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view系列源码分析之三大常用控件之recycleview

2019-12-22  本文已影响0人  暴走的小青春

作为三大常用控件的recycleview和scrollview有着同样广泛的用途,但是在源码分析难度上却不是一个等量级的,可以说,recyclview的控件是我可能过最复杂的控件了,所以务必要带着问题去分析,不然完全没有分析下去的勇气,先说几个问题吧:
1.recycleview有了layoutmanager为何setadapter后就把数据显示在屏幕上
2.recycleview的缓存是如何进行的
3.recycleview的更新流程是怎么样的
4.recycleview的局部更新和传统更新的差别
暂时先列这4点吧,因为recycleview实在是有太多东西要分析的了
1.首先列举最常规的情况:我们在onCreate中初始化,LinearLayoutManager然后在setAdapter了,假设在setadapter时已经有数据了
首先进入到LinearLayoutManager中

 public LinearLayoutManager(Context context, @RecyclerView.Orientation int orientation,
            boolean reverseLayout) {
        setOrientation(orientation);
        setReverseLayout(reverseLayout);
        setAutoMeasureEnabled(true);
    }

很简单这里设置了自动measure为true
然后在setLayoutManager中有个这个方法

void updateViewCacheSize() {
            int extraCache = mLayout != null ? mLayout.mPrefetchMaxCountObserved : 0;
            mViewCacheMax = mRequestedCacheMax + extraCache;

            // first, try the views that can be recycled
            for (int i = mCachedViews.size() - 1;
                    i >= 0 && mCachedViews.size() > mViewCacheMax; i--) {
                recycleCachedViewAt(i);
            }
        }

也就是最大缓存的view,默认是2,记住,等会又会调用到它,其他的方法无非就是重置的操作
而在setAdapter方法里会间接调用

 private void setAdapterInternal(Adapter adapter, boolean compatibleWithPrevious,
            boolean removeAndRecycleViews) {
        if (mAdapter != null) {
            mAdapter.unregisterAdapterDataObserver(mObserver);
            mAdapter.onDetachedFromRecyclerView(this);
        }
        if (!compatibleWithPrevious || removeAndRecycleViews) {
            removeAndRecycleViews();
        }
        mAdapterHelper.reset();
        final Adapter oldAdapter = mAdapter;
        mAdapter = adapter;
        if (adapter != null) {
            adapter.registerAdapterDataObserver(mObserver);
            adapter.onAttachedToRecyclerView(this);
        }
        if (mLayout != null) {
            mLayout.onAdapterChanged(oldAdapter, mAdapter);
        }
        mRecycler.onAdapterChanged(oldAdapter, mAdapter, compatibleWithPrevious);
        mState.mStructureChanged = true;
        setDataSetChangedAfterLayout();
    }

这里由于没有重复调用adapter,所以也没有老数据,简单来说就是注册了观察者以及告诉recycleview,adapter被onAttached了
其中这两个地方都调用了requestLayout,也就是说会调用onMeasure和onLayout方法,在回到onMeasure中

 @Override
    protected void onMeasure(int widthSpec, int heightSpec) {
        if (mLayout == null) {
            defaultOnMeasure(widthSpec, heightSpec);
            return;
        }
        if (mLayout.mAutoMeasure) {
            final int widthMode = MeasureSpec.getMode(widthSpec);
            final int heightMode = MeasureSpec.getMode(heightSpec);
            final boolean skipMeasure = widthMode == MeasureSpec.EXACTLY
                    && heightMode == MeasureSpec.EXACTLY;
            mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
            if (skipMeasure || mAdapter == null) {
                return;
            }
....

由于在xml中给recyclview的宽高是EXACTLY的,所以这里就return了
并执行了onMeasure方法

   void defaultOnMeasure(int widthSpec, int heightSpec) {
        // calling LayoutManager here is not pretty but that API is already public and it is better
        // than creating another method since this is internal.
        final int width = LayoutManager.chooseSize(widthSpec,
                getPaddingLeft() + getPaddingRight(),
                ViewCompat.getMinimumWidth(this));
        final int height = LayoutManager.chooseSize(heightSpec,
                getPaddingTop() + getPaddingBottom(),
                ViewCompat.getMinimumHeight(this));

        setMeasuredDimension(width, height);
    }

可以看到一开始确定了recyclview的宽高了,不了解onMeaure和requestLayout人可以看下关于onmeasure,onLayout, requestLayout ,invalidate你可能忽视的细节
既然在onMeasure中没做啥,来分析下onLayout方法

