LruCache为GridView异步加载大量网络图片

MainActivity如下:

import android.os.Bundle;
import android.widget.GridView;
import android.app.Activity;
/**
 * Demo描述:
 * 在GridView中采用LruCache异步加载大量图片,避免OOM
 *
 * 学习资料:
 * http://blog.csdn.net/guolin_blog/article/details/9526203
 * Thank you very much
 */

public class MainActivity extends Activity {
    private GridView mGridView;
    private GridViewAdapter mGridViewAdapter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        init();
    }

    private void init(){
        mGridView = (GridView) findViewById(R.id.gridView);
        mGridViewAdapter = new GridViewAdapter(this, 0, ImagesUrl.Urls, mGridView);
        mGridView.setAdapter(mGridViewAdapter);
    }

    //取消所有的下载任务
    @Override
    protected void onDestroy() {
        super.onDestroy();
        mGridViewAdapter.cancelAllTasks();
    }

GridViewAdapter如下:

import java.net.HttpURLConnection;
import java.net.URL;
import java.util.HashSet;
import android.annotation.SuppressLint;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.support.v4.util.LruCache;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AbsListView;
import android.widget.AbsListView.OnScrollListener;
import android.widget.ArrayAdapter;
import android.widget.GridView;
import android.widget.ImageView;
/**
 * LruCache的流程分析:
 * 我们从第一次进入应用的情况下开始
 * 1 依据图片的Url从LruCache缓存中取图片.
 *   若图片存在缓存中,则显示该图片;否则显示默认图片
 * 2 因为是第一次进入该界面所以会执行:
 *   loadBitmaps(firstVisibleItem, visibleItemCount);
 *   我们从loadBitmaps()方法作为切入点,继续往下梳理
 * 3 尝试从LruCache缓存中取图片.如果在显示即可,否则进入4
 * 4 开启一个异步任务下载图片.下载完成后显示图片,并且将
 *   该图片存入LruCache缓存中
 *
 * 在停止滑动时,会调用loadBitmaps(firstVisibleItem, visibleItemCount)
 * 情况与上类似
 */
@SuppressLint("NewApi")
public class GridViewAdapter extends ArrayAdapter<String> {

    private GridView mGridView;
    //图片缓存类
    private LruCache<String, Bitmap> mLruCache;
    //记录所有正在下载或等待下载的任务
    private HashSet<DownloadBitmapAsyncTask> mDownloadBitmapAsyncTaskHashSet;
    //GridView中可见的第一张图片的下标
    private int mFirstVisibleItem;
    //GridView中可见的图片的数量
    private int mVisibleItemCount;
    //记录是否是第一次进入该界面
    private boolean isFirstEnterThisActivity = true;

    public GridViewAdapter(Context context, int textViewResourceId,String[] objects, GridView gridView) {
        super(context, textViewResourceId, objects);

        mGridView = gridView;
        mGridView.setOnScrollListener(new ScrollListenerImpl());

        mDownloadBitmapAsyncTaskHashSet = new HashSet<DownloadBitmapAsyncTask>();

        // 获取应用程序最大可用内存
        int maxMemory = (int) Runtime.getRuntime().maxMemory();
        // 设置图片缓存大小为maxMemory的1/6
        int cacheSize = maxMemory/6;

        mLruCache = new LruCache<String, Bitmap>(cacheSize) {
            @Override
            protected int sizeOf(String key, Bitmap bitmap) {
                return bitmap.getByteCount();
            }
        };

    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        String url = getItem(position);
        View view;
        if (convertView == null) {
            view = LayoutInflater.from(getContext()).inflate(R.layout.gridview_item, null);
        } else {
            view = convertView;
        }
        ImageView imageView = (ImageView) view.findViewById(R.id.imageView);
        //为该ImageView设置一个Tag,防止图片错位
        imageView.setTag(url);
        //为该ImageView设置显示的图片
        setImageForImageView(url, imageView);
        return view;
    }

    /**
     * 为ImageView设置图片(Image)
     * 1 从缓存中获取图片
     * 2 若图片不在缓存中则为其设置默认图片
     */
    private void setImageForImageView(String imageUrl, ImageView imageView) {
        Bitmap bitmap = getBitmapFromLruCache(imageUrl);
        if (bitmap != null) {
            imageView.setImageBitmap(bitmap);
        } else {
            imageView.setImageResource(R.drawable.default_image);
        }
    }

    /**
     * 将图片存储到LruCache
     */
    public void addBitmapToLruCache(String key, Bitmap bitmap) {
        if (getBitmapFromLruCache(key) == null) {
            mLruCache.put(key, bitmap);
        }
    }

    /**
     * 从LruCache缓存获取图片
     */
    public Bitmap getBitmapFromLruCache(String key) {
        return mLruCache.get(key);
    }



   /**
    * 为GridView的item加载图片
    *
    * @param firstVisibleItem
    * GridView中可见的第一张图片的下标
    *
    * @param visibleItemCount
    * GridView中可见的图片的数量
    *
    */
    private void loadBitmaps(int firstVisibleItem, int visibleItemCount) {
        try {
            for (int i = firstVisibleItem; i < firstVisibleItem + visibleItemCount; i++) {
                String imageUrl = ImagesUrl.Urls[i];
                Bitmap bitmap = getBitmapFromLruCache(imageUrl);
                if (bitmap == null) {
                    DownloadBitmapAsyncTask downloadBitmapAsyncTask = new DownloadBitmapAsyncTask();
                    mDownloadBitmapAsyncTaskHashSet.add(downloadBitmapAsyncTask);
                    downloadBitmapAsyncTask.execute(imageUrl);
                } else {
                    //依据Tag找到对应的ImageView显示图片
                    ImageView imageView = (ImageView) mGridView.findViewWithTag(imageUrl);
                    if (imageView != null && bitmap != null) {
                        imageView.setImageBitmap(bitmap);
                    }
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * 取消所有正在下载或等待下载的任务
     */
    public void cancelAllTasks() {
        if (mDownloadBitmapAsyncTaskHashSet != null) {
            for (DownloadBitmapAsyncTask task : mDownloadBitmapAsyncTaskHashSet) {
                task.cancel(false);
            }
        }
    }


