[Android] 对自定义图片浏览器经常内存溢出的一些优化

Whenever you first start an Android application, a thread called "main" is automatically created. The main thread, also called the UI thread, is very important because it is in charge of dispatching the events to the appropriate widgets and this includes the drawing events. It is also the thread you interact with Android widgets on. For instance, if you touch the a button on screen, the UI thread dispatches the touch event to the widget which in turn sets its pressed state and posts an invalidate request to the event queue. The UI thread dequeues the request and notifies the widget to redraw itself.

This single thread model can yield poor performance in Android applications that do not consider the implications. Since everything happens on a single thread performing long operations, like network access or database queries, on this thread will block the whole user interface. No event can be dispatched, including drawing events, while the long operation is underway. From the user‘s perspective, the application appears hung. Even worse, if the UI thread is blocked for more than a few seconds (about 5 seconds currently) the user is presented with the infamous "application not responding" (ANR) dialog.

http://android-developers.blogspot.jp/2009/05/painless-threading.html

 1 new Thread(new Runnable() {
 2     @Override
 3     public void run() {
 4         Uri mImageUri = MediaStore.Images.Media.EXTERNAL_CONTENT_URI;// 外部存储的url
 5         ContentResolver mContentResolver = mActivity.getContentResolver();// 创建ContentResolver实例
 6 
 7         Cursor mCursor = mContentResolver.query(
 8                 mImageUri,
 9                 null, // 从URL中查找相关类型的文件保存并用cursor指向它
10                 MediaStore.Images.Media.MIME_TYPE + "=? or " + MediaStore.Images.Media.MIME_TYPE + "=?",
11                 new String[] { "image/jpeg", "image/png" },
12                 MediaStore.Images.Media.DATE_MODIFIED);
13                 
14         while (mCursor.moveToNext()) {
15             // 获取图片路径
16             String path = mCursor.getString(mCursor.getColumnIndex(MediaStore.Images.Media.DATA));
17 
18             // 说明1
19             File file = new File(path);
20             if (file.exists()) {
21                 // get folder name
22                 String folder = new File(path).getParentFile().getName();
23 
24                 // put image in to HashMap according to the folder name
25                 if (!mFolderMap.containsKey(folder)) {
26                     List<String> childList = new ArrayList<String>();
27                     childList.add(path);
28                     mFolderMap.put(folder, childList);
29                 } else {
30                     mFolderMap.get(folder).add(path);
31                 }
32             }
33 
34         }
35         mCursor.close();
36         mHandler.sendEmptyMessage(SCAN_OK);
37     }
38 }).start();

private Handler mHandler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
        switch (msg.what) {
            case SCAN_OK:
                mProgressDialog.dismiss();
                mFolderAdapter = new FolderAdapter(mActivity, mList = subGroupOfImage(mFolderMap), mGridShowFolder);
                mGridShowFolder.setAdapter(mFolderAdapter);
                break;
        }
    }
};

 1 /**
 2  * 根据View(主要是ImageView)的宽和高来计算Bitmap缩放比例。默认不缩放
 3  * 
 4  * @param options
 5  * @param width
 6  * @param height
 7  */
 8 private int computeScale(BitmapFactory.Options options, int viewWidth, int viewHeight) {
 9     int inSampleSize = 1; // 默认缩放比例1 即不缩放
10     if (viewWidth == 0 || viewHeight == 0) { // 如果宽高都为0 即不存在 返回1
11         return inSampleSize;
12     }
13     int bitmapWidth = options.outWidth;// 获得图片的宽和高
14     int bitmapHeight = options.outHeight;
15 
16     // 假如Bitmap的宽度或高度大于我们设定图片的View的宽高，则计算缩放比例
17     if (bitmapWidth > viewWidth || bitmapHeight > viewWidth) {
18         int widthScale = Math.round((float) bitmapWidth / (float) viewWidth);
19         int heightScale = Math.round((float) bitmapHeight / (float) viewWidth);
20 
21         // 为了保证图片不缩放变形，我们取宽高比例最小的那个
22         inSampleSize = widthScale < heightScale ? widthScale : heightScale;
23     }
24     return inSampleSize;
25 }