 @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        TraceCompat.beginSection(TRACE_ON_LAYOUT_TAG);
        dispatchLayout();
        TraceCompat.endSection();
        mFirstLayoutComplete = true;
    }

其中dispatchLayout方法是重点要分析的

void dispatchLayout() {
        if (mAdapter == null) {
            Log.e(TAG, "No adapter attached; skipping layout");
            // leave the state in START
            return;
        }
        if (mLayout == null) {
            Log.e(TAG, "No layout manager attached; skipping layout");
            // leave the state in START
            return;
        }
        mState.mIsMeasuring = false;
        if (mState.mLayoutStep == State.STEP_START) {
            dispatchLayoutStep1();
            mLayout.setExactMeasureSpecsFrom(this);
            dispatchLayoutStep2();
        } else if (mAdapterHelper.hasUpdates() || mLayout.getWidth() != getWidth()
                || mLayout.getHeight() != getHeight()) {
            // First 2 steps are done in onMeasure but looks like we have to run again due to
            // changed size.
            mLayout.setExactMeasureSpecsFrom(this);
            dispatchLayoutStep2();
        } else {
            // always make sure we sync them (to ensure mode is exact)
            mLayout.setExactMeasureSpecsFrom(this);
        }
        dispatchLayoutStep3();
    }

可以看到这里有三个重要的方法,先来看下dispatchLayoutStep1方法

private void dispatchLayoutStep1() {
        mState.assertLayoutStep(State.STEP_START);
        fillRemainingScrollValues(mState);
        mState.mIsMeasuring = false;
        eatRequestLayout();
        mViewInfoStore.clear();
        onEnterLayoutOrScroll();
        processAdapterUpdatesAndSetAnimationFlags();
        saveFocusInfo();
        mState.mTrackOldChangeHolders = mState.mRunSimpleAnimations && mItemsChanged;
        mItemsAddedOrRemoved = mItemsChanged = false;
        mState.mInPreLayout = mState.mRunPredictiveAnimations;
        mState.mItemCount = mAdapter.getItemCount();
        findMinMaxChildLayoutPositions(mMinMaxLayoutPositions);
...

这里有一个关键点是processAdapterUpdatesAndSetAnimationFlags方法和
第一个从名字上来看就是当adapter被局部更新时候所调用的方法,这个等会分析
紧接着分析dispatchLayoutStep2方法

 private void dispatchLayoutStep2() {
        eatRequestLayout();
        onEnterLayoutOrScroll();
        mState.assertLayoutStep(State.STEP_LAYOUT | State.STEP_ANIMATIONS);
        mAdapterHelper.consumeUpdatesInOnePass();
        mState.mItemCount = mAdapter.getItemCount();
        mState.mDeletedInvisibleItemCountSincePreviousLayout = 0;

        // Step 2: Run layout
        mState.mInPreLayout = false;
        mLayout.onLayoutChildren(mRecycler, mState);

        mState.mStructureChanged = false;
        mPendingSavedState = null;

        // onLayoutChildren may have caused client code to disable item animations; re-check
        mState.mRunSimpleAnimations = mState.mRunSimpleAnimations && mItemAnimator != null;
        mState.mLayoutStep = State.STEP_ANIMATIONS;
        onExitLayoutOrScroll();
        resumeRequestLayout(false);
    }

重点在mLayout.onLayoutChildren方法
首先找到他的

 @Override
    public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
        // layout algorithm:
        // 1) by checking children and other variables, find an anchor coordinate and an anchor
        //  item position.
        // 2) fill towards start, stacking from bottom
        // 3) fill towards end, stacking from top
        // 4) scroll to fulfill requirements like stack from bottom.
        // create layout state
        if (DEBUG) {
            Log.d(TAG, "is pre layout:" + state.isPreLayout());
        }
        if (mPendingSavedState != null || mPendingScrollPosition != NO_POSITION) {
            if (state.getItemCount() == 0) {
                removeAndRecycleAllViews(recycler);
                return;
            }
        }
        if (mPendingSavedState != null && mPendingSavedState.hasValidAnchor()) {
            mPendingScrollPosition = mPendingSavedState.mAnchorPosition;
        }

        ensureLayoutState();
        mLayoutState.mRecycle = false;
        // resolve layout direction
        resolveShouldLayoutReverse();

        final View focused = getFocusedChild();
        if (!mAnchorInfo.mValid || mPendingScrollPosition != NO_POSITION
                || mPendingSavedState != null) {
            mAnchorInfo.reset();
            mAnchorInfo.mLayoutFromEnd = mShouldReverseLayout ^ mStackFromEnd;
            // calculate anchor position and coordinate
            updateAnchorInfoForLayout(recycler, state, mAnchorInfo);
            mAnchorInfo.mValid = true;
        }

...

