RecyclerView(三) 一、預加載流程 二、代碼展示

Android知識總結

一、預加載流程

有RecyclerView.onTouchEvent的ACTION_MOVE事件進入分析

GapWorker mGapWorker;

public boolean onTouchEvent(MotionEvent e) {
    case MotionEvent.ACTION_MOVE: {
        if (mGapWorker != null && (dx != 0 || dy != 0)) {
            //開始進入預加載過程
            mGapWorker.postFromTraversal(this, dx, dy);
        }
    }
}

調用GapWorker的postFromTraversal方法。GapWorker只要做間隙工作(即View 刷新間隙)，對RecyclerView的item做預加載處理

final class GapWorker implements Runnable {
    //GapWorker 的靜態實例
    static final ThreadLocal<GapWorker> sGapWorker = new ThreadLocal<>();
    //RecyclerView的緩存
    ArrayList<RecyclerView> mRecyclerViews = new ArrayList<>();

    void postFromTraversal(RecyclerView recyclerView, int prefetchDx, int prefetchDy) {
        if (recyclerView.isAttachedToWindow()) {
            if (RecyclerView.DEBUG && !mRecyclerViews.contains(recyclerView)) {
                throw new IllegalStateException("attempting to post unregistered view!");
            }
            if (mPostTimeNs == 0) {
                mPostTimeNs = recyclerView.getNanoTime();
                //會調用View的Post方法,通過Handler處理任務，活到Runable中
                recyclerView.post(this);
            }
        }

        recyclerView.mPrefetchRegistry.setPrefetchVector(prefetchDx, prefetchDy);
    }
}

View.Post()方法

    public boolean post(Runnable action) {
        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            return attachInfo.mHandler.post(action);
        }
        getRunQueue().post(action);
        return true;
    }

執行GapWorker的任務run中執行

    public void run() {
        try {
            TraceCompat.beginSection(RecyclerView.TRACE_PREFETCH_TAG);
            if (mRecyclerViews.isEmpty()) {
                // abort - no work to do
                return;
            }
            final int size = mRecyclerViews.size();
            long latestFrameVsyncMs = 0;
            for (int i = 0; i < size; i++) {
                RecyclerView view = mRecyclerViews.get(i);
                if (view.getWindowVisibility() == View.VISIBLE) {
                    latestFrameVsyncMs = Math.max(view.getDrawingTime(), latestFrameVsyncMs);
                }
            }

            if (latestFrameVsyncMs == 0) {
                // abort - either no views visible, or couldn't get last vsync for estimating next
                return;
            }
            long nextFrameNs = TimeUnit.MILLISECONDS.toNanos(latestFrameVsyncMs) + mFrameIntervalNs;
            prefetch(nextFrameNs);
        } finally {
            mPostTimeNs = 0;
            TraceCompat.endSection();
        }
    }

GapWorker.prefetch方法

    void prefetch(long deadlineNs) {
        buildTaskList();
        flushTasksWithDeadline(deadlineNs);
    }

    private void flushTasksWithDeadline(long deadlineNs) {
        for (int i = 0; i < mTasks.size(); i++) {
            final Task task = mTasks.get(i);
            if (task.view == null) {
                break; // done with populated tasks
            }
            flushTaskWithDeadline(task, deadlineNs);
            task.clear();
        }
    }

    private void flushTaskWithDeadline(Task task, long deadlineNs) {
        long taskDeadlineNs = task.immediate ? RecyclerView.FOREVER_NS : deadlineNs;
        RecyclerView.ViewHolder holder = prefetchPositionWithDeadline(task.view,
                task.position, taskDeadlineNs);
        if (holder != null
                && holder.mNestedRecyclerView != null
                && holder.isBound()
                && !holder.isInvalid()) {
            prefetchInnerRecyclerViewWithDeadline(holder.mNestedRecyclerView.get(), deadlineNs);
        }
    }

    private void prefetchInnerRecyclerViewWithDeadline(@Nullable RecyclerView innerView,
            long deadlineNs) {
        final LayoutPrefetchRegistryImpl innerPrefetchRegistry = innerView.mPrefetchRegistry;
        innerPrefetchRegistry.collectPrefetchPositionsFromView(innerView, true);
     ...
    }

