【原創】Android記憶體管理-OnTrimMemory，androidontrimmemory

【原創】Android記憶體管理-OnTrimMemory，androidontrimmemory

Application中有兩個與記憶體管理相關的方法：onLowMemory()和 onTrimMemory(int level)，源碼如下

  @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);                }            }        }    }

從源碼我們可以看到Application收到這兩個回調時會通知它的監聽者，而Activity和Service都註冊了監聽，

因此我們可以Application中重寫這兩個方法，也可以在組件中重寫這兩個方法。

 

先重點介紹一下onTrimMemory

為了更好的管理記憶體，OnTrimMemory 方法在 API-14 被引入。這個回調可以在所有組件中擷取到（Activity, Service, ContentProvider, and Application）。

你應該根據當前裝置的限制複寫 onTrimMemory(int) 來逐步的釋放記憶體。通過複寫這個方法釋放資源可以協助你的app更好的響應系統整體，同時通過讓你的app

在系統中存活更久來提高使用者體驗。如果在系統記憶體很低時，你仍舊不釋放記憶體，系統將會優先殺死你在的進程。這樣當使用者返回app時需要重啟影響使用者體驗。

 

onTrimMemory(int)的level值並不成線性關係，它只是提供了記憶體不同狀態的線索。

1. 回調時機

  /**     * 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);

當作業系統認為這是一個進程釋放無用記憶體的好時機時，會調用此方法。比如說當已經沒有足夠的記憶體來維持目前所有的後台進程，而此進程正好處於後台。
非常不推薦用一個精確的值來與level作比較，因為可能會增加新的差值，推薦的做法是判斷一個值是否大於或者等於你感興趣的level.

為了擷取所有進程目前的level,你可以調用{@link android.app.ActivityManager#getMyMemoryState* ActivityManager.getMyMemoryState(RunningAppProcessInfo)}

 

2.level值的具體含義

 

 /**     * 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;

 

當你的app在後台時：

TRIM_MEMORY_COMPLETE ：當前進程在LRU列表的尾部，如果沒有足夠的記憶體，它將很快被殺死。這時候你應該釋放任何不影響app啟動並執行資源。

TRIM_MEMORY_MODERATE ：當前進程在LRU列表的中部，如果系統進一步需要記憶體，你的進程可能會被殺死。

TRIM_MEMORY_BACKGROUND：當前進程在LRU列表的頭部，雖然你的進程不會被高優殺死，但是系統已經開始準備殺死LRU列表中的其他進程了，

                   因此你應該盡量的釋放能夠快速回複的資源，以保證當使用者返回你的app時可以快速恢複。                   。

 

當你的app的可見度改變時：

TRIM_MEMORY_UI_HIDDEN：當前進程的介面已經不可見，這時是釋放UI相關的資源的好時機。

 

當你的app正在運行時：

TRIM_MEMORY_RUNNING_CRITICAL：雖然你的進程不會被殺死，但是系統已經開始準備殺死其他的後台進程了，這時候你應該釋放無用資源以防止效能下降。

                 下一個階段就是調用"onLowMemory()"來報告開始殺死後台進程了，特別是狀況已經開始影響到使用者。

TRIM_MEMORY_RUNNING_LOW：雖然你的進程不會被殺死，但是系統已經開始準備殺死其他的後台進程了，你應該釋放不必要的資源來提供系統效能，否則會

                 影響使用者體驗。

TRIM_MEMORY_RUNNING_MODERATE：系統已經進入了低記憶體的狀態，你的進程正在運行但是不會被殺死。

 

再來說一下onLowMemory

在引入OnTrimMemory之前都是使用OnLowMemory方法。如果你的app運行在API-14+的機器上，應該使用 OnTrimMemory(int),OnLowMemory的調用時機大概等同於TRIM_MEMORY_COMPLETE．