这里会走到if条件里面去,目的就是给开始放置位置赋值,由于没有设置reverse属性,所以 mAnchorInfo.mLayoutFromEnd的boolean是false
并且mAnchorInfo.mPosition的值是0

  anchorInfo.assignCoordinateFromPadding();
        anchorInfo.mPosition = mStackFromEnd ? state.getItemCount() - 1 : 0;

而后面会走到mAnchorInfo.mLayoutFromEnd为false的判断里,接调用fill方法,在此期间,会拿到recyclview的高度,然后赋值给mLayoutState.mAvailable属性

  private void updateLayoutStateToFillEnd(int itemPosition, int offset) {
        mLayoutState.mAvailable = mOrientationHelper.getEndAfterPadding() - offset;
        mLayoutState.mItemDirection = mShouldReverseLayout ? LayoutState.ITEM_DIRECTION_HEAD :
                LayoutState.ITEM_DIRECTION_TAIL;
        mLayoutState.mCurrentPosition = itemPosition;
        mLayoutState.mLayoutDirection = LayoutState.LAYOUT_END;
        mLayoutState.mOffset = offset;
        mLayoutState.mScrollingOffset = LayoutState.SCROLLING_OFFSET_NaN;
    }

来看下fill方法

int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
            RecyclerView.State state, boolean stopOnFocusable) {
        // max offset we should set is mFastScroll + available
        final int start = layoutState.mAvailable;
        if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
            // TODO ugly bug fix. should not happen
            if (layoutState.mAvailable < 0) {
                layoutState.mScrollingOffset += layoutState.mAvailable;
            }
            recycleByLayoutState(recycler, layoutState);
        }
        int remainingSpace = layoutState.mAvailable + layoutState.mExtra;
        LayoutChunkResult layoutChunkResult = mLayoutChunkResult;
        while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
            layoutChunkResult.resetInternal();
            if (VERBOSE_TRACING) {
                TraceCompat.beginSection("LLM LayoutChunk");
            }
            layoutChunk(recycler, state, layoutState, layoutChunkResult);
            if (VERBOSE_TRACING) {
                TraceCompat.endSection();
            }
            if (layoutChunkResult.mFinished) {
                break;
            }
            layoutState.mOffset += layoutChunkResult.mConsumed * layoutState.mLayoutDirection;
            /**
             * 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;
            }

            if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
                layoutState.mScrollingOffset += layoutChunkResult.mConsumed;
                if (layoutState.mAvailable < 0) {
                    layoutState.mScrollingOffset += layoutState.mAvailable;
                }
                recycleByLayoutState(recycler, layoutState);
            }
            if (stopOnFocusable && layoutChunkResult.mFocusable) {
                break;
            }
        }
        if (DEBUG) {
            validateChildOrder();
        }
        return start - layoutState.mAvailable;
    }

其中会走到while循环里去,调用layoutChunk(recycler, state, layoutState, layoutChunkResult);方法,此时在recycle里只有一个mViewCacheMax,其他的recycleviewpool和cach都没有创建呢

void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
            LayoutState layoutState, LayoutChunkResult result) {
//关键点一
        View view = layoutState.next(recycler);
        if (view == null) {
            if (DEBUG && layoutState.mScrapList == null) {
                throw new RuntimeException("received null view when unexpected");
            }
            // 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)) {

