使用AsyncTask加载图片

最新推荐文章于 2017-07-22 11:12:59 发布

翻译最新推荐文章于 2017-07-22 11:12:59 发布 · 446 阅读

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#android

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本文介绍了一种在Android中高效加载图片的方法，通过使用AsyncTask异步加载Bitmap，并结合BitmapDrawable来处理ListView和GridView中的图片复用问题，有效避免了内存泄漏及提高应用性能。

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https://developer.android.com/training/displaying-bitmaps/process-bitmap.html

加载图片基本操作

一、创建AsyncTask子类

将ImageView的弱引用设置为成员变量，创建构造函数传入ImageView对象。
调用指定大小解析Bitmap方法。
因为是弱引用，所以必须判断引用是否被回收。如果异步任务完成前，用户离开Activity或者设置发生改变，ImageView也可能不存在。

class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
    private final WeakReference<ImageView> imageViewReference;
    private int data = 0;

    public BitmapWorkerTask(ImageView imageView) {
        // Use a WeakReference to ensure the ImageView can be garbage collected
        imageViewReference = new WeakReference<ImageView>(imageView);
    }

    // Decode image in background.
    @Override
    protected Bitmap doInBackground(Integer... params) {
        data = params[0];
        return decodeSampledBitmapFromResource(getResources(), data, 100, 100));
    }

    // Once complete, see if ImageView is still around and set bitmap.
    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if (imageViewReference != null && bitmap != null) {
            final ImageView imageView = imageViewReference.get();
            if (imageView != null) {
                imageView.setImageBitmap(bitmap);
            }
        }
    }
}

二、创建异步任务实例对象，开始执行

public void loadBitmap(int resId, ImageView imageView) {
    BitmapWorkerTask task = new BitmapWorkerTask(imageView);
    task.execute(resId);
}

处理并发

因为ListView和GridView复用子布局，无法保证异步任务完成时，相关的布局没有被回收。也无法保证异步任务完成时间的先后顺序。所以必须处理并发事件。

一、创建BitmapDrawable子类

该类专门用来保存Bitmap及其对应的异步任务

static class AsyncDrawable extends BitmapDrawable {
    private final WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference;

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

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

二、绑定AsyncDrawable

创建AsyncDrawable并传入BitmapWorkerTask对象，imageView设置为该AsyncDrawable再开始异步任务

public void loadBitmap(int resId, ImageView imageView) {
    if (cancelPotentialWork(resId, imageView)) {
        final BitmapWorkerTask task = new BitmapWorkerTask(imageView);
        final AsyncDrawable asyncDrawable =
                new AsyncDrawable(getResources(), mPlaceHolderBitmap, task);
        imageView.setImageDrawable(asyncDrawable);
        task.execute(resId);
    }
}

cancelPotentialWork这个方法用于调用方法获取控件对应异步任务，判断是否与当前任务一致

public static boolean cancelPotentialWork(int data, ImageView imageView) {
    final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);

    if (bitmapWorkerTask != null) {
        final int bitmapData = bitmapWorkerTask.data;
        // If bitmapData is not yet set or it differs from the new data
        if (bitmapData == 0 || bitmapData != data) {
            // Cancel previous task
            bitmapWorkerTask.cancel(true);
        } else {
            // The same work is already in progress
            return false;
        }
    }
    // No task associated with the ImageView, or an existing task was cancelled
    return true;
}

这个getBitmapWorkerTask()方法用于获取图片对应异步任务

private static BitmapWorkerTask getBitmapWorkerTask(ImageView imageView) {
   if (imageView != null) {
       final Drawable drawable = imageView.getDrawable();
       if (drawable instanceof AsyncDrawable) {
           final AsyncDrawable asyncDrawable = (AsyncDrawable) drawable;
           return asyncDrawable.getBitmapWorkerTask();
       }
    }
    return null;
}