注：本文翻译自Google官方的Android Developers Training文档，译者技术一般，由于喜爱安卓而产生了翻译的念头，纯属个人兴趣爱好。

原文链接：http://developer.android.com/training/displaying-bitmaps/display-bitmap.html

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这节课程将结合之前所有课程所学习的知识，向你展示如何使用后台线程和位图缓存，在ViewPager和GridView中展示多幅图片，并解决并发和配置变更的问题。

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该滑动视图模式（swipe view pattern）用来作为图库的阅览室再好不过的了。你可以通过PagerAdapter来支持ViewPager实现这个阅览模式。然而， 一个更合适的支持适配器是其子类FragmentStatePagerAdapter，它自动销毁ViewPager中的Fragments，并保存其状态（当它从屏幕消失时），从而保持内存使用不会太高。

Note：

如果你有更小数量的图片，并确信他们能满足应用的内存限制，那么使用平常的PagerAdapter或FragmentPagerAdapter会更合适一些。

下面是一个具有ImageView的ViewPager实现。主activity持有ViewPager和它的适配器：

public class ImageDetailActivity extends FragmentActivity {
public static final String EXTRA_IMAGE = "extra_image";

private ImagePagerAdapter mAdapter;  
private ViewPager mPager;

// A static dataset to back the ViewPager adapter  
public final static Integer\[\] imageResIds = new Integer\[\] {  
        R.drawable.sample\_image\_1, R.drawable.sample\_image\_2, R.drawable.sample\_image\_3,  
        R.drawable.sample\_image\_4, R.drawable.sample\_image\_5, R.drawable.sample\_image\_6,  
        R.drawable.sample\_image\_7, R.drawable.sample\_image\_8, R.drawable.sample\_image\_9};

@Override  
public void onCreate(Bundle savedInstanceState) {  
    super.onCreate(savedInstanceState);  
    setContentView(R.layout.image\_detail\_pager); // Contains just a ViewPager

    mAdapter = new ImagePagerAdapter(getSupportFragmentManager(), imageResIds.length);  
    mPager = (ViewPager) findViewById(R.id.pager);  
    mPager.setAdapter(mAdapter);  
}

public static class ImagePagerAdapter extends FragmentStatePagerAdapter {  
    private final int mSize;

    public ImagePagerAdapter(FragmentManager fm, int size) {  
        super(fm);  
        mSize = size;  
    }

    @Override  
    public int getCount() {  
        return mSize;  
    }

    @Override  
    public Fragment getItem(int position) {  
        return ImageDetailFragment.newInstance(position);  
    }  
}  

}

之后是详细Fragment的实现，它持有子ImageView。这看起来像是一个完美的实现，但是从中你能看出缺陷来吗？有什么办法可以改进它？

public class ImageDetailFragment extends Fragment {
private static final String IMAGE_DATA_EXTRA = "resId";
private int mImageNum;
private ImageView mImageView;

static ImageDetailFragment newInstance(int imageNum) {  
    final ImageDetailFragment f = new ImageDetailFragment();  
    final Bundle args = new Bundle();  
    args.putInt(IMAGE\_DATA\_EXTRA, imageNum);  
    f.setArguments(args);  
    return f;  
}

// Empty constructor, required as per Fragment docs  
public ImageDetailFragment() {}

@Override  
public void onCreate(Bundle savedInstanceState) {  
    super.onCreate(savedInstanceState);  
    mImageNum = getArguments() != null ? getArguments().getInt(IMAGE\_DATA\_EXTRA) : -1;  
}

@Override  
public View onCreateView(LayoutInflater inflater, ViewGroup container,  
        Bundle savedInstanceState) {  
    // image\_detail\_fragment.xml contains just an ImageView  
    final View v = inflater.inflate(R.layout.image\_detail\_fragment, container, false);  
    mImageView = (ImageView) v.findViewById(R.id.imageView);  
    return v;  
}

