android列表下載圖片listview緩存
2021-09-16 11:48:36

在使用listview下載圖片的時候，經常遇到圖片錯亂問題，快速滑動會導致圖片位置不對，這里提供一種思路

實現效果圖：

（1）activity_main.xml

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<ListView
    android:id="@+id/list_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

（2）images_item.xml

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<ImageView
    android:id="@+id/image"
    android:layout_width="match_parent"
    android:layout_height="120dp"
    android:src="@mipmap/empty_photo"
    android:scaleType="fitXY"/>

（3）在清單文件AndroidManifest.xml里添加聯(lián)網權限

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<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

（4）創(chuàng)建一個圖片類Images，用于放置圖片數組

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public class Images {

    public final static String[] imageUrls = new String[] {
            "https://img-my.csdn.net/uploads/201508/05/1438760758_3497.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760758_6667.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760757_3588.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760756_3304.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760755_6715.jpeg",
            "https://img-my.csdn.net/uploads/201508/05/1438760726_5120.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760726_8364.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760725_4031.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760724_9463.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760724_2371.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760707_4653.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760706_6864.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760706_9279.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760704_2341.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760704_5707.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760685_5091.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760685_4444.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760684_8827.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760683_3691.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760683_7315.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760663_7318.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760662_3454.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760662_5113.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760661_3305.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760661_7416.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760589_2946.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760589_1100.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760588_8297.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760587_2575.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760587_8906.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760550_2875.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760550_9517.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760549_7093.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760549_1352.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760548_2780.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760531_1776.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760531_1380.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760530_4944.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760530_5750.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760529_3289.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760500_7871.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760500_6063.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760499_6304.jpeg",
            "https://img-my.csdn.net/uploads/201508/05/1438760499_5081.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760498_7007.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760478_3128.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760478_6766.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760477_1358.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760477_3540.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760476_1240.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760446_7993.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760446_3641.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760445_3283.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760444_8623.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760444_6822.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760422_2224.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760421_2824.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760420_2660.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760420_7188.jpg",
            "https://img-my.csdn.net/uploads/201508/05/1438760419_4123.jpg",
    };
}

（5）在MainActivity里面引用適配器，設置顯示的內容

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public class MainActivity extends AppCompatActivity {
    private ListView listView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        setContentView(R.layout.activity_main);
        listView = (ListView) findViewById(R.id.list_view);
        ImageAdapter adapter = new ImageAdapter(this, 0, Images.imageUrls);
        listView.setAdapter(adapter);
    }
}

（6）適配器ImageAdapter

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public class ImageAdapter extends ArrayAdapter<String> {

    private ListView mListView;

    private Bitmap mLoadingBitmap;

    /**
     * 圖片緩存技術的核心類，用于緩存所有下載好的圖片，在程序內存達到設定值時會將最少最近使用的圖片移除掉。
     */
    private LruCache<String, BitmapDrawable> mMemoryCache;

    public ImageAdapter(Context context, int resource, String[] objects) {
        super(context, resource, objects);
        mLoadingBitmap = BitmapFactory.decodeResource(context.getResources(),
                R.mipmap.empty_photo);
        // 獲取應用程序最大可用內存
        int maxMemory = (int) Runtime.getRuntime().maxMemory();
        int cacheSize = maxMemory / 8;
        mMemoryCache = new LruCache<String, BitmapDrawable>(cacheSize) {
            @Override
            protected int sizeOf(String key, BitmapDrawable drawable) {
                return drawable.getBitmap().getByteCount();
            }
        };
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        if (mListView == null) {
            mListView = (ListView) parent;
        }
        String url = getItem(position);
        View view;
        if (convertView == null) {
            view = LayoutInflater.from(getContext()).inflate(R.layout.images_item, null);
        } else {
            view = convertView;
        }
        ImageView image = (ImageView) view.findViewById(R.id.image);
        BitmapDrawable drawable = getBitmapFromMemoryCache(url);
        if (drawable != null) {
            image.setImageDrawable(drawable);
        } else if (cancelPotentialWork(url, image)) {
            BitmapWorkerTask task = new BitmapWorkerTask(image);
            AsyncDrawable asyncDrawable = new AsyncDrawable(getContext()
                    .getResources(), mLoadingBitmap, task);
            image.setImageDrawable(asyncDrawable);
            task.execute(url);
        }
        return view;
    }

