Application中有两个与内存管理相关的方法：onLowMemory()和 onTrimMemory(int level)，源码如下
@CallSuper
public void onLowMemory() {
Object[] callbacks = collectComponentCallbacks();
if (callbacks != null) {
for (int i=0; i<callbacks.length; i++) {
((ComponentCallbacks)callbacks[i]).onLowMemory();
}
}
}
@CallSuper
public void onTrimMemory(int level) {
Object[] callbacks = collectComponentCallbacks();
if (callbacks != null) {
for (int i=0; i<callbacks.length; i++) {
Object c = callbacks[i];
if (c instanceof ComponentCallbacks2) {
((ComponentCallbacks2)c).onTrimMemory(level);
}
}
}
}
从源码我们可以看到Application收到这两个回调时会通知它的监听者，而Activity和Service都注册了监听，
因此我们可以Application中重写这两个方法，也可以在组件中重写这两个方法。
先重点介绍一下onTrimMemory
为了更好的管理内存，OnTrimMemory 方法在 API-14 被引入。这个回调可以在所有组件中获取到（Activity, Service, ContentProvider, and Application）。
你应该根据当前设备的限制复写 onTrimMemory(int) 来逐步的释放内存。通过复写这个方法释放资源可以帮助你的app更好的响应系统整体，同时通过让你的app
在系统中存活更久来提高用户体验。如果在系统内存很低时，你仍旧不释放内存，系统将会优先杀死你在的进程。这样当用户返回app时需要重启影响用户体验。
onTrimMemory(int)的level值并不成线性关系，它只是提供了内存不同状态的线索。
1. 回调时机
/**
* Called when the operating system has determined that it is a good
* time for a process to trim unneeded memory from its process. This will
* happen for example when it goes in the background and there is not enough
* memory to keep as many background processes running as desired. You
* should never compare to exact values of the level, since new intermediate
* values may be added -- you will typically want to compare if the value
* is greater or equal to a level you are interested in.
*
* <p>To retrieve the processes current trim level at any point, you can
* use {@link android.app.ActivityManager#getMyMemoryState
* ActivityManager.getMyMemoryState(RunningAppProcessInfo)}.
*
* @param level The context of the trim, giving a hint of the amount of
* trimming the application may like to perform. May be
* {@link #TRIM_MEMORY_COMPLETE}, {@link #TRIM_MEMORY_MODERATE},
* {@link #TRIM_MEMORY_BACKGROUND}, {@link #TRIM_MEMORY_UI_HIDDEN},
* {@link #TRIM_MEMORY_RUNNING_CRITICAL}, {@link #TRIM_MEMORY_RUNNING_LOW},
* or {@link #TRIM_MEMORY_RUNNING_MODERATE}.
*/
void onTrimMemory(int level);
当操作系统认为这是一个进程释放无用内存的好时机时，会调用此方法。比如说当已经没有足够的内存来维持目前所有的后台进程，而此进程正好处于后台。
非常不推荐用一个精确的值来与level作比较，因为可能会增加新的差值，推荐的做法是判断一个值是否大于或者等于你感兴趣的level.
为了获取所有进程目前的level,你可以调用{@link android.app.ActivityManager#getMyMemoryState* ActivityManager.getMyMemoryState(RunningAppProcessInfo)}
2.level值的具体含义
/**
* Level for {@link #onTrimMemory(int)}: the process is nearing the end
* of the background LRU list, and if more memory isn't found soon it will
* be killed.
*/
static final int TRIM_MEMORY_COMPLETE = 80;
/**
* Level for {@link #onTrimMemory(int)}: the process is around the middle
* of the background LRU list; freeing memory can help the system keep
* other processes running later in the list for better overall performance.
*/
static final int TRIM_MEMORY_MODERATE = 60;
/**
* Level for {@link #onTrimMemory(int)}: the process has gone on to the
* LRU list. This is a good opportunity to clean up resources that can
* efficiently and quickly be re-built if the user returns to the app.
*/
static final int TRIM_MEMORY_BACKGROUND = 40;
/**
* Level for {@link #onTrimMemory(int)}: the process had been showing
* a user interface, and is no longer doing so. Large allocations with
* the UI should be released at this point to allow memory to be better
* managed.
*/
static final int TRIM_MEMORY_UI_HIDDEN = 20;
/**
* Level for {@link #onTrimMemory(int)}: the process is not an expendable
* background process, but the device is running extremely low on memory
* and is about to not be able to keep any background processes running.
* Your running process should free up as many non-critical resources as it
* can to allow that memory to be used elsewhere. The next thing that
* will happen after this is {@link #onLowMemory()} called to report that
* nothing at all can be kept in the background, a situation that can start
* to notably impact the user.
*/
static final int TRIM_MEMORY_RUNNING_CRITICAL = 15;
/**
* Level for {@link #onTrimMemory(int)}: the process is not an expendable
* background process, but the device is running low on memory.
* Your running process should free up unneeded resources to allow that
* memory to be used elsewhere.
*/
static final int TRIM_MEMORY_RUNNING_LOW = 10;
/**
* Level for {@link #onTrimMemory(int)}: the process is not an expendable
* background process, but the device is running moderately low on memory.
* Your running process may want to release some unneeded resources for
* use elsewhere.
*/
static final int TRIM_MEMORY_RUNNING_MODERATE = 5;
当你的app在后台时：
TRIM_MEMORY_COMPLETE ：当前进程在LRU列表的尾部，如果没有足够的内存，它将很快被杀死。这时候你应该释放任何不影响app运行的资源。
TRIM_MEMORY_MODERATE ：当前进程在LRU列表的中部，如果系统进一步需要内存，你的进程可能会被杀死。
TRIM_MEMORY_BACKGROUND：当前进程在LRU列表的头部，虽然你的进程不会被高优杀死，但是系统已经开始准备杀死LRU列表中的其他进程了，
因此你应该尽量的释放能够快速回复的资源，以保证当用户返回你的app时可以快速恢复。 。
当你的app的可见性改变时：
TRIM_MEMORY_UI_HIDDEN：当前进程的界面已经不可见，这时是释放UI相关的资源的好时机。
当你的app正在运行时：
TRIM_MEMORY_RUNNING_CRITICAL：虽然你的进程不会被杀死，但是系统已经开始准备杀死其他的后台进程了，这时候你应该释放无用资源以防止性能下降。
下一个阶段就是调用"onLowMemory()"来报告开始杀死后台进程了，特别是状况已经开始影响到用户。
TRIM_MEMORY_RUNNING_LOW：虽然你的进程不会被杀死，但是系统已经开始准备杀死其他的后台进程了，你应该释放不必要的资源来提供系统性能，否则会
影响用户体验。
TRIM_MEMORY_RUNNING_MODERATE：系统已经进入了低内存的状态，你的进程正在运行但是不会被杀死。
再来说一下onLowMemory
在引入OnTrimMemory之前都是使用OnLowMemory方法。如果你的app运行在API-14+的机器上，应该使用 OnTrimMemory(int),OnLowMemory的调用时机大概等同于TRIM_MEMORY_COMPLETE．