@Override  
public void onActivityCreated(Bundle savedInstanceState) {  
    super.onActivityCreated(savedInstanceState);  
    final int resId = ImageDetailActivity.imageResIds\[mImageNum\];  
    mImageView.setImageResource(resId); // Load image into ImageView  
}  

}

很好，你发现了问题所在：图片时在UI线程上进行读取的，因此可能会导致应用停止响应从而崩溃。使用一个在Processing Bitmaps Off the UI Thread（博客链接：http://www.cnblogs.com/jdneo/p/3521195.html）中说的AsyncTask，它直接将图片加载和处理移动到后台线程中：

public class ImageDetailActivity extends FragmentActivity {
…

public void loadBitmap(int resId, ImageView imageView) {  
    mImageView.setImageResource(R.drawable.image\_placeholder);  
    BitmapWorkerTask task = new BitmapWorkerTask(mImageView);  
    task.execute(resId);  
}

... // include BitmapWorkerTask class  

}

public class ImageDetailFragment extends Fragment {
…

@Override  
public void onActivityCreated(Bundle savedInstanceState) {  
    super.onActivityCreated(savedInstanceState);  
    if (ImageDetailActivity.class.isInstance(getActivity())) {  
        final int resId = ImageDetailActivity.imageResIds\[mImageNum\];  
        // Call out to ImageDetailActivity to load the bitmap in a background thread  
        ((ImageDetailActivity) getActivity()).loadBitmap(resId, mImageView);  
    }  
}  

}

任何额外的处理（比如改变尺寸或者从网络获取图片）可以在[BitmapWorkerTask](http://developer.android.com/training/displaying-bitmaps/process-bitmap.html#BitmapWorkerTask)中发生而不会影响到主UI的响应性。如果后台线程所做的不仅仅是直接从磁盘读取图片，那么添加一个内存缓存或磁盘缓存也是有益处的（博客链接：[http://www.cnblogs.com/jdneo/p/3522538.html](http://www.cnblogs.com/jdneo/p/3522538.html)）。下面是内存缓存的修改代码：

public class ImageDetailActivity extends FragmentActivity {
…
private LruCache mMemoryCache;

@Override  
public void onCreate(Bundle savedInstanceState) {  
    ...  
    // initialize LruCache as per Use a Memory Cache section  
}

public void loadBitmap(int resId, ImageView imageView) {  
    final String imageKey = String.valueOf(resId);

    final Bitmap bitmap = mMemoryCache.get(imageKey);  
    if (bitmap != null) {  
        mImageView.setImageBitmap(bitmap);  
    } else {  
        mImageView.setImageResource(R.drawable.image\_placeholder);  
        BitmapWorkerTask task = new BitmapWorkerTask(mImageView);  
        task.execute(resId);  
    }  
}

... // include updated BitmapWorkerTask from Use a Memory Cache section  

}

将所有代码片段放在一起，组成了响应良好的ViewPager实现，它有很小的图片加载延迟，并且有能力在后台根据你的需要处理图片。

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二）. 实现向一个GridView加载位图

grid list building block对于展示图片数据集是很有用的，并且可以通过一个GridView组件来实现。GridView组件允许同一时间在屏幕上显示许多图片，同时还有很多备用图片（当用户上下滑动页面时，备用图片将会被显示）。当实现这个控制类型时，你必须保证UI的流畅性，内存使用可控，并发处理正确（根据GridView回收它的子View）。

我们首先来看一下一个标准的GridView实现，它有一个置于Fragment中的子ImageView。还是像之前一样，请读者思考这一看上去完美的实现是否有进一步提升的空间？

public class ImageGridFragment extends Fragment implements AdapterView.OnItemClickListener {
private ImageAdapter mAdapter;