              //关键点二
                addView(view);
            } else {
                addView(view, 0);
            }
        } else {
            if (mShouldReverseLayout == (layoutState.mLayoutDirection
                    == LayoutState.LAYOUT_START)) {
                addDisappearingView(view);
            } else {
                addDisappearingView(view, 0);
            }
        }
        measureChildWithMargins(view, 0, 0);
        result.mConsumed = mOrientationHelper.getDecoratedMeasurement(view);
        int left, top, right, bottom;
        if (mOrientation == VERTICAL) {
            if (isLayoutRTL()) {
                right = getWidth() - getPaddingRight();
                left = right - mOrientationHelper.getDecoratedMeasurementInOther(view);
            } else {
                left = getPaddingLeft();
                right = left + mOrientationHelper.getDecoratedMeasurementInOther(view);
            }
            if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
                bottom = layoutState.mOffset;
                top = layoutState.mOffset - result.mConsumed;
            } else {
                top = layoutState.mOffset;
                bottom = layoutState.mOffset + result.mConsumed;
            }
        } else {
            top = getPaddingTop();
            bottom = top + mOrientationHelper.getDecoratedMeasurementInOther(view);

            if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
                right = layoutState.mOffset;
                left = layoutState.mOffset - result.mConsumed;
            } else {
                left = layoutState.mOffset;
                right = layoutState.mOffset + result.mConsumed;
            }
        }
        // We calculate everything with View's bounding box (which includes decor and margins)
        // To calculate correct layout position, we subtract margins.
        layoutDecoratedWithMargins(view, left, top, right, bottom);
        if (DEBUG) {
            Log.d(TAG, "laid out child at position " + getPosition(view) + ", with l:"
                    + (left + params.leftMargin) + ", t:" + (top + params.topMargin) + ", r:"
                    + (right - params.rightMargin) + ", b:" + (bottom - params.bottomMargin));
        }
        // Consume the available space if the view is not removed OR changed
        if (params.isItemRemoved() || params.isItemChanged()) {
            result.mIgnoreConsumed = true;
        }
        result.mFocusable = view.hasFocusable();
    }

在这方法里有两个关键点,第一个是获取到view第一个view,这里关键代码在缓存这块细说,主要记住在这方法里没有任何缓存的话会调用我们熟悉的onCreateViewHolder和onBindViewHolder方法, 关键点二就是被加到了recycleview里面了,然后会获取每个item的高度,从而把remainingSpace和layoutState.mAvailable重新赋值,当不满足条件的情况下,就退出循环了,而在fill结束以后 startOffset和endOffset就是所加view实际的高度了

if (getChildCount() > 0) {
            // because layout from end may be changed by scroll to position
            // we re-calculate it.
            // find which side we should check for gaps.
            if (mShouldReverseLayout ^ mStackFromEnd) {
                int fixOffset = fixLayoutEndGap(endOffset, recycler, state, true);
                startOffset += fixOffset;
                endOffset += fixOffset;
                fixOffset = fixLayoutStartGap(startOffset, recycler, state, false);
                startOffset += fixOffset;
                endOffset += fixOffset;
            } else {
                int fixOffset = fixLayoutStartGap(startOffset, recycler, state, true);
                startOffset += fixOffset;
                endOffset += fixOffset;
                fixOffset = fixLayoutEndGap(endOffset, recycler, state, false);
                startOffset += fixOffset;
                endOffset += fixOffset;
            }
        }

也就是说最基本的dispatchLayoutStep2被分析完了,在没有动画和任何缓存的情况下还是很简单的,至此第一个问题也就有答案了,主要就是添加显示在屏幕view,然后add进去,而dispatchLayoutStep3里面主要对动画的处理和恢复初始值,这里就不分析了,接下来分析比较绕的缓存这一块

缓存

缓存的view的获取主要在tryGetViewHolderForPositionByDeadline里

@Nullable
        ViewHolder tryGetViewHolderForPositionByDeadline(int position,
                boolean dryRun, long deadlineNs) {
            if (position < 0 || position >= mState.getItemCount()) {
                throw new IndexOutOfBoundsException("Invalid item position " + position
                        + "(" + position + "). Item count:" + mState.getItemCount()
                        + exceptionLabel());
            }
            boolean fromScrapOrHiddenOrCache = false;
            ViewHolder holder = null;
            // 0) If there is a changed scrap, try to find from there
            if (mState.isPreLayout()) {
                holder = getChangedScrapViewForPosition(position);
                fromScrapOrHiddenOrCache = holder != null;
            }
            // 1) Find by position from scrap/hidden list/cache
            if (holder == null) {
                holder = getScrapOrHiddenOrCachedHolderForPosition(position, dryRun);
                if (holder != null) {
                    if (!validateViewHolderForOffsetPosition(holder)) {
                        // recycle holder (and unscrap if relevant) since it can't be used
                        if (!dryRun) {
                            // we would like to recycle this but need to make sure it is not used by
                            // animation logic etc.
                            holder.addFlags(ViewHolder.FLAG_INVALID);
                            if (holder.isScrap()) {
                                removeDetachedView(holder.itemView, false);
                                holder.unScrap();
                            } else if (holder.wasReturnedFromScrap()) {
                                holder.clearReturnedFromScrapFlag();
                            }
                            recycleViewHolderInternal(holder);
                        }
                        holder = null;
                    } else {
                        fromScrapOrHiddenOrCache = true;
                    }
                }
            }
            if (holder == null) {
                final int offsetPosition = mAdapterHelper.findPositionOffset(position);
                if (offsetPosition < 0 || offsetPosition >= mAdapter.getItemCount()) {
                    throw new IndexOutOfBoundsException("Inconsistency detected. Invalid item "
                            + "position " + position + "(offset:" + offsetPosition + ")."
                            + "state:" + mState.getItemCount() + exceptionLabel());
                }

