基于Android官方AsyncListUtil优化改进RecyclerView分页加载机制(一)

基于Android官方AsyncListUtil优化改进RecyclerView分页加载机制(一)


Android AsyncListUtil是Android官方提供的专为列表这样的数据更新加载提供的异步加载组件。基于AsyncListUtil组件,可以轻易实现常见的RecyclerView分页加载技术。AsyncListUtil技术涉及的细节比较繁复,因此我将分别写若干篇文章,分点、分解AsyncListUtil技术。

先给出一个可运行的例子,MainActivity.java:

package zhangphil.app;

import android.graphics.Color;
import android.os.Bundle;
import android.os.SystemClock;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.util.AsyncListUtil;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.text.TextUtils;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.LinearLayout;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    private String TAG = "调试";

    private final int NULL = -1;

    private RecyclerView mRecyclerView;
    private AsyncListUtil mAsyncListUtil;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mRecyclerView = findViewById(R.id.recycler_view);

        LinearLayoutManager mLayoutManager = new LinearLayoutManager(this);
        mLayoutManager.setOrientation(LinearLayout.VERTICAL);
        mRecyclerView.setLayoutManager(mLayoutManager);

        RecyclerView.Adapter mAdapter = new MyAdapter();
        mRecyclerView.setAdapter(mAdapter);

        MyDataCallback mDataCallback = new MyDataCallback();
        MyViewCallback mViewCallback = new MyViewCallback();
        mAsyncListUtil = new AsyncListUtil(String.class, 20, mDataCallback, mViewCallback);

        mRecyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
            @Override
            public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
                super.onScrollStateChanged(recyclerView, newState);

                Log.d(TAG, "onRangeChanged");
                mAsyncListUtil.onRangeChanged();
            }
        });

        findViewById(R.id.button).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Log.d(TAG, "refresh");
                mAsyncListUtil.refresh();
            }
        });
    }

    private class MyDataCallback extends AsyncListUtil.DataCallback<String> {

        @Override
        public int refreshData() {
            //更新数据的元素个数。
            //假设预先设定更新若干条。
            int count = Integer.MAX_VALUE;
            Log.d(TAG, "refreshData:" + count);
            return count;
        }

        /**
         * 在这里完成数据加载的耗时任务。
         *
         * @param data
         * @param startPosition
         * @param itemCount
         */
        @Override
        public void fillData(String[] data, int startPosition, int itemCount) {
            Log.d(TAG, "fillData:" + startPosition + "," + itemCount);
            for (int i = 0; i < itemCount; i++) {
                data[i] = String.valueOf(System.currentTimeMillis());

                //模拟耗时任务,故意休眠一定时延。
                SystemClock.sleep(100);
            }
        }
    }

    private class MyViewCallback extends AsyncListUtil.ViewCallback {

        /**
         * @param outRange
         */
        @Override
        public void getItemRangeInto(int[] outRange) {
            getOutRange(outRange);

            /**
             * 如果当前的RecyclerView为空,主动为用户加载数据.
             * 假设预先加载若干条数据
             *
             */
            if (outRange[0] == NULL && outRange[1] == NULL) {
                Log.d(TAG, "当前RecyclerView为空!");
                outRange[0] = 0;
                outRange[1] = 9;
            }

            Log.d(TAG, "getItemRangeInto,当前可见position: " + outRange[0] + " ~ " + outRange[1]);
        }

        @Override
        public void onDataRefresh() {
            int[] outRange = new int[2];
            getOutRange(outRange);
            mRecyclerView.getAdapter().notifyItemRangeChanged(outRange[0], outRange[1] - outRange[0] + 1);

            Log.d(TAG, "onDataRefresh:"+outRange[0]+","+outRange[1]);
        }

        @Override
        public void onItemLoaded(int position) {
            mRecyclerView.getAdapter().notifyItemChanged(position);
            Log.d(TAG, "onItemLoaded:" + position);
        }
    }

    private void    getOutRange(int[] outRange){
        outRange[0] = ((LinearLayoutManager) mRecyclerView.getLayoutManager()).findFirstVisibleItemPosition();
        outRange[1] = ((LinearLayoutManager) mRecyclerView.getLayoutManager()).findLastVisibleItemPosition();
    }

    private class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
        public MyAdapter() {
            super();
        }