    /**
     * 自定義的一個Drawable，讓這個Drawable持有BitmapWorkerTask的弱引用。
     */
    class AsyncDrawable extends BitmapDrawable {

        private WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference;

        public AsyncDrawable(Resources res, Bitmap bitmap,
                             BitmapWorkerTask bitmapWorkerTask) {
            super(res, bitmap);
            bitmapWorkerTaskReference = new WeakReference<>(
                    bitmapWorkerTask);
        }

        public BitmapWorkerTask getBitmapWorkerTask() {
            return bitmapWorkerTaskReference.get();
        }

    }

    /**
     * 獲取傳入的ImageView它所對應的BitmapWorkerTask。
     */
    private BitmapWorkerTask getBitmapWorkerTask(ImageView imageView) {
        if (imageView != null) {
            Drawable drawable = imageView.getDrawable();
            if (drawable instanceof AsyncDrawable) {
                AsyncDrawable asyncDrawable = (AsyncDrawable) drawable;
                return asyncDrawable.getBitmapWorkerTask();
            }
        }
        return null;
    }

    /**
     * 取消掉后臺的潛在任務，當認為當前ImageView存在著一個另外圖片請求任務時
     * ，則把它取消掉并返回true，否則返回false。
     */
    public boolean cancelPotentialWork(String url, ImageView imageView) {
        BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);
        if (bitmapWorkerTask != null) {
            String imageUrl = bitmapWorkerTask.imageUrl;
            if (imageUrl == null || !imageUrl.equals(url)) {
                bitmapWorkerTask.cancel(true);
            } else {
                return false;
            }
        }
        return true;
    }

    /**
     * 將一張圖片存儲到LruCache中。
     *
     * @param key
     *            LruCache的鍵，這里傳入圖片的URL地址。
     * @param drawable
     *            LruCache的值，這里傳入從網絡上下載的BitmapDrawable對象。
     */
    public void addBitmapToMemoryCache(String key, BitmapDrawable drawable) {
        if (getBitmapFromMemoryCache(key) == null) {
            mMemoryCache.put(key, drawable);
        }
    }

    /**
     * 從LruCache中獲取一張圖片，如果不存在就返回null。
     *
     * @param key
     *            LruCache的鍵，這里傳入圖片的URL地址。
     * @return 對應傳入鍵的BitmapDrawable對象，或者null。
     */
    public BitmapDrawable getBitmapFromMemoryCache(String key) {
        return mMemoryCache.get(key);
    }

    /**
     * 異步下載圖片的任務。
     *
     * @author guolin
     */
    class BitmapWorkerTask extends AsyncTask<String, Void, BitmapDrawable> {

        String imageUrl;

        private WeakReference<ImageView> imageViewReference;

        public BitmapWorkerTask(ImageView imageView) {
            imageViewReference = new WeakReference<>(imageView);
        }

        @Override
        protected BitmapDrawable doInBackground(String... params) {
            imageUrl = params[0];
            // 在后臺開始下載圖片
            Bitmap bitmap = downloadBitmap(imageUrl);
            BitmapDrawable drawable = new BitmapDrawable(getContext().getResources(), bitmap);
            addBitmapToMemoryCache(imageUrl, drawable);
            return drawable;
        }

        @Override
        protected void onPostExecute(BitmapDrawable drawable) {
            ImageView imageView = getAttachedImageView();
            if (imageView != null && drawable != null) {
                imageView.setImageDrawable(drawable);
            }
        }

        /**
         * 獲取當前BitmapWorkerTask所關聯(lián)的ImageView。
         */
        private ImageView getAttachedImageView() {
            ImageView imageView = imageViewReference.get();
            BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);
            if (this == bitmapWorkerTask) {
                return imageView;
            }
            return null;
        }

        /**
         * 建立HTTP請求，并獲取Bitmap對象。
         *
         * @param imageUrl
         *            圖片的URL地址
         * @return 解析后的Bitmap對象
         */
        private Bitmap downloadBitmap(String imageUrl) {
            Bitmap bitmap = null;
            HttpURLConnection con = null;
            try {
                URL url = new URL(imageUrl);
                con = (HttpURLConnection) url.openConnection();
                con.setConnectTimeout(5 * 1000);
                con.setReadTimeout(10 * 1000);
                bitmap = BitmapFactory.decodeStream(con.getInputStream());
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                if (con != null) {
                    con.disconnect();
                }
            }
            return bitmap;
        }

    }

}

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