SharedPreferences中的commit和apply方法

• preferences:参数
• SharedPreferences：共享参数

---

1. SharedPreferences对象获取

context类中的getSharedPreferences方法可以获取一个SharedPreferences对象

private File makeFilename(File base, String name) {
        if (name.indexOf(File.separatorChar) < 0) {
            return new File(base, name);
        }
        throw new IllegalArgumentException(
                "File " + name + " contains a path separator");
    }

其本质也是在文件不存在的情况下new一个File

2. commit方法

源码注释：

 /**
 * Commit your preferences changes back from this Editor to the
 * {@link SharedPreferences} object it is editing.  This atomically
 * performs the requested modifications, replacing whatever is currently
 * in the SharedPreferences.
 *
 * 
Note that when two editors are modifying preferences at the same
 * time, the last one to call commit wins.
 *
 * 
If you don't care about the return value and you're
 * using this from your application's main thread, consider
 * using {@link #apply} instead.
 *
 * @return Returns true if the new values were successfully written
 * to persistent storage.
 */
boolean commit();

• commit方法是有一个boolean的返回值
• 当数据变化进行存储时是一个原子性的操作
• 当两个editor对象同时对一个共享的preferences参数进行操作时，永远都是最后一个调用commit方法的editor变更了最后的数据值

3. apply方法

源码注释：

/**
* Commit your preferences changes back from this Editor to the
* {@link SharedPreferences} object it is editing.  This atomically
* performs the requested modifications, replacing whatever is currently
* in the SharedPreferences.
*
* 
Note that when two editors are modifying preferences at the same
* time, the last one to call apply wins.
*
* 
Unlike {@link #commit}, which writes its preferences out
* to persistent storage synchronously, {@link #apply}
* commits its changes to the in-memory
* {@link SharedPreferences} immediately but starts an
* asynchronous commit to disk and you won't be notified of
* any failures.  If another editor on this
* {@link SharedPreferences} does a regular {@link #commit}
* while a {@link #apply} is still outstanding, the
* {@link #commit} will block until all async commits are
* completed as well as the commit itself.
*
* 
As {@link SharedPreferences} instances are singletons within
* a process, it's safe to replace any instance of {@link #commit} with
* {@link #apply} if you were already ignoring the return value.
*
* 
You don't need to worry about Android component
* lifecycles and their interaction with apply()
* writing to disk.  The framework makes sure in-flight disk
* writes from apply() complete before switching
* states.
*
* 
The SharedPreferences.Editor interface
* isn't expected to be implemented directly.  However, if you
* previously did implement it and are now getting errors
* about missing apply(), you can simply call
* {@link #commit} from apply().
*/
void apply();

• apply方法是没有返回值的
• 当两个editor同时对preferences对象编辑时，也是最后一个调用apply方法的对象编辑数据
• apply的提交操作也是原子性的

4. commit和apply方法的区别：

1. commit和apply虽然都是原子性操作，但是原子的操作不同，commit是原子提交到数据库，所以从提交数据到存在Disk中都是同步过程，中间不可打断。

2. 而apply方法的原子操作是原子提交的内存中，而非数据库，所以在提交到内存中时不可打断，之后再异步提交数据到数据库中，因此也不会有相应的返回值。

3. 所有commit提交是同步过程，效率会比apply异步提交的速度慢，但是apply没有返回值，永远无法知道存储是否失败。

4. 在不关心提交结果是否成功的情况下，优先考虑apply方法。