低内存优化

public class MainActivity extends AppCompatActivity
    implements ComponentCallbacks2 {
    // Other activity code ...
    /**
     * Release memory when the UI becomes hidden or when system resources become low.
     * @param level the memory-related event that was raised.
     */
    public void onTrimMemory(int level) {
        // Determine which lifecycle or system event was raised.
        switch (level) {
            case ComponentCallbacks2.TRIM_MEMORY_UI_HIDDEN:
                /*
                   Release any UI objects that currently hold memory.
                   The user interface has moved to the background.
                */
                break;
            case ComponentCallbacks2.TRIM_MEMORY_RUNNING_MODERATE:
            case ComponentCallbacks2.TRIM_MEMORY_RUNNING_LOW:
            case ComponentCallbacks2.TRIM_MEMORY_RUNNING_CRITICAL:
                /*
                   Release any memory that your app doesn't need to run.
                   The device is running low on memory while the app is running.
                   The event raised indicates the severity of the memory-related event.
                   If the event is TRIM_MEMORY_RUNNING_CRITICAL, then the system will
                   begin killing background processes.
                */
                break;
            case ComponentCallbacks2.TRIM_MEMORY_BACKGROUND:
            case ComponentCallbacks2.TRIM_MEMORY_MODERATE:
            case ComponentCallbacks2.TRIM_MEMORY_COMPLETE:
                /*
                   Release as much memory as the process can.
                   The app is on the LRU list and the system is running low on memory.
                   The event raised indicates where the app sits within the LRU list.
                   If the event is TRIM_MEMORY_COMPLETE, the process will be one of
                   the first to be terminated.
                */
                break;
            default:
                /*
                  Release any non-critical data structures.
                  The app received an unrecognized memory level value
                  from the system. Treat this as a generic low-memory message.
                */
                break;
        }
    }
}

//再通过Context.registerComponentCallbacks(ComponentCallbacks callback)注册

//level内存状态，紧急程度有高到低
TRIM_MEMORY_COMPLETE = 80：内存不足，并且该进程在后台进程列表最后一个，马上就要被清理。基本与onLowMemory()情况相同；
TRIM_MEMORY_MODERATE = 60：内存不足，并且该进程在后台进程列表的中部；
TRIM_MEMORY_BACKGROUND = 40：内存不足，并且该进程是后台进程；
TRIM_MEMORY_UI_HIDDEN = 20：内存不足，并且该进程的UI已经不可见了

//4.1之后增加的，是当我们的应用程序真正运行时的回调，紧急程度有高到低
TRIM_MEMORY_RUNNING_CRITICAL = 15：内存不足(后台进程不足3个)
TRIM_MEMORY_RUNNING_LOW = 10：内存不足(后台进程不足5个)
TRIM_MEMORY_RUNNING_MODERATE = 5：内存不足(后台进程超过5个)

//ComponentCallbacks2继承ComponentCallbacks，多了一个方法：
void onTrimMemory(int level);

if(bitmap != null && !bitmap.isRecycled()){  
    bitmap.recycle(); 
    bitmap = null; 
}

//主动释放ImageView的图片资源
private static void recycleImageViewBitMap(ImageView imageView) {
    if (imageView != null) {
        BitmapDrawable bd = (BitmapDrawable) imageView.getDrawable();
        rceycleBitmapDrawable(bd);
    }
}
private static void rceycleBitmapDrawable(BitmapDrawable bitmapDrawable) {
    if (bitmapDrawable != null) {
        Bitmap bitmap = bitmapDrawable.getBitmap();
        rceycleBitmap(bitmap);
    }
    bitmapDrawable = null;
}
private static void rceycleBitmap(Bitmap bitmap) {
    if (bitmap != null && !bitmap.isRecycled()) {
        bitmap.recycle();
        bitmap = null;
    }
}

//主动释放ImageView的背景资源
public static void recycleBackgroundBitMap(ImageView view) {
    if (view != null) {
        BitmapDrawable bd = (BitmapDrawable) view.getBackground();
        rceycleBitmapDrawable(bd);
    }
}
public static void recycleImageViewBitMap(ImageView imageView) {
    if (imageView != null) {
        BitmapDrawable bd = (BitmapDrawable) imageView.getDrawable();
        rceycleBitmapDrawable(bd);
    }
}
private static void rceycleBitmapDrawable(BitmapDrawable bitmapDrawable) {
    if (bitmapDrawable != null) {
        Bitmap bitmap = bitmapDrawable.getBitmap();
        rceycleBitmap(bitmap);
    }
    bitmapDrawable = null;
}

//来自Launcher.java:
@Override
public void onTrimMemory(int level) {
    super.onTrimMemory(level);
    if (level >= ComponentCallbacks2.TRIM_MEMORY_MODERATE) {
        mAppsCustomizeTabHost.onTrimMemory();
    }
}

//AppsCustomizeTabHost.java:
public void onTrimMemory() {
    mContent.setVisibility(GONE);
    // Clear the widget pages of all their subviews - this will trigger the widget previews
    // to delete their bitmaps
    mPagedView.clearAllWidgetPages();
}

//AppsCustomizePagedView.java:
public void clearAllWidgetPages() {
    cancelAllTasks();
    int count = getChildCount();
    for (int i = 0; i < count; i++) {
        View v = getPageAt(i);
        if (v instanceof PagedViewGridLayout) {
            ((PagedViewGridLayout) v).removeAllViewsOnPage();
            mDirtyPageContent.set(i, true);
        }
    }
}

//PagedViewGridLayout.java
@Override
public void removeAllViewsOnPage() {
    removeAllViews();
    mOnLayoutListener = null;
    setLayerType(LAYER_TYPE_NONE, null);
}

if(list != null) {
    list.clear();
    list = null; 
}

//Contact，联系人
@Override
public void onTrimMemory(int level) {
    if (level >= ComponentCallbacks2.TRIM_MEMORY_MODERATE) {
        // Clear the caches.  Note all pending requests will be removed too.
        clear();
    }
}
public void clear() {
    mPendingRequests.clear();
    mBitmapHolderCache.evictAll();
    mBitmapCache.evictAll();
}

Cursor cursor = getContentResolver().query(uri ...);
if (cursor.moveToNext()) {
 	... ... 
}
//改为：
Cursor cursor = null;
try {
  	cursor = getContentResolver().query(uri ...);
	if (cursor != null && cursor.moveToNext()) {
	... ... 
	}
	} finally {
		if (cursor != null) {
	try { 
		cursor.close();
	} catch (Exception e) {
		//ignore this
		}
	}
}

public View getView(int position, View convertView, ViewGroup parent) {
     View view = new Xxx(...);
     ... ...
     return view;
}

//改为：
public View getView(int position, View convertView, ViewGroup parent) {
     View view = null;
     if (convertView != null) {
         view = convertView;
         populate(view, getItem(position));
         ...
     } else {
         view = new Xxx(...);
         ...
     }
     return view;
}

public class DemoActivity extends Activity {
	... ...
	private Handler mHandler = ...
	private Object obj;
	public void operation() {
	 obj = initObj();
	 ...
	 [Mark]
	 mHandler.post(new Runnable() {
	        public void run() {
	         useObj(obj);
	        }
	 });
	}
}
//可以在[Mark]的位置释放对象的引用，而代码可以修改为：
public void operation() {
	obj = initObj();
	...
	final Object o = obj;
	obj = null;
	mHandler.post(new Runnable() {
	    public void run() {
	        useObj(o);
	    }
	}
}