Java tutorial
/* * Copyright (C) 2006 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package android.content; import android.annotation.IntDef; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; /** * Extended {@link ComponentCallbacks} interface with a new callback for * finer-grained memory management. This interface is available in all application components * ({@link android.app.Activity}, {@link android.app.Service}, * {@link ContentProvider}, and {@link android.app.Application}). * * <p>You should implement {@link #onTrimMemory} to incrementally release memory based on current * system constraints. Using this callback to release your resources helps provide a more * responsive system overall, but also directly benefits the user experience for * your app by allowing the system to keep your process alive longer. That is, * if you <em>don't</em> trim your resources based on memory levels defined by this callback, * the system is more likely to kill your process while it is cached in the least-recently used * (LRU) list, thus requiring your app to restart and restore all state when the user returns to it. * * <p>The values provided by {@link #onTrimMemory} do not represent a single linear progression of * memory limits, but provide you different types of clues about memory availability:</p> * <ul> * <li>When your app is running: * <ol> * <li>{@link #TRIM_MEMORY_RUNNING_MODERATE} <br>The device is beginning to run low on memory. * Your app is running and not killable. * <li>{@link #TRIM_MEMORY_RUNNING_LOW} <br>The device is running much lower on memory. * Your app is running and not killable, but please release unused resources to improve system * performance (which directly impacts your app's performance). * <li>{@link #TRIM_MEMORY_RUNNING_CRITICAL} <br>The device is running extremely low on memory. * Your app is not yet considered a killable process, but the system will begin killing * background processes if apps do not release resources, so you should release non-critical * resources now to prevent performance degradation. * </ol> * </li> * <li>When your app's visibility changes: * <ol> * <li>{@link #TRIM_MEMORY_UI_HIDDEN} <br>Your app's UI is no longer visible, so this is a good * time to release large resources that are used only by your UI. * </ol> * </li> * <li>When your app's process resides in the background LRU list: * <ol> * <li>{@link #TRIM_MEMORY_BACKGROUND} <br>The system is running low on memory and your process is * near the beginning of the LRU list. Although your app process is not at a high risk of being * killed, the system may already be killing processes in the LRU list, so you should release * resources that are easy to recover so your process will remain in the list and resume * quickly when the user returns to your app. * <li>{@link #TRIM_MEMORY_MODERATE} <br>The system is running low on memory and your process is * near the middle of the LRU list. If the system becomes further constrained for memory, there's a * chance your process will be killed. * <li>{@link #TRIM_MEMORY_COMPLETE} <br>The system is running low on memory and your process is * one of the first to be killed if the system does not recover memory now. You should release * absolutely everything that's not critical to resuming your app state. * <p>To support API levels lower than 14, you can use the {@link #onLowMemory} method as a * fallback that's roughly equivalent to the {@link ComponentCallbacks2#TRIM_MEMORY_COMPLETE} level. * </li> * </ol> * <p class="note"><strong>Note:</strong> When the system begins * killing processes in the LRU list, although it primarily works bottom-up, it does give some * consideration to which processes are consuming more memory and will thus provide more gains in * memory if killed. So the less memory you consume while in the LRU list overall, the better * your chances are to remain in the list and be able to quickly resume.</p> * </li> * </ul> * <p>More information about the different stages of a process lifecycle (such as what it means * to be placed in the background LRU list) is provided in the <a * href="{@docRoot}guide/components/processes-and-threads.html#Lifecycle">Processes and Threads</a> * document. */ public interface ComponentCallbacks2 extends ComponentCallbacks { /** @hide */ @IntDef(prefix = { "TRIM_MEMORY_" }, value = { TRIM_MEMORY_COMPLETE, TRIM_MEMORY_MODERATE, TRIM_MEMORY_BACKGROUND, TRIM_MEMORY_UI_HIDDEN, TRIM_MEMORY_RUNNING_CRITICAL, TRIM_MEMORY_RUNNING_LOW, TRIM_MEMORY_RUNNING_MODERATE, }) @Retention(RetentionPolicy.SOURCE) public @interface TrimMemoryLevel { } /** * 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; /** * 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. */ void onTrimMemory(@TrimMemoryLevel int level); }