Android onLowMemory()和onTrimMemory(...)调整运行过程中的内存消耗

https://blog.youkuaiyun.com/qq_31726827/article/details/50715722

在很多APP经常会因为数据量大,或者一个页面占有内存较高,在切换到另外一个页面的时候,内存不能够暂时释放,导致切换到另外一个页面加载较慢...
Android在4.0版本后推出了onLowMemory()和onTrimMemory(...)两个回调方法,提供给开发者,提示Android APP在低内存的状态.
OnTrimMemory的主要作用就是指导应用程序在不同的情况下进行自身的内存释放，以避免被系统直接杀掉，提高应用程序的用户体验
onLowMemory()方法在使用过程只要低内存状态下,就会回调.
系统回调是在ActivityThread类中进行的:
final void handleLowMemory() {        ArrayList<ComponentCallbacks2> callbacks = collectComponentCallbacks(true, null);         final int N = callbacks.size();        for (int i=0; i<N; i++) {            callbacks.get(i).onLowMemory();        }        // Ask SQLite to free up as much memory as it can, mostly from its page caches.        if (Process.myUid() != Process.SYSTEM_UID) {            int sqliteReleased = SQLiteDatabase.releaseMemory();            EventLog.writeEvent(SQLITE_MEM_RELEASED_EVENT_LOG_TAG, sqliteReleased);        }        // Ask graphics to free up as much as possible (font/image caches)        Canvas.freeCaches();        // Ask text layout engine to free also as much as possible        Canvas.freeTextLayoutCaches();        BinderInternal.forceGc("mem");    }    final void handleTrimMemory(int level) {        if (DEBUG_MEMORY_TRIM) Slog.v(TAG, "Trimming memory to level: " + level);        final WindowManagerGlobal windowManager = WindowManagerGlobal.getInstance();        windowManager.startTrimMemory(level);        ArrayList<ComponentCallbacks2> callbacks = collectComponentCallbacks(true, null);         final int N = callbacks.size();        for (int i = 0; i < N; i++) {            callbacks.get(i).onTrimMemory(level);        }        windowManager.endTrimMemory();    }
这里面要注意onLowMemory()方法中会调用BinderInternal.forceGc("mem");会触发系统垃圾回收,但是onTrimMemory不会,它自会提示开发者当前的内存水平.
public static void forceGc(String reason) {        EventLog.writeEvent(2741, reason);        Runtime.getRuntime().gc();    }

onTrimMemory(level)中level水平说明可以参考ComponentCallbacks2.java类,也可以大致看一下下面的:

这两个回调在下面中都得到实现:

四大组件除了广播,加一个Fragment.

具体程序中实现下面两个回调:
@Override    public void onLowMemory()

{       

super.onLowMemory();   

}    

@Override   

public void onTrimMemory(int level)

{       

super.onTrimMemory(level);        

if(level> ComponentCallbacks2.TRIM_MEMORY_UI_HIDDEN)

{        

}

else if(level>ComponentCallbacks2.TRIM_MEMORY_BACKGROUND)

{        

}

else if(level>ComponentCallbacks2.TRIM_MEMORY_COMPLETE)

{        

}

else if(level>ComponentCallbacks2.TRIM_MEMORY_MODERATE)

{        

}

else if(level>ComponentCallbacks2.TRIM_MEMORY_RUNNING_LOW)

{        

}

else if(level>ComponentCallbacks2.TRIM_MEMORY_RUNNING_CRITICAL)

{        

}else if(level>ComponentCallbacks2.TRIM_MEMORY_RUNNING_MODERATE)

{        

}   

}
在不同的级别阶段处理不同的内存管理.
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作者：喝醉了的熊猫
来源：优快云
原文：https://blog.youkuaiyun.com/qq_31726827/article/details/50715722
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