Android onLowMemory()和onTrimMemory(...)调整运行过程中的内存消耗 <21>

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.