getContext() , getApplicationContext() , getBaseContext()的區別

• 1、View.getContext(): Returns the context the view is currently running in. Usually the currently active Activity.

• 當前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.

• 應用程序的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與2的生命周期不同， 前者的生命周期依賴於所在的Activity，后者的生命周期依賴於整個應用。所以new AlertDialog.Builder(getApplicationContext())時會發生錯誤，AletrDialog依賴的是一個View, 而View對應一個Activity，若傳入getApplicationContext()，其生命周期是整個應用，當退出當前Activity的時候，就會報Unable to add window -- token null is not for an application的錯誤，應該傳入當前Activity的Context。