【Android Developers Training】 60. 在你的UI中显示位图

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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一）. 实现向一个ViewPager加载位图

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）。下面是内存缓存的修改代码：

public class ImageDetailActivity extends FragmentActivity {
    ...
    private LruCache<String, Bitmap> 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