        @Override
        public ViewHolder onCreateViewHolder(ViewGroup viewGroup, int i) {
            View view = LayoutInflater.from(getApplicationContext()).inflate(android.R.layout.simple_list_item_2, null);
            ViewHolder holder = new ViewHolder(view);
            return holder;
        }

        @Override
        public void onBindViewHolder(ViewHolder viewHolder, int i) {
            viewHolder.text1.setText(String.valueOf(i));

            String s = String.valueOf(mAsyncListUtil.getItem(i));
            if (TextUtils.equals(s, "null")) {
                s = "加载中...";
            }

            viewHolder.text2.setText(s);
        }

        @Override
        public int getItemCount() {
            return mAsyncListUtil.getItemCount();
        }

        public class ViewHolder extends RecyclerView.ViewHolder {
            public TextView text1;
            public TextView text2;

            public ViewHolder(View itemView) {
                super(itemView);

                text1 = itemView.findViewById(android.R.id.text1);
                text1.setTextColor(Color.RED);

                text2 = itemView.findViewById(android.R.id.text2);
                text2.setTextColor(Color.BLUE);
            }
        }
    }
}


MainActivity所需布局文件:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="更新" />

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</LinearLayout>


(一)new AsyncListUtil之后Android自动就会启动初次刷新加载。
原因在AsyncListUtil构造函数里面,已经调用refresh方法启动刷新,见AsyncListUtil构造函数源代码:
/**
     * Creates an AsyncListUtil.
     *
     * @param klass Class of the data item.
     * @param tileSize Number of item per chunk loaded at once.
     * @param dataCallback Data access callback.
     * @param viewCallback Callback for querying visible item range and update notifications.
     */
    public AsyncListUtil(Class<T> klass, int tileSize, DataCallback<T> dataCallback,
                         ViewCallback viewCallback) {
        mTClass = klass;
        mTileSize = tileSize;
        mDataCallback = dataCallback;
        mViewCallback = viewCallback;

        mTileList = new TileList<T>(mTileSize);

        ThreadUtil<T> threadUtil = new MessageThreadUtil<T>();
        mMainThreadProxy = threadUtil.getMainThreadProxy(mMainThreadCallback);
        mBackgroundProxy = threadUtil.getBackgroundProxy(mBackgroundCallback);

        refresh();
    }

当代码启动后logcat输出:
11-22 14:41:18.313 32764-447/zhangphil.app D/调试: refreshData:2147483647
11-22 14:41:18.336 32764-32764/zhangphil.app D/调试: onDataRefresh:-1,-1
11-22 14:41:18.336 32764-32764/zhangphil.app D/调试: 当前RecyclerView为空!
11-22 14:41:18.336 32764-32764/zhangphil.app D/调试: getItemRangeInto,当前可见position: 0 ~ 9
11-22 14:41:18.337 32764-449/zhangphil.app D/调试: fillData:0,20
11-22 14:41:20.350 32764-32764/zhangphil.app D/调试: onItemLoaded:0
11-22 14:41:20.351 32764-32764/zhangphil.app D/调试: onItemLoaded:1
11-22 14:41:20.351 32764-32764/zhangphil.app D/调试: onItemLoaded:2
11-22 14:41:20.352 32764-32764/zhangphil.app D/调试: onItemLoaded:3
11-22 14:41:20.353 32764-32764/zhangphil.app D/调试: onItemLoaded:4
11-22 14:41:20.353 32764-32764/zhangphil.app D/调试: onItemLoaded:5
11-22 14:41:20.353 32764-32764/zhangphil.app D/调试: onItemLoaded:6