                final int type = mAdapter.getItemViewType(offsetPosition);
                // 2) Find from scrap/cache via stable ids, if exists
                if (mAdapter.hasStableIds()) {
                    holder = getScrapOrCachedViewForId(mAdapter.getItemId(offsetPosition),
                            type, dryRun);
                    if (holder != null) {
                        // update position
                        holder.mPosition = offsetPosition;
                        fromScrapOrHiddenOrCache = true;
                    }
                }
                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) {
                            throw new IllegalArgumentException("getViewForPositionAndType returned"
                                    + " a view which does not have a ViewHolder"
                                    + exceptionLabel());
                        } else if (holder.shouldIgnore()) {
                            throw new IllegalArgumentException("getViewForPositionAndType returned"
                                    + " a view that is ignored. You must call stopIgnoring before"
                                    + " returning this view." + exceptionLabel());
                        }
                    }
                }
                if (holder == null) { // fallback to pool
                    if (DEBUG) {
                        Log.d(TAG, "tryGetViewHolderForPositionByDeadline("
                                + position + ") fetching from shared pool");
                    }
                    holder = getRecycledViewPool().getRecycledView(type);
                    if (holder != null) {
                        holder.resetInternal();
                        if (FORCE_INVALIDATE_DISPLAY_LIST) {
                            invalidateDisplayListInt(holder);
                        }
                    }
                }
                if (holder == null) {
                    long start = getNanoTime();
                    if (deadlineNs != FOREVER_NS
                            && !mRecyclerPool.willCreateInTime(type, start, deadlineNs)) {
                        // abort - we have a deadline we can't meet
                        return null;
                    }
                    holder = mAdapter.createViewHolder(RecyclerView.this, type);
                    if (ALLOW_THREAD_GAP_WORK) {
                        // only bother finding nested RV if prefetching
                        RecyclerView innerView = findNestedRecyclerView(holder.itemView);
                        if (innerView != null) {
                            holder.mNestedRecyclerView = new WeakReference<>(innerView);
                        }
                    }

                    long end = getNanoTime();
                    mRecyclerPool.factorInCreateTime(type, end - start);
                    if (DEBUG) {
                        Log.d(TAG, "tryGetViewHolderForPositionByDeadline created new ViewHolder");
                    }
                }
            }

            // This is very ugly but the only place we can grab this information
            // before the View is rebound and returned to the LayoutManager for post layout ops.
            // We don't need this in pre-layout since the VH is not updated by the LM.
            if (fromScrapOrHiddenOrCache && !mState.isPreLayout() && holder
                    .hasAnyOfTheFlags(ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST)) {
                holder.setFlags(0, ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST);
                if (mState.mRunSimpleAnimations) {
                    int changeFlags = ItemAnimator
                            .buildAdapterChangeFlagsForAnimations(holder);
                    changeFlags |= ItemAnimator.FLAG_APPEARED_IN_PRE_LAYOUT;
                    final ItemHolderInfo info = mItemAnimator.recordPreLayoutInformation(mState,
                            holder, changeFlags, holder.getUnmodifiedPayloads());
                    recordAnimationInfoIfBouncedHiddenView(holder, info);
                }
            }

            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
                            + exceptionLabel());
                }
                final int offsetPosition = mAdapterHelper.findPositionOffset(position);
                bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
            }

            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 = fromScrapOrHiddenOrCache && bound;
            return holder;
        }

