Android SharedPreferences理解

摘要: SharedPreferences

SharedPreferences是单例对象

只在第一次调用getSharedPreferences方法时打开文件时，创建一个实例对象。并放入map中，第二次获取数据从map 中获取，不需再解析。

写操作有两步，一是把数据先写入内存中，即map集合，二是把数据写入硬盘文件中。

SharedPreferences.Editor是用来修改SharedPreferences对象值的接口。你在editor 做出的修改都是待处理的，并没有被复制到SharedPreferences里，直到你调用commit()或apply()修改才会被执行。

commit()：线程安全，性能慢，一般来说在当前线程完成写文件操作apply()：线程不安全，性能高，异步处理IO操作，一定会把这个写文件操作放入一个SingleThreadExecutor线程池中处理不过apply()方法有个坑，那就是如果在一个Activity调用过多的apply()方法，在Activity关闭时，系统会等待所有放到内存map的都写完磁盘，才关闭Activity。所以一个Activity如果频繁操作SP，最好放到线程中，使用commit提交

ActivityThread.java

private void handleStopActivity(IBinder token, boolean show, int configChanges) {
.....................
// Make sure any pending writes are now committed.
if (!r.isPreHoneycomb()) {
QueuedWork.waitToFinish();
}
.....................
}

/**
* Internal utility class to keep track of process-global work that's
* outstanding and hasn't been finished yet.
*
* This was created for writing SharedPreference edits out
* asynchronously so we'd have a mechanism to wait for the writes in
* Activity.onPause and similar places, but we may use this mechanism
* for other things in the future.
*
* @hide
*/
public class QueuedWork {

/**
* Finishes or waits for async operations to complete.
* (e.g. SharedPreferences$Editor#startCommit writes)
*
* Is called from the Activity base class's onPause(), after
* BroadcastReceiver's onReceive, after Service command handling,
* etc.  (so async work is never lost)
*/
public static void waitToFinish() {
Runnable toFinish;
while ((toFinish = sPendingWorkFinishers.poll()) != null) {
toFinish.run();
}
}

clear清除数据，必须要commit或apply