【Google官方教程】第四课：在UI中显示Bitmap

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http://my.oschina.net/ryanhoo/blog/88484

译者：Ryan Hoo

来源：https://developer.android.com/develop/index.html

译者按： 在Google最新的文档中，提供了一系列含金量至关高的教程。由于种种缘由而不为人知，真是惋惜！Ryan将会细心整理，将之翻译成中文，但愿对开发者有所帮助。

        本系列是Google关于展现大Bitmap（位图）的官方演示，能够有效的解决内存限制，更加有效的加载并显示图片，同时避免让人头疼的OOM（Out Of Memory）。

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译文：

        这节课将咱们前面几节课学习的东西都整合起来，向你展现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，它能够在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：若是你只有少许的图片，而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";

    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);
        }
    }
}

        这里有一个用来持有ImageView并显示详细信息的Fragment的实现类。看起来这彷佛是很是合理的方法，可是你可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
    }
}

        但愿你能注意到：这些图片是在UI线程从资源中读取过来的，而这极有可能致使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)能够放在BitmapWorkerTask中而不会影响到主UI线程的响应性。若是后台线程作的不单单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
}

        将上面的代码片断整合在一块儿会让你的ViewPager具有优良的响应性能，能够实现最小的加载延迟，根据你的图片加载须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很是有用，也可使用GridView组件来实现，若是用户上下滚动的话，有不少图片处于就绪状态，随时能够显示在屏幕上。若是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并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。咱们再一次思考这个问题，这个方法看起来彷佛很是完美且合乎情理，可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;
        }
    }
}

        固然，问题仍是这个方法在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)，若是须要作任何的处理，UI就会被阻塞(甚至引发ANR(Application Not Responding))。

        和前一节相同的处理方式，咱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，你须要谨慎处理并发问题。可使用“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

        注意：相同的代码也能够很好的适配ListView。

        这里的实现方法容许灵活地处理和加载图片，而且不会影响UI的流畅性。在后台线程中，你能够从网络加载图片，调整大幅的数码相机照片的大小，并在处理任务结束的时候将图片显示在UI界面中。

        要了解在本节课中讨论到的概念和完整代码，请参阅示例程序。

        BitmapFun:http://vdisk.weibo.com/s/hNgFB