); if (isHardwareAccelerated()) { invalidateViewProperty(false, false); invalidateParentIfNeededAndWasQuickRejected(); } else { if (!matrixIsIdentity) { invalidateViewProperty(false, true); } invalidateParentIfNeeded(); } notifySubtreeAccessibilityStateChangedIfNeeded(); } }

总结

RecyclerView整体来说，还是和ListView比较相似的（屁话，他们的需求接近）

RecyclerView最大的两个特色： 1. 更高程度的解耦，方便灵活的组合和扩展 2. Recycler相较于ListView更加复杂的缓存机制，可以带来更高的View复用性能 3. 细节实现更加出色，对于API detachViewFromParent & attachViewToParent的使用，有助于提高性能