(二)在RecyclerView里面的onScrollStateChanged增加onRangeChanged方法,触发AsyncListUtil的关键函数getItemRangeInto。
触发getItemRangeInto的方法有很多种,通常在RecyclerView里面,分页加载常常会由用户的上下翻动RecyclerView触发。因此自然的就想到在RecyclerView的onScrollStateChanged触发AsyncListUtil分页更新加载逻辑。
getItemRangeInto参数outRange维护两个整型元素,前者outRange[0]表示列表顶部可见元素的位置position,后者outRange[1]表示最底部可见元素的position,开发者对这两个值进行计算,通常就是获取当前RecyclerView顶部outRange[0]的FirstVisibleItemPosition,

outRange[1]是LastVisibleItemPosition。当这两个参数赋值后,将直接触发fillData,fillData是AsyncListUtil进行长期耗时后台任务的地方,开发者可以在这里处理自己的后台线程任务。

比如现在手指在屏幕上从下往上翻滚RecyclerView,故意翻到没有数据的地方(position=21 ~ position=28)然后加载出来,logcat输出:

11-22 14:42:35.543 32764-32764/zhangphil.app D/调试: getItemRangeInto,当前可见position: 0 ~ 6
11-22 14:42:36.012 32764-32764/zhangphil.app D/调试: onRangeChanged
11-22 14:42:36.012 32764-32764/zhangphil.app D/调试: getItemRangeInto,当前可见position: 5 ~ 12
11-22 14:42:36.013 32764-1011/zhangphil.app D/调试: fillData:20,20
11-22 14:42:36.844 32764-32764/zhangphil.app D/调试: onRangeChanged
11-22 14:42:36.844 32764-32764/zhangphil.app D/调试: getItemRangeInto,当前可见position: 10 ~ 16
11-22 14:42:37.067 32764-32764/zhangphil.app D/调试: onRangeChanged
11-22 14:42:37.067 32764-32764/zhangphil.app D/调试: getItemRangeInto,当前可见position: 13 ~ 20
11-22 14:42:38.020 32764-32764/zhangphil.app D/调试: onItemLoaded:20
11-22 14:42:38.020 32764-32764/zhangphil.app D/调试: onItemLoaded:21
11-22 14:42:38.020 32764-32764/zhangphil.app D/调试: onItemLoaded:22
11-22 14:42:38.020 32764-32764/zhangphil.app D/调试: onItemLoaded:23
11-22 14:42:38.021 32764-32764/zhangphil.app D/调试: onItemLoaded:24
11-22 14:42:38.021 32764-32764/zhangphil.app D/调试: onItemLoaded:25
11-22 14:42:38.021 32764-32764/zhangphil.app D/调试: onItemLoaded:26
11-22 14:42:38.021 32764-32764/zhangphil.app D/调试: onItemLoaded:27
11-22 14:42:38.021 32764-32764/zhangphil.app D/调试: onItemLoaded:28
11-22 14:42:38.784 32764-32764/zhangphil.app D/调试: onRangeChanged
11-22 14:42:38.784 32764-32764/zhangphil.app D/调试: getItemRangeInto,当前可见position: 21 ~ 28

(三)fillData分页加载。
fillData将实现最终的分页加载,通常开发者在这里把数据从网络/数据库/文件系统把数据读出来。本例fillData每次读取20条数据,原因是在AsyncListUtil构造时候,指定了tileSize=20。tileSize决定每次分页加载的数据量。由此,每一次AsyncListUtil分页加载的startPosition位置依次是:0,20,40,60……

(四)onItemLoaded数据装载成功后回调。
当fillData把数据加载完成后,会主动的加载到getItemRangeInto所限定的第一个到最后一个可见范围内的item,此时在RecyclerView里面用notifyItemChanged更新UI即可。

(五)fillData加载的数据覆盖getItemRangeInto返回的第一个到最后一个可见范围内的RecyclerView列表项目。
比如,如果getItemRangeInto返回的两个position:outRange[0]=0,outRange[1]=9,那么fillData将一如既往的加载第0个位置开始的20条数据。即fillData的设计目的将为把用户可见区域内容的所有项目数据均加载完成,保证用户可见区域内的数据是优先加载的。

随后当用户在上下翻动RecyclerView时候,onRangeChanged 触发getItemRangeInto返回变化的outRange,如果历史的数据已经加载,即便用户翻回去,亦不会重新加载即fillData。