 1 private Bitmap loadBitmap(String path, int n) {    //n 缩放比例
 2     InputStream inputStream = null;
 3     if (!path.isEmpty()) {
 4         try {
 5             inputStream = new FileInputStream(path);
 6         } catch (FileNotFoundException e) {
 7             // TODO Auto-generated catch block
 8             e.printStackTrace();
 9         }
10     }
11 
12     BitmapFactory.Options opts = new BitmapFactory.Options();
13     opts.inPreferredConfig = Bitmap.Config.RGB_565; // 减小画面的精细度，每个像素占内存减少
14     opts.inPurgeable = true; // 可以被回收
15     opts.inInputShareable = true;
16     if (n != 0) {
17         opts.inSampleSize = n;
18     }
19     Bitmap bmp = BitmapFactory.decodeStream(inputStream, null, opts);
20     return bmp;
21 }

If this is non-null, the decoder will try to decode into this
 internal configuration. If it is null, or the request cannot be met,
 the decoder will try to pick the best matching config based on the
 system‘s screen depth, and characteristics of the original image such
 as if it has per-pixel alpha (requiring a config that also does).

Image are loaded with the  Bitmap.Config.ARGB_8888} config by default.

//Bitmap.Config   Because of the poor quality of ARGB_4444, it is advised to use {@link #ARGB_8888} instead.
private static Config sConfigs[] = {
            null, ALPHA_8, null, RGB_565, ARGB_4444, ARGB_8888
};

Deprecated. As of android.os.Build.VERSION_CODES.LOLLIPOP, this is ignored. In android.os.Build.VERSION_CODES.KITKAT and below, if this is set to true, then the resulting bitmap will allocate its pixels such that they can be purged if the system needs to reclaim memory. In that instance, when the pixels need to be accessed again (e.g. the bitmap is drawn, getPixels() is called), they will be automatically re-decoded.

For the re-decode to happen, the bitmap must have access to the encoded data, either by sharing a reference to the input or by making a copy of it. This distinction is controlled by inInputShareable. If this is true, then the bitmap may keep a shallow reference to the input. If this is false, then the bitmap will explicitly make a copy of the input data, and keep that. Even if sharing is allowed, the implementation may still decide to make a deep copy of the input data.

While inPurgeable can help avoid big Dalvik heap allocations (from API level 11 onward), it sacrifices performance predictability since any image that the view system tries to draw may incur a decode delay which can lead to dropped frames. Therefore, most apps should avoid using inPurgeable to allow for a fast and fluid UI. To minimize Dalvik heap allocations use the inBitmap flag instead.

Note: This flag is ignored when used with decodeResource(Resources, int, android.graphics.BitmapFactory.Options) or decodeFile(String, android.graphics.BitmapFactory.Options).

Deprecated. As of android.os.Build.VERSION_CODES.LOLLIPOP, this is ignored. In android.os.Build.VERSION_CODES.KITKAT and below, this field works in conjuction with inPurgeable. If inPurgeable is false, then this field is ignored. If inPurgeable is true, then this field determines whether the bitmap can share a reference to the input data (inputstream, array, etc.) or if it must make a deep copy.

If set to a value > 1, requests the decoder to subsample the original image, returning a smaller image to save memory. The sample size is the number of pixels in either dimension that correspond to a single pixel in the decoded bitmap. For example, inSampleSize == 4 returns an image that is 1/4 the width/height of the original, and 1/16 the number of pixels. Any value <= 1 is treated the same as 1. Note: the decoder uses a final value based on powers of 2, any other value will be rounded down to the nearest power of 2.

@Override
public Object instantiateItem(ViewGroup container, int position) {
    // TODO Auto-generated method stub
    
    long beforeTime = System.currentTimeMillis();
    
    PicBrowser_ViewPagerItem itemView;
    if (mHashMap.containsKey(position)) {
        String path = mPathList.get(position);
        itemView = mHashMap.get(position);
        itemView.setShowHeight(itemHeight);
        itemView.reload(path, position);

        Log.d("TIME", "reload time: " + (System.currentTimeMillis() - beforeTime) + " position:" + position);
    } else {
        itemView = new PicBrowser_ViewPagerItem(mContext, mMemoryCacheForBmp);
        itemView.setShowHeight(itemHeight);
        String path = mPathList.get(position);
        Bitmap bmp = mMemoryCache.get(path);

        itemView.setData(path, bmp, position);
        // Log.v("viewpager", "setData:" + position);
        mHashMap.put(position, itemView);
        ((ViewPager) container).addView(itemView);
        
        Log.d("TIME", "setData time: " + (System.currentTimeMillis() - beforeTime) + " position:" + position);
    }

    clearHashMap(position);
    
    return itemView;

}