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如果 activity 實現了一個回調接口，然後使用 this 設置給需要回調接口的方法，這種應用場景比較常見，最常見的就是實現 onClickListener 接口，然後 findViewById().setOnClickListenr(this)
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如果這個回調接口設置到了一個靜態對象（單例模式），當 activity finish() 的時候（按返回鍵，回到桌面），則activity 不會被調用 onDestroy() ，原因可能是 activity 對象還在被引用！
此時你再點擊圖標回到應用，onCreate() 再次調用！
很明顯，如果你把資源釋放放在了 onDestroy() 裏面，就會導致內存泄露！
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解決辦法：
1、可以在finish中進行釋放
2、可以在 onPause() 方法裏面判斷 isFinishing() ，正常調用 finish() 後 activity 的回調過程是 onPause、onStop、onDestroy ，倘若出現上面的情況，只到 onPause！但是 isFinishing() 標誌還是爲 true ！你可以釋放資源了。
我們來看下 onDestroy 的官方解釋：
protected void onDestroy ()
Added in API level 1
Perform any final cleanup before an activity is destroyed. This can happen either because the activity is finishing (someone called finish() on it, or because the system is temporarily destroying this instance of the activity to save space. You can distinguish between these two scenarios with the isFinishing() method.
Note: do not count on this method being called as a place for saving data! For example, if an activity is editing data in a content provider, those edits should be committed in either onPause() or onSaveInstanceState(Bundle), not here. This method is usually implemented to free resources like threads that are associated with an activity, so that a destroyed activity does not leave such things around while the rest of its application is still running. There are situations where the system will simply kill the activity's hosting process without calling this method (or any others) in it, so it should not be used to do things that are intended to remain around after the process goes away.
Derived classes must call through to the super class's implementation of this method. If they do not, an exception will be thrown. 0xff
參考：http://blog.csdn.net/z1074971432/article/details/10517449