recreate()方法重新创建Activity

在内存不足时，Activity被回收，recreate()是Activity再次恢复的过程。

recreate()的方法内容为：

/**
 * Cause this Activity to be recreated with a new instance.  This results
 * in essentially the same flow as when the Activity is created due to
 * a configuration change -- the current instance will go through its
 * lifecycle to {@link #onDestroy} and a new instance then created after it.
 */
public void recreate() {
    if (mParent != null) {
        throw new IllegalStateException("Can only be called on top-level activity");
    }
    if (Looper.myLooper() != mMainThread.getLooper()) {
        throw new IllegalStateException("Must be called from main thread");
    }
    mMainThread.requestRelaunchActivity(mToken, null, null, 0, false, null, false);
}

可以看到这个方法有两个IllegalStateException处理。一是，当前需要重建的Activity就处在顶层结构，没有父容器包裹它；二是，必须在主线程里调用。requestRelaunchActivity方法将重新启动Activity。

在ONCE项目中是为了无需重启更新主题所做的处理，即在SettingActivity更换主题之后，通知MainActivity调用recreate方法重新构建MainActivity。

另外可以下面两个方法进行数据的保存和恢复：

@Override  
protected void onSaveInstanceState(Bundle savedInstanceState) {  
    super.onSaveInstanceState(savedInstanceState);  
    savedInstanceState.putInt("DATA", 1);
} 

@Override
protected void onRestoreInstanceState(Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);
    int data = savedInstanceState.getInt("DATA");
}