回調到GapWorker的內部類LayoutPrefetchRegistryImpl的collectPrefetchPositionsFromView方法

    void collectPrefetchPositionsFromView(RecyclerView view, boolean nested) {
        mCount = 0;
        if (mPrefetchArray != null) {
            Arrays.fill(mPrefetchArray, -1);
        }

        final RecyclerView.LayoutManager layout = view.mLayout;
        if (view.mAdapter != null
                && layout != null
                && layout.isItemPrefetchEnabled()) {
            if (nested) {
                // nested prefetch, only if no adapter updates pending. Note: we don't query
                // view.hasPendingAdapterUpdates(), as first layout may not have occurred
                if (!view.mAdapterHelper.hasPendingUpdates()) {
                    layout.collectInitialPrefetchPositions(view.mAdapter.getItemCount(), this);
                }
            } else {
                //1）、 需要預加載
                if (!view.hasPendingAdapterUpdates()) {
                    layout.collectAdjacentPrefetchPositions(mPrefetchDx, mPrefetchDy,
                            view.mState, this);
                }
            }

            if (mCount > layout.mPrefetchMaxCountObserved) {
                //預加載 mPrefetchMaxCountObserved = 1
                layout.mPrefetchMaxCountObserved = mCount;
                //是否預加載了
                layout.mPrefetchMaxObservedInInitialPrefetch = nested;
                //2）、更新CacheSize的大小
                view.mRecycler.updateViewCacheSize();
            }
        }
    }

• 1)、步驟設置mCount的值
  我們看LayoutManager實現了LinearLayoutManager的方法collectInitialPrefetchPositions

    public void collectInitialPrefetchPositions(int adapterItemCount,
            LayoutPrefetchRegistry layoutPrefetchRegistry) {
        final boolean fromEnd;
        final int anchorPos;
        if (mPendingSavedState != null && mPendingSavedState.hasValidAnchor()) {
            // use restored state, since it hasn't been resolved yet
            fromEnd = mPendingSavedState.mAnchorLayoutFromEnd;
            anchorPos = mPendingSavedState.mAnchorPosition;
        } else {
            resolveShouldLayoutReverse();
            fromEnd = mShouldReverseLayout;
            if (mPendingScrollPosition == RecyclerView.NO_POSITION) {
                anchorPos = fromEnd ? adapterItemCount - 1 : 0;
            } else {
                anchorPos = mPendingScrollPosition;
            }
        }

        final int direction = fromEnd
                ? LayoutState.ITEM_DIRECTION_HEAD
                : LayoutState.ITEM_DIRECTION_TAIL;
        int targetPos = anchorPos;
        for (int i = 0; i < mInitialPrefetchItemCount; i++) {
            if (targetPos >= 0 && targetPos < adapterItemCount) {
                //預加載時，添加一個item值
                layoutPrefetchRegistry.addPosition(targetPos, 0);
            } else {
                break; // no more to prefetch
            }
            targetPos += direction;
        }
    }

最後會回到GapWorker中執行addPosition方法

    public void addPosition(int layoutPosition, int pixelDistance) {
        if (layoutPosition < 0) {
            throw new IllegalArgumentException("Layout positions must be non-negative");
        }

        if (pixelDistance < 0) {
            throw new IllegalArgumentException("Pixel distance must be non-negative");
        }

        // allocate or expand array as needed, doubling when needed
        final int storagePosition = mCount * 2;
        if (mPrefetchArray == null) {
            mPrefetchArray = new int[4];
            Arrays.fill(mPrefetchArray, -1);
        } else if (storagePosition >= mPrefetchArray.length) {
            final int[] oldArray = mPrefetchArray;
            mPrefetchArray = new int[storagePosition * 2];
            System.arraycopy(oldArray, 0, mPrefetchArray, 0, oldArray.length);
        }

        // add position
        mPrefetchArray[storagePosition] = layoutPosition;
        mPrefetchArray[storagePosition + 1] = pixelDistance;
        // mCount = 1，可以回到 collectPrefetchPositionsFromView方法中再看執行結果
        mCount++;
    }

• 2）、更新CacheSize的大小
  執行RecyclerView的updateViewCacheSize方法

    void updateViewCacheSize() {
        int extraCache = mLayout != null ? mLayout.mPrefetchMaxCountObserved : 0;
        //mViewCacheMax = 3
        mViewCacheMax = mRequestedCacheMax + extraCache;

