activity的启动模式，想进阶不得不知道的东西

在开始今天的博客之前强调一点，android系统是通过返回栈来管理activity的，这个一定要知道。


　　activity一共有四种启动模式：standard，singleTop，singleTask和singleInstance。


　　standard是系统默认的启动模式，在这个模式下，系统不会问你在返回栈中是否已经有该activity的实例，只要你启动一个它就仍一个进去返回栈并处于栈顶。


　　singleTop 在这个模式下，如果你新启动的activity在返回栈中的栈顶，那么系统就不会去重新创建新的实例，而是直接拿这个来用了。这也就导致，如果你在一个activity中启动它本身的话，你会觉得没有反应。


　　singleTask 这个模式是最合理的也是我们经常能用到的，在这个模式下，只要你启动一个activity，系统都会先去返回栈中查看一遍，若已存在，则直接拿来使用，若不存在则创建。


　　singleInstance 其实这个也是最复杂最难理解的一个。因为这个模式下，你启动activity系统会新创建一个返回栈给这个activity专用。然并卵。。。这个模式的最大作用就是可以和启动程序来共享这个activity。然并卵。。。到现在还没使用过，效果好不好用过了才知道。


　　这几个模式说起来都很简单，也好理解。但是书到用时方恨少，每次到用的时候我都需要去查一遍，真是无语了。

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