// A static dataset to back the GridView adapter  
public final static Integer\[\] imageResIds = new Integer\[\] {  
        R.drawable.sample\_image\_1, R.drawable.sample\_image\_2, R.drawable.sample\_image\_3,  
        R.drawable.sample\_image\_4, R.drawable.sample\_image\_5, R.drawable.sample\_image\_6,  
        R.drawable.sample\_image\_7, R.drawable.sample\_image\_8, R.drawable.sample\_image\_9};

// Empty constructor as per Fragment docs  
public ImageGridFragment() {}

@Override  
public void onCreate(Bundle savedInstanceState) {  
    super.onCreate(savedInstanceState);  
    mAdapter = new ImageAdapter(getActivity());  
}

@Override  
public View onCreateView(  
        LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {  
    final View v = inflater.inflate(R.layout.image\_grid\_fragment, container, false);  
    final GridView mGridView = (GridView) v.findViewById(R.id.gridView);  
    mGridView.setAdapter(mAdapter);  
    mGridView.setOnItemClickListener(this);  
    return v;  
}

@Override  
public void onItemClick(AdapterView<?> parent, View v, int position, long id) {  
    final Intent i = new Intent(getActivity(), ImageDetailActivity.class);  
    i.putExtra(ImageDetailActivity.EXTRA\_IMAGE, position);  
    startActivity(i);  
}

private class ImageAdapter extends BaseAdapter {  
    private final Context mContext;

    public ImageAdapter(Context context) {  
        super();  
        mContext = context;  
    }

    @Override  
    public int getCount() {  
        return imageResIds.length;  
    }

    @Override  
    public Object getItem(int position) {  
        return imageResIds\[position\];  
    }

    @Override  
    public long getItemId(int position) {  
        return position;  
    }

    @Override  
    public View getView(int position, View convertView, ViewGroup container) {  
        ImageView imageView;  
        if (convertView == null) { // if it's not recycled, initialize some attributes  
            imageView = new ImageView(mContext);  
            imageView.setScaleType(ImageView.ScaleType.CENTER\_CROP);  
            imageView.setLayoutParams(new GridView.LayoutParams(  
                    LayoutParams.MATCH\_PARENT, LayoutParams.MATCH\_PARENT));  
        } else {  
            imageView = (ImageView) convertView;  
        }  
        imageView.setImageResource(imageResIds\[position\]); // Load image into ImageView  
        return imageView;  
    }  
}  

}

再一次地，这个实现的问题在于图片设置是在UI线程中执行的。虽然这样做对于简单，尺寸小的图片来说没有问题（当然还和系统加载和缓存的资源有关），如果还有其他额外的处理要做，那么你的UI会戛然而止。

之前章节的异步处理和缓存的方法可以在这里实现。然而，你也需要警惕并发的问题，因为GridView会回收它的子View。要处理这个问题，使用在Processing Bitmaps Off the UI Thread课程（博客链接：http://www.cnblogs.com/jdneo/p/3521195.html）中所讲的知识。下面是升级后的解决方案：

public class ImageGridFragment extends Fragment implements AdapterView.OnItemClickListener {
…

private class ImageAdapter extends BaseAdapter {  
    ...

    @Override  
    public View getView(int position, View convertView, ViewGroup container) {  
        ...  
        loadBitmap(imageResIds\[position\], imageView)  
        return imageView;  
    }  
}

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);  
    }  
}

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();  
    }  
}

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

    if (bitmapWorkerTask != null) {  
        final int bitmapData = bitmapWorkerTask.data;  
        if (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;  
}

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;  
}

... // include updated BitmapWorkerTask class

Note：

同样的代码也可以应用于ListView

这样的实现对于如何处理和加载图片保留了足够的弹性，从而不会引起UI的卡顿。在后台任务中，你可以从网络上读取图片或者将大尺寸的数码照片进行缩放，在任务完成后，图像会显示出来。

要看这系列课程的完整代码样例，请下载http://www.cnblogs.com/jdneo/p/3512517.html中的样例代码