(六)AsyncListUtil的refresh强制刷新。

常见的RecyclerView可能需要强制刷新的功能,比如,当用户长期停留而不做任何滑动时候,如果仍然要保证数据最新,那么就要刷新一次获取。AsyncListUtil的refresh为此设计。refresh触发新的一轮由getItemRangeInto决定的、fillData完成的数据更新。但是要注意,这时候的RecyclerView若更新是由refresh触发,需要在onDataRefresh调用RecyclerView的notifyItemRangeChanged更新UI。但是要注意这里面要时刻注意fillData每次加载数据都是分页的按照startPosition:0,20,40,60……这样的刻度,每次取20条。







附录Android官方实现的AsyncListUtil.java源代码:

/*
 * Copyright (C) 2015 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.support.v7.util;

import android.support.annotation.UiThread;
import android.support.annotation.WorkerThread;
import android.util.Log;
import android.util.SparseBooleanArray;
import android.util.SparseIntArray;

/**
 * A utility class that supports asynchronous content loading.
 * <p>
 * It can be used to load Cursor data in chunks without querying the Cursor on the UI Thread while
 * keeping UI and cache synchronous for better user experience.
 * <p>
 * It loads the data on a background thread and keeps only a limited number of fixed sized
 * chunks in memory at all times.
 * <p>
 * {@link AsyncListUtil} queries the currently visible range through {@link ViewCallback},
 * loads the required data items in the background through {@link DataCallback}, and notifies a
 * {@link ViewCallback} when the data is loaded. It may load some extra items for smoother
 * scrolling.
 * <p>
 * Note that this class uses a single thread to load the data, so it suitable to load data from
 * secondary storage such as disk, but not from network.
 * <p>
 * This class is designed to work with {@link android.support.v7.widget.RecyclerView}, but it does
 * not depend on it and can be used with other list views.
 *
 */
public class AsyncListUtil<T> {
    static final String TAG = "AsyncListUtil";

    static final boolean DEBUG = false;

    final Class<T> mTClass;
    final int mTileSize;
    final DataCallback<T> mDataCallback;
    final ViewCallback mViewCallback;

    final TileList<T> mTileList;

    final ThreadUtil.MainThreadCallback<T> mMainThreadProxy;
    final ThreadUtil.BackgroundCallback<T> mBackgroundProxy;

    final int[] mTmpRange = new int[2];
    final int[] mPrevRange = new int[2];
    final int[] mTmpRangeExtended = new int[2];

    boolean mAllowScrollHints;
    private int mScrollHint = ViewCallback.HINT_SCROLL_NONE;

    int mItemCount = 0;

    int mDisplayedGeneration = 0;
    int mRequestedGeneration = mDisplayedGeneration;

    final SparseIntArray mMissingPositions = new SparseIntArray();

    void log(String s, Object... args) {
        Log.d(TAG, "[MAIN] " + String.format(s, args));
    }

    /**
     * Creates an AsyncListUtil.
     *
     * @param klass Class of the data item.
     * @param tileSize Number of item per chunk loaded at once.
     * @param dataCallback Data access callback.
     * @param viewCallback Callback for querying visible item range and update notifications.
     */
    public AsyncListUtil(Class<T> klass, int tileSize, DataCallback<T> dataCallback,
                         ViewCallback viewCallback) {
        mTClass = klass;
        mTileSize = tileSize;
        mDataCallback = dataCallback;
        mViewCallback = viewCallback;

        mTileList = new TileList<T>(mTileSize);

        ThreadUtil<T> threadUtil = new MessageThreadUtil<T>();
        mMainThreadProxy = threadUtil.getMainThreadProxy(mMainThreadCallback);
        mBackgroundProxy = threadUtil.getBackgroundProxy(mBackgroundCallback);

        refresh();
    }

    private boolean isRefreshPending() {
        return mRequestedGeneration != mDisplayedGeneration;
    }

    /**
     * Updates the currently visible item range.
     *
     * <p>
     * Identifies the data items that have not been loaded yet and initiates loading them in the
     * background. Should be called from the view‘s scroll listener (such as
     * {@link android.support.v7.widget.RecyclerView.OnScrollListener#onScrolled}).
     */
    public void onRangeChanged() {
        if (isRefreshPending()) {
            return;  // Will update range will the refresh result arrives.
        }
        updateRange();
        mAllowScrollHints = true;
    }