        // first, try the views that can be recycled
        //是否滿足向 pool 中加入值
        for (int i = mCachedViews.size() - 1;
             i >= 0 && mCachedViews.size() > mViewCacheMax; i--) {
            //向 pool 中加入VH
            recycleCachedViewAt(i);
        }
    }

二、代碼展示

下面是一個簡單的代碼來講解預加載的打開或關閉效果

public class CheckCacheSizeActivity extends AppCompatActivity {
    private RecyclerView mRecyclerView;

    public static void startCheckCacheSizeActivity(Activity activity) {
        Intent intent = new Intent(activity, CheckCacheSizeActivity.class);
        activity.startActivity(intent);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_check_cache_size);
        mRecyclerView = findViewById(R.id.cache_rv);
        LinearLayoutManager layoutManager = new LinearLayoutManager(this);
        layoutManager.setOrientation(RecyclerView.VERTICAL);
        mRecyclerView.setLayoutManager(layoutManager);

        mRecyclerView.setAdapter(new MyCheckCacheAdapter(initDate()));

        //關閉預加載使能
        layoutManager.setItemPrefetchEnabled(false);

        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                try {
                    Class<?> clazz = Class.forName("androidx.recyclerview.widget.LinearLayoutManager");
                    //獲取預加載的個數
                    Field mPrefetchMaxCountObserved = clazz.getSuperclass()
                            .getDeclaredField("mPrefetchMaxCountObserved");
                    mPrefetchMaxCountObserved.setAccessible(true);
                    Log.e("--->", "mPrefetchMaxCountObserved = " + mPrefetchMaxCountObserved.get(layoutManager));
                } catch (Exception e) {
                    e.printStackTrace();
                }

                try {
                    Class<?> clazz = Class.forName("androidx.recyclerview.widget.RecyclerView");
                    Field mRecyclerField = clazz.getDeclaredField("mRecycler");
                    mRecyclerField.setAccessible(true);
                    RecyclerView.Recycler recycler  = (RecyclerView.Recycler) mRecyclerField.get(mRecyclerView);
                    Class<?> recyclerClass = recycler.getClass();
                    //獲取緩存 mCachedViews
                    Field mCachedViews = recyclerClass.getDeclaredField("mCachedViews");
                    mCachedViews.setAccessible(true);
                    ArrayList<RecyclerView.ViewHolder> views = (ArrayList<RecyclerView.ViewHolder>) mCachedViews.get(recycler);
                    Log.e("--->", "mCachedViews.size = " + views.size());
                } catch (Exception e) {
                    e.printStackTrace();
                }

            }
        }).start();

    }
    
    private List<String> initDate() {
        List<String> list = new ArrayList<>();
        for (int i =0; i < 40; i ++){
            list.add("This is item " + i);
        }
        return list;
    }

    class MyCheckCacheAdapter extends RecyclerView.Adapter<MyCheckCacheAdapter.MyViewHolder> {
        private List<String> mList;

        public MyCheckCacheAdapter(List<String> list) {
            mList = list;
        }

        @NonNull
        @Override
        public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
            View view = LayoutInflater.from(parent.getContext())
                    .inflate(R.layout.check_cache_rv_layout, null);
            return new MyViewHolder(view);
        }

        @Override
        public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
            holder.mTextView.setText(mList.get(position));
        }

        @Override
        public int getItemCount() {
            return mList != null ? mList.size() : 0;
        }

        class MyViewHolder extends RecyclerView.ViewHolder {
            private TextView mTextView;
            public MyViewHolder(@NonNull View itemView) {
                super(itemView);
                mTextView = itemView.findViewById(R.id.name_tv);
            }
        }
    }
}

2.1、打開預加載

layoutManager.setItemPrefetchEnabled(true);打開預加載

• 效果

//預加載個數
 E/--->: mPrefetchMaxCountObserved = 1
//mCachedViews 的大小
 E/--->: mCachedViews.size = 3

2.2、關閉預加載

layoutManager.setItemPrefetchEnabled(false);關閉預加載

• 效果

E/--->: mPrefetchMaxCountObserved = 0
E/--->: mCachedViews.size = 2

由上面可見 mCachedViews的大小等於 DEFAULT_CACHE_SIZE + mPrefetchMaxCountObserved的大小。