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这是 的第六期。本期我们将来探讨一个有趣的东西 —— SharePrefrence 的两种提交方式 apply() 和 commit()。
往期内容传递：
其实非常有趣，我们经常在开发中使用 SharePrefrence 保存一些轻量级数据，比如判断是否是首次启动，首次启动进入引导页，否则直接到主页面，或者是其它的一些应用场景。
而我们也耳熟能详这样的写法。
public class SplashActivity extends AppCompatActivity { public static final String SP_KEY = "com.zxedu.myapplication.SplashActivity_SP_KEY"; public static final String IS_FIRST_IN = "com.zxedu.myapplication.SplashActivity_IS_FIRST_IN"; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); // 一些业务代码 ...... SharedPreferences preferences = getSharedPreferences("name",MODE_PRIVATE); if (preferences.getBoolean(IS_FIRST_IN,true)){ // 跳转引导页面 startActivity(new Intent(this,GuideActivity.class)); finish(); }else{ // 跳转主页面 startActivity(new Intent(this,MainActivity.class)); } }}public class GuideActivity extends AppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_splash); getSharedPreferences(SplashActivity.SP_KEY,MODE_PRIVATE).edit().putBoolean(SplashActivity.IS_FIRST_IN,false).apply(); }} 从代码中可以看到，一阵混乱操作，没啥特别的地方，但早期开发的人员应该知道，之前我们都是比较青睐 commit() 进行提交的。而现在 Android Studio 在我们使用 commit() 直接提交的时候会直接报黄色警告。
先说说相同点：
不同点直接上源码吧，先看看 commit() 方法的定义：
/** * Commit your preferences changes back from this Editor to the * {@link 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 commit wins. * *
If you don't care about the return value and you're * using this from your application's main thread, consider * using {@link #apply} instead. * * @return Returns true if the new values were successfully written * to persistent storage. */ boolean commit();
综合一下 commit() 方法的注释也就是两点：
apply()。再看看 apply() 方法的定义：
/** * Commit your preferences changes back from this Editor to the * {@link 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 {@link #commit}, which writes its preferences out * to persistent storage synchronously, {@link #apply} * commits its changes to the in-memory * {@link SharedPreferences} immediately but starts an * asynchronous commit to disk and you won't be notified of * any failures. If another editor on this * {@link SharedPreferences} does a regular {@link #commit} * while a {@link #apply} is still outstanding, the * {@link #commit} will block until all async commits are * completed as well as the commit itself. * *
As {@link SharedPreferences} instances are singletons within * a process, it's safe to replace any instance of {@link #commit} with * {@link #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 * {@link #commit} from apply(). */ void apply();
略微有点长，大概意思就是 apply() 跟 commit() 不一样的地方是，它使用的是异步而不是同步，它会立即将更改提交到内存，然后异步提交到硬盘，并且如果失败将没有任何提示。
总结一下不同点：
commit() 是直接同步地提交到硬件磁盘，因此，多个并发的采用 commit() 做提交的时候，它们会等待正在处理的 commit() 保存到磁盘后再进行操作，从而降低了效率。而 apply() 只是原子的提交到内容，后面再调用 apply() 的函数进行异步操作。apply() 返回值为 void，而 commit() 返回一个 boolean 值代表是否提交成功。apply() 方法不会有任何失败的提示。大多数情况下，我们都是在同一个进程中，这时候的 SharedPrefrence 都是单实例，一般不会出现并发冲突，如果对提交的结果不关心的话，我们非常建议用 apply() ，当然需要确保操作成功且有后续操作的话，还是需要用 commit() 的。
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