    /**
     * Forces reloading the data.
     * <p>
     * Discards all the cached data and reloads all required data items for the currently visible
     * range. To be called when the data item count and/or contents has changed.
     */
    public void refresh() {
        mMissingPositions.clear();
        mBackgroundProxy.refresh(++mRequestedGeneration);
    }

    /**
     * Returns the data item at the given position or <code>null</code> if it has not been loaded
     * yet.
     *
     * <p>
     * If this method has been called for a specific position and returned <code>null</code>, then
     * {@link ViewCallback#onItemLoaded(int)} will be called when it finally loads. Note that if
     * this position stays outside of the cached item range (as defined by
     * {@link ViewCallback#extendRangeInto} method), then the callback will never be called for
     * this position.
     *
     * @param position Item position.
     *
     * @return The data item at the given position or <code>null</code> if it has not been loaded
     *         yet.
     */
    public T getItem(int position) {
        if (position < 0 || position >= mItemCount) {
            throw new IndexOutOfBoundsException(position + " is not within 0 and " + mItemCount);
        }
        T item = mTileList.getItemAt(position);
        if (item == null && !isRefreshPending()) {
            mMissingPositions.put(position, 0);
        }
        return item;
    }

    /**
     * Returns the number of items in the data set.
     *
     * <p>
     * This is the number returned by a recent call to
     * {@link DataCallback#refreshData()}.
     *
     * @return Number of items.
     */
    public int getItemCount() {
        return mItemCount;
    }

    void updateRange() {
        mViewCallback.getItemRangeInto(mTmpRange);
        if (mTmpRange[0] > mTmpRange[1] || mTmpRange[0] < 0) {
            return;
        }
        if (mTmpRange[1] >= mItemCount) {
            // Invalid range may arrive soon after the refresh.
            return;
        }

        if (!mAllowScrollHints) {
            mScrollHint = ViewCallback.HINT_SCROLL_NONE;
        } else if (mTmpRange[0] > mPrevRange[1] || mPrevRange[0] > mTmpRange[1]) {
            // Ranges do not intersect, long leap not a scroll.
            mScrollHint = ViewCallback.HINT_SCROLL_NONE;
        } else if (mTmpRange[0] < mPrevRange[0]) {
            mScrollHint = ViewCallback.HINT_SCROLL_DESC;
        } else if (mTmpRange[0] > mPrevRange[0]) {
            mScrollHint = ViewCallback.HINT_SCROLL_ASC;
        }

        mPrevRange[0] = mTmpRange[0];
        mPrevRange[1] = mTmpRange[1];

        mViewCallback.extendRangeInto(mTmpRange, mTmpRangeExtended, mScrollHint);
        mTmpRangeExtended[0] = Math.min(mTmpRange[0], Math.max(mTmpRangeExtended[0], 0));
        mTmpRangeExtended[1] =
                Math.max(mTmpRange[1], Math.min(mTmpRangeExtended[1], mItemCount - 1));

        mBackgroundProxy.updateRange(mTmpRange[0], mTmpRange[1],
                mTmpRangeExtended[0], mTmpRangeExtended[1], mScrollHint);
    }

    private final ThreadUtil.MainThreadCallback<T>
            mMainThreadCallback = new ThreadUtil.MainThreadCallback<T>() {
        @Override
        public void updateItemCount(int generation, int itemCount) {
            if (DEBUG) {
                log("updateItemCount: size=%d, gen #%d", itemCount, generation);
            }
            if (!isRequestedGeneration(generation)) {
                return;
            }
            mItemCount = itemCount;
            mViewCallback.onDataRefresh();
            mDisplayedGeneration = mRequestedGeneration;
            recycleAllTiles();

            mAllowScrollHints = false;  // Will be set to true after a first real scroll.
            // There will be no scroll event if the size change does not affect the current range.
            updateRange();
        }

