getContext() , getApplicationContext() , getBaseContext()的区别

• 1、View.getContext(): Returns the context the view is currently running in. Usually the currently active Activity.
• 当前Activity的上下文
• 2、Activity.getApplicationContext(): Returns the context for the entire application (the process all the Activities are running inside of). Use this instead of the current Activity context if you need a context tied to the lifecycle of the entire application, not just the current Activity.
• 应用程序的context
• 3、ContextWrapper.getBaseContext(): If you need access to a Context from within another context, you use a ContextWrapper. The Context referred to from inside that ContextWrapper is accessed via getBaseContext().
• getBaseContext() is the method of ContextWrapper. And ContextWrapper is, "Proxying implementation of Context that simply delegates all of its calls to another Context. Can be subclassed to modify behavior without changing the original Context."

　　1与2的生命周期不同， 前者的生命周期依赖于所在的Activity，后者的生命周期依赖于整个应用。所以new AlertDialog.Builder(getApplicationContext())时会发生错误，AletrDialog依赖的是一个View, 而View对应一个Activity，若传入getApplicationContext()，其生命周期是整个应用，当退出当前Activity的时候，就会报Unable to add window -- token null is not for an application的错误，应该传入当前Activity的Context。