协慌网

我使用ListView来显示与这些图像相关的一些图像和标题。我从互联网上获取图像。有没有办法延迟加载图像，以便文本显示时，UI 不会被锁定，图像会在下载时显示？

图像总数不固定。

这是我创建的用于保存我的应用当前正在显示的图像的内容。请注意，这里使用的 “Log” 对象是围绕 Android 内部最终 Log 类的自定义包装器。

package com.wilson.android.library;

/*
 Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements.  See the NOTICE file
distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you 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.
*/
import java.io.IOException;

public class DrawableManager {
    private final Map<String, Drawable> drawableMap;

    public DrawableManager() {
        drawableMap = new HashMap<String, Drawable>();
    }

    public Drawable fetchDrawable(String urlString) {
        if (drawableMap.containsKey(urlString)) {
            return drawableMap.get(urlString);
        }

        Log.d(this.getClass().getSimpleName(), "image url:" + urlString);
        try {
            InputStream is = fetch(urlString);
            Drawable drawable = Drawable.createFromStream(is, "src");


            if (drawable != null) {
                drawableMap.put(urlString, drawable);
                Log.d(this.getClass().getSimpleName(), "got a thumbnail drawable: " + drawable.getBounds() + ", "
                        + drawable.getIntrinsicHeight() + "," + drawable.getIntrinsicWidth() + ", "
                        + drawable.getMinimumHeight() + "," + drawable.getMinimumWidth());
            } else {
              Log.w(this.getClass().getSimpleName(), "could not get thumbnail");
            }

            return drawable;
        } catch (MalformedURLException e) {
            Log.e(this.getClass().getSimpleName(), "fetchDrawable failed", e);
            return null;
        } catch (IOException e) {
            Log.e(this.getClass().getSimpleName(), "fetchDrawable failed", e);
            return null;
        }
    }

    public void fetchDrawableOnThread(final String urlString, final ImageView imageView) {
        if (drawableMap.containsKey(urlString)) {
            imageView.setImageDrawable(drawableMap.get(urlString));
        }

        final Handler handler = new Handler() {
            @Override
            public void handleMessage(Message message) {
                imageView.setImageDrawable((Drawable) message.obj);
            }
        };

        Thread thread = new Thread() {
            @Override
            public void run() {
                //TODO : set imageView to a "pending" image
                Drawable drawable = fetchDrawable(urlString);
                Message message = handler.obtainMessage(1, drawable);
                handler.sendMessage(message);
            }
        };
        thread.start();
    }

    private InputStream fetch(String urlString) throws MalformedURLException, IOException {
        DefaultHttpClient httpClient = new DefaultHttpClient();
        HttpGet request = new HttpGet(urlString);
        HttpResponse response = httpClient.execute(request);
        return response.getEntity().getContent();
    }
}

我用图像做了一个懒惰列表 （位于 GitHub） 的简单演示 。这可能对某人有帮助。它在后台线程中下载图像。图像正在缓存在 SD 卡和内存中。缓存实现非常简单，仅适用于演示。我用 inSampleSize 解码图像以减少内存消耗。我也尝试正确处理回收的视图。

我推荐开源仪器Universal Image Loader 。它最初基于 Fedor Vlasov 的项目LazyList ，并从那时起大大改进。

• 多线程图像加载
• 可以广泛调整 ImageLoader 的配置（线程执行器，下载器，解码器，内存和光盘缓存，显示图像选项等）
• 在内存和 / 或设备的文件系统（或 SD 卡）上进行图像缓存的可能性
• 可以 “监听” 加载过程
• 可以使用单独的选项自定义每个显示图像调用
• 小工具支持
• Android 2.0 + 支持