        @Override
        public void addTile(int generation, TileList.Tile<T> tile) {
            if (!isRequestedGeneration(generation)) {
                if (DEBUG) {
                    log("recycling an older generation tile @%d", tile.mStartPosition);
                }
                mBackgroundProxy.recycleTile(tile);
                return;
            }
            TileList.Tile<T> duplicate = mTileList.addOrReplace(tile);
            if (duplicate != null) {
                Log.e(TAG, "duplicate tile @" + duplicate.mStartPosition);
                mBackgroundProxy.recycleTile(duplicate);
            }
            if (DEBUG) {
                log("gen #%d, added tile @%d, total tiles: %d",
                        generation, tile.mStartPosition, mTileList.size());
            }
            int endPosition = tile.mStartPosition + tile.mItemCount;
            int index = 0;
            while (index < mMissingPositions.size()) {
                final int position = mMissingPositions.keyAt(index);
                if (tile.mStartPosition <= position && position < endPosition) {
                    mMissingPositions.removeAt(index);
                    mViewCallback.onItemLoaded(position);
                } else {
                    index++;
                }
            }
        }

        @Override
        public void removeTile(int generation, int position) {
            if (!isRequestedGeneration(generation)) {
                return;
            }
            TileList.Tile<T> tile = mTileList.removeAtPos(position);
            if (tile == null) {
                Log.e(TAG, "tile not found @" + position);
                return;
            }
            if (DEBUG) {
                log("recycling tile @%d, total tiles: %d", tile.mStartPosition, mTileList.size());
            }
            mBackgroundProxy.recycleTile(tile);
        }

        private void recycleAllTiles() {
            if (DEBUG) {
                log("recycling all %d tiles", mTileList.size());
            }
            for (int i = 0; i < mTileList.size(); i++) {
                mBackgroundProxy.recycleTile(mTileList.getAtIndex(i));
            }
            mTileList.clear();
        }

        private boolean isRequestedGeneration(int generation) {
            return generation == mRequestedGeneration;
        }
    };

    private final ThreadUtil.BackgroundCallback<T>
            mBackgroundCallback = new ThreadUtil.BackgroundCallback<T>() {

        private TileList.Tile<T> mRecycledRoot;

        final SparseBooleanArray mLoadedTiles = new SparseBooleanArray();

        private int mGeneration;
        private int mItemCount;

        private int mFirstRequiredTileStart;
        private int mLastRequiredTileStart;

        @Override
        public void refresh(int generation) {
            mGeneration = generation;
            mLoadedTiles.clear();
            mItemCount = mDataCallback.refreshData();
            mMainThreadProxy.updateItemCount(mGeneration, mItemCount);
        }

        @Override
        public void updateRange(int rangeStart, int rangeEnd, int extRangeStart, int extRangeEnd,
                int scrollHint) {
            if (DEBUG) {
                log("updateRange: %d..%d extended to %d..%d, scroll hint: %d",
                        rangeStart, rangeEnd, extRangeStart, extRangeEnd, scrollHint);
            }

            if (rangeStart > rangeEnd) {
                return;
            }

            final int firstVisibleTileStart = getTileStart(rangeStart);
            final int lastVisibleTileStart = getTileStart(rangeEnd);

            mFirstRequiredTileStart = getTileStart(extRangeStart);
            mLastRequiredTileStart = getTileStart(extRangeEnd);
            if (DEBUG) {
                log("requesting tile range: %d..%d",
                        mFirstRequiredTileStart, mLastRequiredTileStart);
            }

            // All pending tile requests are removed by ThreadUtil at this point.
            // Re-request all required tiles in the most optimal order.
            if (scrollHint == ViewCallback.HINT_SCROLL_DESC) {
                requestTiles(mFirstRequiredTileStart, lastVisibleTileStart, scrollHint, true);
                requestTiles(lastVisibleTileStart + mTileSize, mLastRequiredTileStart, scrollHint,
                        false);
            } else {
                requestTiles(firstVisibleTileStart, mLastRequiredTileStart, scrollHint, false);
                requestTiles(mFirstRequiredTileStart, firstVisibleTileStart - mTileSize, scrollHint,
                        true);
            }
        }

        private int getTileStart(int position) {
            return position - position % mTileSize;
        }

        private void requestTiles(int firstTileStart, int lastTileStart, int scrollHint,
                                  boolean backwards) {
            for (int i = firstTileStart; i <= lastTileStart; i += mTileSize) {
                int tileStart = backwards ? (lastTileStart + firstTileStart - i) : i;
                if (DEBUG) {
                    log("requesting tile @%d", tileStart);
                }
                mBackgroundProxy.loadTile(tileStart, scrollHint);
            }
        }

