在android 中儲存資料時經常用SharedPreference, 並且在提交資料時一直用的是Editor的commit方法, 今天無意了看到了系統用了apply,看了方法的介紹, 原來這個方法也是可以提交資料的.

apply方法在官方SDK說明如下：

Commit your preferences changes back from this Editor to the SharedPreferences object it is editing. This atomically performs the requested modifications, replacing whatever is currently in the SharedPreferences.

Note that when two editors are modifying preferences at the same time, the last one to call apply wins.

Unlike commit, which writes its preferences out to persistent storage synchronously, apply commits its changes to the in-memory SharedPreferences immediately but starts an asynchronous commit to disk and you won’t be notified of any failures. If another editor on this SharedPreferences does a regular commit while a apply is still outstanding, the commit will block until all async commits are completed as well as the commit itself.

As SharedPreferences instances are singletons within a process, it’s safe to replace any instance of commit with apply if you were already ignoring the return value.

You don’t need to worry about Android component lifecycles and their interaction with apply() writing to disk. The framework makes sure in-flight disk writes from apply() complete before switching states.

The SharedPreferences.Editor interface isn’t expected to be implemented directly. However, if you previously did implement it and are now getting errors about missing apply(), you can simply call commit from apply().

這兩個方法的區別在於： 
1. apply沒有返回值而commit返回boolean表明修改是否提交成功 
2. apply是將修改資料原子提交到記憶體, 而後非同步真正提交到硬體磁碟, 而commit是同步的提交到硬體磁碟，因此，在多個併發的提交commit的時候，他們會等待正在處理的commit儲存到磁碟後在操作，從而降低了效率。而apply只是原子的提交到內容，後面有呼叫apply的函式的將會直接覆蓋前面的記憶體資料，這樣從一定程度上提高了很多效率。 
3. apply方法不會提示任何失敗的提示。 
由於在一個程序中，sharedPreference是單例項，一般不會出現併發衝突，如果對提交的結果不關心的話，建議使用apply，當然需要確保提交成功且有後續操作的話，還是需要用commit的。