android mediaplayer prepare

         android 的mediaplayer的prepare過程有很多動作，比方demuxer，find codec,如果時間較長就會出現ANR，android的系統會在5秒檢測主線程是否有響應，沒有就會出現錯誤提示，使用者體驗非常的不好。所以系統如果有parepareAsync提供的話，可以使用這個函數，然後再檢測prepare狀態，如果沒有prepareAsync提供的話，prepare不能在主線程來使用。         下面是從SDK摘出來的

Asynchronous Preparation

Using MediaPlayer can be straightforward in principle. However,
it's important to keep in mind that a few more things are necessary to integrate it correctly with a typical Android application. For example, the call to prepare() can
take a long time to execute, because it might involve fetching and decoding media data. So, as is the case with any method that may take long to execute, you should never call it from your application's UI thread. Doing that will cause the
UI to hang until the method returns, which is a very bad user experience and can cause an ANR (Application Not Responding) error. Even if you expect your resource to load quickly, remember that anything that takes more than a tenth of a second to respond in
the UI will cause a noticeable pause and will give the user the impression that your application is slow.

To avoid hanging your UI thread, spawn another thread to prepare the MediaPlayer and
notify the main thread when done. However, while you could write the threading logic yourself, this pattern is so common when using MediaPlayer that
the framework supplies a convenient way to accomplish this task by using the prepareAsync() method.
This method starts preparing the media in the background and returns immediately. When the media is done preparing, the onPrepared() method
of the MediaPlayer.OnPreparedListener, configured
throughsetOnPreparedListener() is
called