        @Override
        public void loadTile(int position, int scrollHint) {
            if (isTileLoaded(position)) {
                if (DEBUG) {
                    log("already loaded tile @%d", position);
                }
                return;
            }
            TileList.Tile<T> tile = acquireTile();
            tile.mStartPosition = position;
            tile.mItemCount = Math.min(mTileSize, mItemCount - tile.mStartPosition);
            mDataCallback.fillData(tile.mItems, tile.mStartPosition, tile.mItemCount);
            flushTileCache(scrollHint);
            addTile(tile);
        }

        @Override
        public void recycleTile(TileList.Tile<T> tile) {
            if (DEBUG) {
                log("recycling tile @%d", tile.mStartPosition);
            }
            mDataCallback.recycleData(tile.mItems, tile.mItemCount);

            tile.mNext = mRecycledRoot;
            mRecycledRoot = tile;
        }

        private TileList.Tile<T> acquireTile() {
            if (mRecycledRoot != null) {
                TileList.Tile<T> result = mRecycledRoot;
                mRecycledRoot = mRecycledRoot.mNext;
                return result;
            }
            return new TileList.Tile<T>(mTClass, mTileSize);
        }

        private boolean isTileLoaded(int position) {
            return mLoadedTiles.get(position);
        }

        private void addTile(TileList.Tile<T> tile) {
            mLoadedTiles.put(tile.mStartPosition, true);
            mMainThreadProxy.addTile(mGeneration, tile);
            if (DEBUG) {
                log("loaded tile @%d, total tiles: %d", tile.mStartPosition, mLoadedTiles.size());
            }
        }

        private void removeTile(int position) {
            mLoadedTiles.delete(position);
            mMainThreadProxy.removeTile(mGeneration, position);
            if (DEBUG) {
                log("flushed tile @%d, total tiles: %s", position, mLoadedTiles.size());
            }
        }

        private void flushTileCache(int scrollHint) {
            final int cacheSizeLimit = mDataCallback.getMaxCachedTiles();
            while (mLoadedTiles.size() >= cacheSizeLimit) {
                int firstLoadedTileStart = mLoadedTiles.keyAt(0);
                int lastLoadedTileStart = mLoadedTiles.keyAt(mLoadedTiles.size() - 1);
                int startMargin = mFirstRequiredTileStart - firstLoadedTileStart;
                int endMargin = lastLoadedTileStart - mLastRequiredTileStart;
                if (startMargin > 0 && (startMargin >= endMargin ||
                        (scrollHint == ViewCallback.HINT_SCROLL_ASC))) {
                    removeTile(firstLoadedTileStart);
                } else if (endMargin > 0 && (startMargin < endMargin ||
                        (scrollHint == ViewCallback.HINT_SCROLL_DESC))){
                    removeTile(lastLoadedTileStart);
                } else {
                    // Could not flush on either side, bail out.
                    return;
                }
            }
        }

        private void log(String s, Object... args) {
            Log.d(TAG, "[BKGR] " + String.format(s, args));
        }
    };

    /**
     * The callback that provides data access for {@link AsyncListUtil}.
     *
     * <p>
     * All methods are called on the background thread.
     */
    public static abstract class DataCallback<T> {

        /**
         * Refresh the data set and return the new data item count.
         *
         * <p>
         * If the data is being accessed through {@link android.database.Cursor} this is where
         * the new cursor should be created.
         *
         * @return Data item count.
         */
        @WorkerThread
        public abstract int refreshData();

