在UI中显示Bitmap

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本文介绍如何在Android中使用后台线程和Bitmap缓存加载图片到ViewPager和GridView，避免UI阻塞，提升用户体验。

这节课将我们前面几节课学习的东西都整合起来，向你展示如何使用后台线程和Bitmap缓存加载多个Bitmap(位图)到ViewPager和GridView组件中，并学习如何处理并发和配置变化问题。 实现加载Bitmap到ViewPager

滑动浏览模式(Swipe View Pattern)是一种很好的浏览详细图片的方式。你可以使用ViewPager组件配合PagerAdapter(适配器)来实现这种模式。然而，更加合适的适配器是FragmentStatePagerAdapter，它可以在ViewPager退出屏幕的时候自动销毁并存储Fragments的状态，使得内存依然保留下来。

注意：如果你只有少量的图片，并且确信它们不会超出程序的内存限制，使用常规的PagerAdapter或者FragmentPagerAdapter或许更加合适。

这里有一个包含ImageView的ViewPager的实现类，Main Activity(主活动)持有这个ViewPager和Adapter

public class ImageDetailActivity extends FragmentActivity {

 
    public static final String EXTRA_IMAGE = "extra_image";
 

03

 
    private ImagePagerAdapter mAdapter;
 

 
    private ViewPager mPager;
 

06

 
    // 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};
 

12

 
    @Override
 

 
    public void onCreate(Bundle savedInstanceState) {
 

 
        super.onCreate(savedInstanceState);
 

 
        setContentView(R.layout.image_detail_pager); // Contains just a ViewPager
 

17

 
        mAdapter = new ImagePagerAdapter(getSupportFragmentManager(), imageResIds.length);
 

 
        mPager = (ViewPager) findViewById(R.id.pager);
 

 
        mPager.setAdapter(mAdapter);
 

}

22

 
    public static class ImagePagerAdapter extends FragmentStatePagerAdapter {
 

 
        private final int mSize;
 

25

 
        public ImagePagerAdapter(FragmentManager fm, int size) {
 

 
            super(fm);
 

 
            mSize = size;
 

}

30

 
        @Override
 

 
        public int getCount() {
 

 
            return mSize;
 

}

35

 
        @Override
 

 
        public Fragment getItem(int position) {
 

 
            return ImageDetailFragment.newInstance(position);
 

}

}

}

这里有一个用来持有ImageView并显示详细信息的Fragment的实现类。看起来这似乎是非常合理的方法，但是你能否看到这个方案的缺点呢？应该如何改善它呢？

 
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
 
    }
 
}

希望你能注意到：这些图片是在UI线程从资源中读取过来的，而这极有可能导致应用挂起甚至被强制关闭。使用在“非UI线程处理Bitmap”一课中提到的AsyncTask，直接将图片加载和处理移到后台线程中。

任何额外的处理(例如调整大小或者从网络获取图片)可以放在BitmapWorkerTask中而不会影响到主UI线程的响应性。如果后台线程做的不仅仅是直接从硬盘直接加载图片，那么如“缓存Bitmap”一课中说的，将图片缓存到内存或者硬盘是有利于程序优化的。这里是对内存缓存的一些额外修改：

 
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
 
}

将上面的代码片段整合在一起会让你的ViewPager具备优良的响应性能，可以实现最小的加载延迟，根据你的图片加载需要或多或少的进行后台处理。

实现加载Bitmap到GridView

网格列表控件(Grid List Building Block)对于显示图片数据集非常有用，也可以使用GridView组件来实现，如果用户上下滚动的话，有很多图片处于就绪状态，随时可以显示在屏幕上。如果要实现这种类型的控制，你必须确保UI保持流畅性，内存使用处于控制之中而且并发也要被正确地处理(取决于GridView回收子视图的方式)。

首先，这里有一个标准的GridView实现，将ImageView子控件存放在Fragment中。我们再一次思考这个问题，这个方法看起来似乎非常完美且合乎情理，但是有没有办法让它便得更好呢？

 
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;
 
        }
 
    }
 
}

当然，问题还是这个方法在UI线程中处理图片。这种方式或许是和处理小而简单的图片(系统资源的加载和缓存)，如果需要做任何的处理，UI就会被阻塞(甚至引起ANR(Application Not Responding))。

和前一节相同的处理方式，我们在异步线程中进行处理和缓存。然而，考虑到GridView回收子视图的方式，你需要谨慎处理并发问题。可以使用“在非UI线程中处理Bitmap”一课中提到的技巧。这里是更新的解决方案：

 
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