    private class ScrollListenerImpl implements OnScrollListener{
        /**
         *
         * 我们的本意是通过onScrollStateChanged获知:每次GridView停止滑动时加载图片
         * 但是存在一个特殊情况:
         * 当第一次入应用的时候,此时并没有滑动屏幕的操作即不会调用onScrollStateChanged,但应该加载图片.
         * 所以在此处做一个特殊的处理.
         * 即代码:
         * if (isFirstEnterThisActivity && visibleItemCount > 0) {
         *      loadBitmaps(firstVisibleItem, visibleItemCount);
         *      isFirstEnterThisActivity = false;
         *    }
         *
         * ------------------------------------------------------------
         *
         * 其余的都是正常情况.
         * 所以我们需要不断保存:firstVisibleItem和visibleItemCount
         * 从而便于中在onScrollStateChanged()判断当停止滑动时加载图片
         *
         */
        @Override
        public void onScroll(AbsListView view, int firstVisibleItem,int visibleItemCount, int totalItemCount) {
            mFirstVisibleItem = firstVisibleItem;
            mVisibleItemCount = visibleItemCount;
            if (isFirstEnterThisActivity && visibleItemCount > 0) {
                loadBitmaps(firstVisibleItem, visibleItemCount);
                isFirstEnterThisActivity = false;
            }
        }

        /**
         *  GridView停止滑动时下载图片
         *  其余情况下取消所有正在下载或者等待下载的任务
         */
        @Override
        public void onScrollStateChanged(AbsListView view, int scrollState) {
            if (scrollState == SCROLL_STATE_IDLE) {
                loadBitmaps(mFirstVisibleItem, mVisibleItemCount);
            } else {
                cancelAllTasks();
            }
        }

    }

    /**
     * 下载图片的异步任务
     */
    class DownloadBitmapAsyncTask extends AsyncTask<String, Void, Bitmap> {
        private String imageUrl;
        @Override
        protected Bitmap doInBackground(String... params) {
            imageUrl = params[0];
            Bitmap bitmap = downloadBitmap(params[0]);
            if (bitmap != null) {
                //下载完后,将其缓存到LrcCache
                addBitmapToLruCache(params[0], bitmap);
            }
            return bitmap;
        }

        @Override
        protected void onPostExecute(Bitmap bitmap) {
            super.onPostExecute(bitmap);
            //下载完成后,找到其对应的ImageView显示图片
            ImageView imageView = (ImageView) mGridView.findViewWithTag(imageUrl);
            if (imageView != null && bitmap != null) {
                imageView.setImageBitmap(bitmap);
            }
            mDownloadBitmapAsyncTaskHashSet.remove(this);
        }
    }

    // 获取Bitmap
    private Bitmap downloadBitmap(String imageUrl) {
        Bitmap bitmap = null;
        HttpURLConnection httpURLConnection = null;
        try {
            URL url = new URL(imageUrl);
            httpURLConnection = (HttpURLConnection) url.openConnection();
            httpURLConnection.setConnectTimeout(5 * 1000);
            httpURLConnection.setReadTimeout(10 * 1000);
            httpURLConnection.setDoInput(true);
            httpURLConnection.setDoOutput(true);
            bitmap = BitmapFactory.decodeStream(httpURLConnection.getInputStream());
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (httpURLConnection != null) {
                httpURLConnection.disconnect();
            }
        }
        return bitmap;
    }

}

ImagesUrl如下:

public class ImagesUrl {

    public final static String[] Urls = new String[] {
            "https://lh6.googleusercontent.com/-55osAWw3x0Q/URquUtcFr5I/AAAAAAAAAbs/rWlj1RUKrYI/s160-c/A%252520Photographer.jpg",
            "https://lh4.googleusercontent.com/--dq8niRp7W4/URquVgmXvgI/AAAAAAAAAbs/-gnuLQfNnBA/s160-c/A%252520Song%252520of%252520Ice%252520and%252520Fire.jpg",
            "https://lh5.googleusercontent.com/-7qZeDtRKFKc/URquWZT1gOI/AAAAAAAAAbs/hqWgteyNXsg/s160-c/Another%252520Rockaway%252520Sunset.jpg",
            "https://lh3.googleusercontent.com/--L0Km39l5J8/URquXHGcdNI/AAAAAAAAAbs/3ZrSJNrSomQ/s160-c/Antelope%252520Butte.jpg",
            "https://lh6.googleusercontent.com/-8HO-4vIFnlw/URquZnsFgtI/AAAAAAAAAbs/WT8jViTF7vw/s160-c/Antelope%252520Hallway.jpg",
            "https://lh4.googleusercontent.com/-WIuWgVcU3Qw/URqubRVcj4I/AAAAAAAAAbs/YvbwgGjwdIQ/s160-c/Antelope%252520Walls.jpg",
            "https://lh6.googleusercontent.com/-UBmLbPELvoQ/URqucCdv0kI/AAAAAAAAAbs/IdNhr2VQoQs/s160-c/Apre%2525CC%252580s%252520la%252520Pluie.jpg"｝；

｝