首先我们来大概分析一下,这里在源码中也有注释,第一步是获取改变的废弃的view,前提是是在预布局阶段,那何时会有预布局呢,其实你如果单个view notify的话就会有预布局了,具体的后文会说,正常的显然不走此流程,第二种情况是按照位置获取废弃的view,这其实是当你滑动的时候,会将部分view放进去,具体的看后文,现在都是获取不到的,第三种是按照id获取废弃的view,这基本用不到可以跳过,第四种是根据ViewCacheExtension获取缓存的,当然源码中并不会给我们实现,显然意义不大,最后是根据getRecycledViewPool也就是相同的type类型来获取holder,当都获取不到的时候就调用createViewHolder方法来获取,当获取完以后会把此type放进scrapdata中,会间接调用getScrapDataForType方法

 private ScrapData getScrapDataForType(int viewType) {
            ScrapData scrapData = mScrap.get(viewType);
            if (scrapData == null) {
                scrapData = new ScrapData();
                mScrap.put(viewType, scrapData);
            }
            return scrapData;
        }

而后会调用bindViewHolder从而调用onBindViewHolder方法

public final void bindViewHolder(VH holder, int position) {
            holder.mPosition = position;
            if (hasStableIds()) {
                holder.mItemId = getItemId(position);
            }
            holder.setFlags(ViewHolder.FLAG_BOUND,
                    ViewHolder.FLAG_BOUND | ViewHolder.FLAG_UPDATE | ViewHolder.FLAG_INVALID
                            | ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN);
            TraceCompat.beginSection(TRACE_BIND_VIEW_TAG);
            onBindViewHolder(holder, position, holder.getUnmodifiedPayloads());
            holder.clearPayload();
            final ViewGroup.LayoutParams layoutParams = holder.itemView.getLayoutParams();
            if (layoutParams instanceof RecyclerView.LayoutParams) {
                ((LayoutParams) layoutParams).mInsetsDirty = true;
            }
            TraceCompat.endSection();
        }

在分析出了缓存以后,我们来看下这些主要的缓存是如何触发的,我们轻轻滑动recycleview,很明显在其onTouchEvent中,然后会间接调用到
scrollVerticallyBy方法

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

从而调用scrollBy方法,间接的会调用fill方法,在fill方法中

if (layoutState.mScrollingOffset != LayoutState.SCROLLING_OFFSET_NaN) {
            // TODO ugly bug fix. should not happen
            if (layoutState.mAvailable < 0) {
                layoutState.mScrollingOffset += layoutState.mAvailable;
            }
            recycleByLayoutState(recycler, layoutState);
        }

有这样一个分之,触发条件是比如在屏幕上的view是0,1,2,在把第三个view完全滑出来的时候,第0个view就会被回收了,从而调用了removeAndRecycleViewAt方法

 public void removeAndRecycleViewAt(int index, Recycler recycler) {
            final View view = getChildAt(index);
            // 注释一
            removeViewAt(index);
            // 注释二
            recycler.recycleView(view);
        }

注释一主要是给adapter回掉的,回调的adapter的onViewDetachedFromWindow方法,注释二是添加此view到recycler里去的,着重看下注释二,会间接调用recycleViewHolderInternal方法

void recycleViewHolderInternal(ViewHolder holder) {
            if (holder.isScrap() || holder.itemView.getParent() != null) {
                throw new IllegalArgumentException(
                        "Scrapped or attached views may not be recycled. isScrap:"
                                + holder.isScrap() + " isAttached:"
                                + (holder.itemView.getParent() != null) + exceptionLabel());
            }

            if (holder.isTmpDetached()) {
                throw new IllegalArgumentException("Tmp detached view should be removed "
                        + "from RecyclerView before it can be recycled: " + holder
                        + exceptionLabel());
            }

            if (holder.shouldIgnore()) {
                throw new IllegalArgumentException("Trying to recycle an ignored view holder. You"
                        + " should first call stopIgnoringView(view) before calling recycle."
                        + exceptionLabel());
            }
            //noinspection unchecked
            final boolean transientStatePreventsRecycling = holder
                    .doesTransientStatePreventRecycling();
            final boolean forceRecycle = mAdapter != null
                    && transientStatePreventsRecycling
                    && mAdapter.onFailedToRecycleView(holder);
            boolean cached = false;
            boolean recycled = false;
            if (DEBUG && mCachedViews.contains(holder)) {
                throw new IllegalArgumentException("cached view received recycle internal? "
                        + holder + exceptionLabel());
            }
            if (forceRecycle || holder.isRecyclable()) {
                if (mViewCacheMax > 0
                        && !holder.hasAnyOfTheFlags(ViewHolder.FLAG_INVALID
                        | ViewHolder.FLAG_REMOVED
                        | ViewHolder.FLAG_UPDATE
                        | ViewHolder.FLAG_ADAPTER_POSITION_UNKNOWN)) {
                    // Retire oldest cached view
                    int cachedViewSize = mCachedViews.size();
                    if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
                        recycleCachedViewAt(0);
                        cachedViewSize--;
                    }

