編寫(xiě):kesenhoo - 原文:http://developer.android.com/training/displaying-bitmaps/display-bitmap.html

這一課會(huì)演示如何運(yùn)用前面幾節(jié)課的內(nèi)容，使用后臺(tái)線程與緩存機(jī)制將圖片加載到ViewPager與GridView控件，并且學(xué)習(xí)處理并發(fā)與配置改變問(wèn)題。

實(shí)現(xiàn)加載圖片到ViewPager

Swipe View Pattern是一個(gè)使用滑動(dòng)來(lái)切換顯示不同詳情頁(yè)面的設(shè)計(jì)模型。（關(guān)于這種效果請(qǐng)先參看Android Design: Swipe Views）。我們可以通過(guò)PagerAdapter與ViewPager控件來(lái)實(shí)現(xiàn)這個(gè)效果。 不過(guò)，一個(gè)更加合適的Adapter是PagerAdapter的一個(gè)子類(lèi)，叫做FragmentStatePagerAdapter：它可以在某個(gè)ViewPager中的子視圖切換出屏幕時(shí)自動(dòng)銷(xiāo)毀與保存Fragments的狀態(tài)。這樣能夠保持更少的內(nèi)存消耗。

Note: 如果只有為數(shù)不多的圖片并且確保不會(huì)超出程序內(nèi)存限制，那么使用PagerAdapter或 FragmentPagerAdapter會(huì)更加合適。

下面是一個(gè)使用ViewPager與ImageView作為子視圖的示例。主Activity包含有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);
        }
    }
}

Fragment里面包含了ImageView控件:

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
    }
}

希望你有發(fā)現(xiàn)上面示例存在的問(wèn)題：在UI線程中讀取圖片可能會(huì)導(dǎo)致應(yīng)用無(wú)響應(yīng)。因此使用在第二課中學(xué)習(xí)的AsyncTask會(huì)更好。

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);
        }
    }
}

在BitmapWorkerTask中做一些例如重設(shè)圖片大小，從網(wǎng)絡(luò)拉取圖片的任務(wù)，可以確保不會(huì)阻塞UI線程。如果后臺(tái)線程不僅僅是一個(gè)簡(jiǎn)單的加載操作，增加一個(gè)內(nèi)存緩存或者磁盤(pán)緩存會(huì)比較好（請(qǐng)參考第三課：緩存Bitmap），下面是一些為了內(nèi)存緩存而附加的內(nèi)容：

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
}

把前面學(xué)習(xí)到的所有技巧合并起來(lái)，我們將得到一個(gè)響應(yīng)性良好的ViewPager實(shí)現(xiàn)：它擁有最小的加載延遲，同時(shí)可以根據(jù)實(shí)際需求執(zhí)行不同的后臺(tái)處理任務(wù)。

實(shí)現(xiàn)加載圖片到GridView

Grid List Building Block是一種有效顯示大量圖片的方式。它能夠一次顯示許多圖片，同時(shí)即將被顯示的圖片會(huì)處于準(zhǔn)備顯示的狀態(tài)。如果我們想要實(shí)現(xiàn)這種效果，必須確保UI是流暢的，能夠控制內(nèi)存使用，并且正確處理并發(fā)問(wèn)題（因?yàn)镚ridView會(huì)循環(huán)使用子視圖)。

下面是一個(gè)典型的使用場(chǎng)景，在Fragment里面內(nèi)置GridView，其中GridView的子視圖是ImageView：

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;
            }
        //請(qǐng)注意下面的代碼
        imageView.setImageResource(imageResIds[position]); // Load image into ImageView
        return imageView;
    }
}

這里同樣有一個(gè)問(wèn)題，上面的代碼實(shí)現(xiàn)中，犯了把圖片加載放在UI線程進(jìn)行處理的錯(cuò)誤。如果只是加載一些很小的圖片，或者是經(jīng)過(guò)Android系統(tǒng)縮放并緩存過(guò)的圖片，上面的代碼在運(yùn)行時(shí)不會(huì)有太大問(wèn)題，但是如果加載的圖片稍微復(fù)雜耗時(shí)一點(diǎn)，這都會(huì)導(dǎo)致你的UI卡頓甚至應(yīng)用無(wú)響應(yīng)。

與前面加載圖片到ViewPager一樣，如果setImageResource的操作會(huì)比較耗時(shí)，也有可能會(huì)阻塞UI線程。不過(guò)我們可以使用類(lèi)似前面異步處理圖片與增加緩存的方法來(lái)解決這個(gè)問(wèn)題。然而，我們還需要考慮GridView的循環(huán)機(jī)制所帶來(lái)的并發(fā)問(wèn)題。為了處理這個(gè)問(wèn)題，可以參考前面的課程 。下面是一個(gè)更新過(guò)后的解決方案：

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 bitmapWorkerTaskReference;

        public AsyncDrawable(Resources res, Bitmap bitmap,
                BitmapWorkerTask bitmapWorkerTask) {
            super(res, bitmap);
            bitmapWorkerTaskReference =
                new WeakReference(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:對(duì)于ListView同樣可以套用上面的方法。

上面的方法提供了足夠的彈性，使得我們可以做從網(wǎng)絡(luò)下載圖片，并對(duì)大尺寸大的數(shù)碼照片做縮放等操作而不至于阻塞UI線程。