正确的在ViewPager中使用Bitmap图像

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在实现image gallery的详情查看时使用swipe view模式是很好的选择,你可以使用ViewPager和对应的PagerAdapter来实现这个模式。不过对于adapter你还有更好的选择:FragmentStatePagerAdapter,当屏幕关闭的时候这个子类可以自动的销毁和保存ViewPager的Fragments状态,以节省内存。

注意:如果你只需要使用很少的图片,并确定其不会超出程序内存的限制的话,使用PagerAdapter或者FragmentPagerAdapter对于你来说是最适合的。

下面是一个ViewPager的实现,他里面包含了数个ImageView。在main activity中使用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);
        }
    }
}

下面是使用Fragment和ImageView的一个实现,看上去是十分合理的,你能发现它的缺点吗?并且如何进行改进?

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
    }
}

希望你已经注意到了这个问题:所有的图片都是在UI线程中处理的,这样将导致程序挂掉,强制退出。这时需要使用AsyncTask(在“Processing Bitmaps Off the UI Thread”这一篇文章中介绍过的)来处理,直接将图片的加载和处理放在后台线程中去做。

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);
        }
    }
}

任何多余的处理(比如调整图像大小,从网络获取图片)都移到BitmapWorkerTask中,这样就不会影响到UI主线程。如果后台线程除了在硬盘中直接读取图片之外还有其他处理,在内存或硬盘中加入一个缓存是十分有用的(在“Caching Bitmaps”这篇文章中有介绍)。下面是一个内存缓存的实现:

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
}

现在将上面的代码片段整理一下,就是一个完美的ViewPager,它实现了最小化的图片资源加载延迟,可以在后台线程中实现尽量多或尽量少的处理(根据你的需求)。

下面一篇文章会介绍如何在GridView中做类似的处理。

翻译自:Displaying Bitmaps in Your UI

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