        /**
         * Fill the given tile.
         *
         * <p>
         * The provided tile might be a recycled tile, in which case it will already have objects.
         * It is suggested to re-use these objects if possible in your use case.
         *
         * @param startPosition The start position in the list.
         * @param itemCount The data item count.
         * @param data The data item array to fill into. Should not be accessed beyond
         *             <code>itemCount</code>.
         */
        @WorkerThread
        public abstract void fillData(T[] data, int startPosition, int itemCount);

        /**
         * Recycle the objects created in {@link #fillData} if necessary.
         *
         *
         * @param data Array of data items. Should not be accessed beyond <code>itemCount</code>.
         * @param itemCount The data item count.
         */
        @WorkerThread
        public void recycleData(T[] data, int itemCount) {
        }

        /**
         * Returns tile cache size limit (in tiles).
         *
         * <p>
         * The actual number of cached tiles will be the maximum of this value and the number of
         * tiles that is required to cover the range returned by
         * {@link ViewCallback#extendRangeInto(int[], int[], int)}.
         * <p>
         * For example, if this method returns 10, and the most
         * recent call to {@link ViewCallback#extendRangeInto(int[], int[], int)} returned
         * {100, 179}, and the tile size is 5, then the maximum number of cached tiles will be 16.
         * <p>
         * However, if the tile size is 20, then the maximum number of cached tiles will be 10.
         * <p>
         * The default implementation returns 10.
         *
         * @return Maximum cache size.
         */
        @WorkerThread
        public int getMaxCachedTiles() {
            return 10;
        }
    }

    /**
     * The callback that links {@link AsyncListUtil} with the list view.
     *
     * <p>
     * All methods are called on the main thread.
          */
    public static abstract class ViewCallback {

        /**
         * No scroll direction hint available.
         */
        public static final int HINT_SCROLL_NONE = 0;

        /**
         * Scrolling in descending order (from higher to lower positions in the order of the backing
         * storage).
         */
        public static final int HINT_SCROLL_DESC = 1;

        /**
         * Scrolling in ascending order (from lower to higher positions in the order of the backing
         * storage).
         */
        public static final int HINT_SCROLL_ASC = 2;

        /**
         * Compute the range of visible item positions.
         * <p>
         * outRange[0] is the position of the first visible item (in the order of the backing
         * storage).
         * <p>
         * outRange[1] is the position of the last visible item (in the order of the backing
         * storage).
         * <p>
         * Negative positions and positions greater or equal to {@link #getItemCount} are invalid.
         * If the returned range contains invalid positions it is ignored (no item will be loaded).
         *
         * @param outRange The visible item range.
         */
        @UiThread
        public abstract void getItemRangeInto(int[] outRange);

        /**
         * Compute a wider range of items that will be loaded for smoother scrolling.
         *
         * <p>
         * If there is no scroll hint, the default implementation extends the visible range by half
         * its length in both directions. If there is a scroll hint, the range is extended by
         * its full length in the scroll direction, and by half in the other direction.
         * <p>
         * For example, if <code>range</code> is <code>{100, 200}</code> and <code>scrollHint</code>
         * is {@link #HINT_SCROLL_ASC}, then <code>outRange</code> will be <code>{50, 300}</code>.
         * <p>
         * However, if <code>scrollHint</code> is {@link #HINT_SCROLL_NONE}, then
         * <code>outRange</code> will be <code>{50, 250}</code>
         *
         * @param range Visible item range.
         * @param outRange Extended range.
         * @param scrollHint The scroll direction hint.
         */
        @UiThread
        public void extendRangeInto(int[] range, int[] outRange, int scrollHint) {
            final int fullRange = range[1] - range[0] + 1;
            final int halfRange = fullRange / 2;
            outRange[0] = range[0] - (scrollHint == HINT_SCROLL_DESC ? fullRange : halfRange);
            outRange[1] = range[1] + (scrollHint == HINT_SCROLL_ASC ? fullRange : halfRange);
        }

        /**
         * Called when the entire data set has changed.
         */
        @UiThread
        public abstract void onDataRefresh();

        /**
         * Called when an item at the given position is loaded.
         * @param position Item position.
         */
        @UiThread
        public abstract void onItemLoaded(int position);
    }
}

基于Android官方AsyncListUtil优化改进RecyclerView分页加载机制(一)

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