                    int targetCacheIndex = cachedViewSize;
                    if (ALLOW_THREAD_GAP_WORK
                            && cachedViewSize > 0
                            && !mPrefetchRegistry.lastPrefetchIncludedPosition(holder.mPosition)) {
                        // when adding the view, skip past most recently prefetched views
                        int cacheIndex = cachedViewSize - 1;
                        while (cacheIndex >= 0) {
                            int cachedPos = mCachedViews.get(cacheIndex).mPosition;
                            if (!mPrefetchRegistry.lastPrefetchIncludedPosition(cachedPos)) {
                                break;
                            }
                            cacheIndex--;
                        }
                        targetCacheIndex = cacheIndex + 1;
                    }
                    mCachedViews.add(targetCacheIndex, holder);
                    cached = true;
                }
                if (!cached) {
                    addViewHolderToRecycledViewPool(holder, true);
                    recycled = true;
                }
            } else {
                // NOTE: A view can fail to be recycled when it is scrolled off while an animation
                // runs. In this case, the item is eventually recycled by
                // ItemAnimatorRestoreListener#onAnimationFinished.

                // TODO: consider cancelling an animation when an item is removed scrollBy,
                // to return it to the pool faster
                if (DEBUG) {
                    Log.d(TAG, "trying to recycle a non-recycleable holder. Hopefully, it will "
                            + "re-visit here. We are still removing it from animation lists"
                            + exceptionLabel());
                }
            }
            // even if the holder is not removed, we still call this method so that it is removed
            // from view holder lists.
            mViewInfoStore.removeViewHolder(holder);
            if (!cached && !recycled && transientStatePreventsRecycling) {
                holder.mOwnerRecyclerView = null;
            }
        }

可以看到由于第一次的cachedViewSize是0,而且mViewCacheMax的个数由于预加载的原因(recycleview在25以后做了优化,会在同一帧提前加载布局)这里本来是二的cache,变成了三,最后会走到里

  mCachedViews.add(targetCacheIndex, holder);
                    cached = true;

可以说第一个view被划出去的时候放到了mCachedViews里面,现在展示的列表的view是1,2,3,当我们重新滑到第0个时,此时view就会从
mCachedViews里取出来,并且从列表中移除掉此view

ViewHolder getScrapOrHiddenOrCachedHolderForPosition(int position, boolean dryRun) {
            final int scrapCount = mAttachedScrap.size();

            // Try first for an exact, non-invalid match from scrap.
            for (int i = 0; i < scrapCount; i++) {
                final ViewHolder holder = mAttachedScrap.get(i);
                if (!holder.wasReturnedFromScrap() && holder.getLayoutPosition() == position
                        && !holder.isInvalid() && (mState.mInPreLayout || !holder.isRemoved())) {
                    holder.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP);
                    return holder;
                }
            }

            if (!dryRun) {
                View view = mChildHelper.findHiddenNonRemovedView(position);
                if (view != null) {
                    // This View is good to be used. We just need to unhide, detach and move to the
                    // scrap list.
                    final ViewHolder vh = getChildViewHolderInt(view);
                    mChildHelper.unhide(view);
                    int layoutIndex = mChildHelper.indexOfChild(view);
                    if (layoutIndex == RecyclerView.NO_POSITION) {
                        throw new IllegalStateException("layout index should not be -1 after "
                                + "unhiding a view:" + vh + exceptionLabel());
                    }
                    mChildHelper.detachViewFromParent(layoutIndex);
                    scrapView(view);
                    vh.addFlags(ViewHolder.FLAG_RETURNED_FROM_SCRAP
                            | ViewHolder.FLAG_BOUNCED_FROM_HIDDEN_LIST);
                    return vh;
                }
            }

            // Search in our first-level recycled view cache.
            final int cacheSize = mCachedViews.size();
            for (int i = 0; i < cacheSize; i++) {
                final ViewHolder holder = mCachedViews.get(i);
                // invalid view holders may be in cache if adapter has stable ids as they can be
                // retrieved via getScrapOrCachedViewForId
                if (!holder.isInvalid() && holder.getLayoutPosition() == position) {
                    if (!dryRun) {
                        mCachedViews.remove(i);
                    }
                    if (DEBUG) {
                        Log.d(TAG, "getScrapOrHiddenOrCachedHolderForPosition(" + position
                                + ") found match in cache: " + holder);
                    }
                    return holder;
                }
            }
            return null;
        }

这里主要走的是cacheSize的方法,也就是说从mCachedViews里恢复的view是可以直接add的,不用调用任何的回掉,也就是最高效的
也就是recycleview的view都是先add在remove的
此时我们把视图初始化,回到0,1,2的位置,我这里手动关闭了预加载,也就是说mViewCacheMax的是二,来分析下,回收池的回收情况
这里滑到了2,3,4的位置,显然cache的size是二了,我们在往上滑
这里走到了如下代码中

 if (cachedViewSize >= mViewCacheMax && cachedViewSize > 0) {
                        recycleCachedViewAt(0);
                        cachedViewSize--;
                    }

来看下recycleCachedViewAt的方法

 void recycleCachedViewAt(int cachedViewIndex) {
            if (DEBUG) {
                Log.d(TAG, "Recycling cached view at index " + cachedViewIndex);
            }
            ViewHolder viewHolder = mCachedViews.get(cachedViewIndex);
            if (DEBUG) {
                Log.d(TAG, "CachedViewHolder to be recycled: " + viewHolder);
            }
            addViewHolderToRecycledViewPool(viewHolder, true);
            mCachedViews.remove(cachedViewIndex);
        }

这里我们看到会把第一个的cached从列表里移出去,同时把第一个的view放进了recycledViewPool中

   void addViewHolderToRecycledViewPool(ViewHolder holder, boolean dispatchRecycled) {
            clearNestedRecyclerViewIfNotNested(holder);
            if (holder.hasAnyOfTheFlags(ViewHolder.FLAG_SET_A11Y_ITEM_DELEGATE)) {
                holder.setFlags(0, ViewHolder.FLAG_SET_A11Y_ITEM_DELEGATE);
                ViewCompat.setAccessibilityDelegate(holder.itemView, null);
            }
            if (dispatchRecycled) {
                dispatchViewRecycled(holder);
            }
            holder.mOwnerRecyclerView = null;
            getRecycledViewPool().putRecycledView(holder);
        }

这个dispatchViewRecycled是放到缓存池的标志

   public void putRecycledView(ViewHolder scrap) {
            final int viewType = scrap.getItemViewType();
            final ArrayList<ViewHolder> scrapHeap = getScrapDataForType(viewType).mScrapHeap;
            if (mScrap.get(viewType).mMaxScrap <= scrapHeap.size()) {
                return;
            }
            if (DEBUG && scrapHeap.contains(scrap)) {
                throw new IllegalArgumentException("this scrap item already exists");
            }
            scrap.resetInternal();
            scrapHeap.add(scrap);
        }

这里我们看到其实是每一个type最大有5个scrap,因为recycleview的缓存池是和type相关的,并不是和位置相关的,继续看下recycleViewHolderInternal方法会发现,此时它又把第二个位置给放到缓存队列里了,所以现在的结果是
第0个在缓存池里,第一和第二个在cache里,第三,第四,第五个在列表的显示,继续滑动,会发现从第六个开始,就不再调用onCreateViewHolder方法了,但是由于是缓存池,缓存的是type,所以
onBindViewHolder方法还是会调用的原因如下

tryGetViewHolderForPositionByDeadline方法
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
                            + exceptionLabel());
                }
                final int offsetPosition = mAdapterHelper.findPositionOffset(position);
                bound = tryBindViewHolderByDeadline(holder, offsetPosition, position, deadlineNs);
            }

由于是缓存池会让此holder需要更新,所以会走到tryBindViewHolderByDeadline从而调用onBindViewHolder方法,而从cache恢复的却不会
现在出一个小问题:
recycleview的数量有20个都是同一个类型的type,界面上有3个itme,一共会调用几次onCreateViewHolder呢?(这里不考虑25+的预加载的情况)
当然第二个的答案也就出来了,第三和第四个的情况下篇分析把,不然篇幅太长了

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