Source share: SharedPreferencesCompat.java

Firstly look at the code:

/**
 * Reflection utils to call SharedPreferences$Editor.apply when possible,
 * falling back to commit when apply isn't available.
 */
public class SharedPreferencesCompat {
    private static final Method sApplyMethod = findApplyMethod();

    private static Method findApplyMethod() {
        try {
            Class cls = SharedPreferences.Editor.class;
            return cls.getMethod("apply");
        } catch (NoSuchMethodException unused) {
            // fall through
        }
        return null;
    }

    public static void apply(SharedPreferences.Editor editor) {
        if (sApplyMethod != null) {
            try {
                sApplyMethod.invoke(editor);
                return;
            } catch (InvocationTargetException unused) {
                // fall through
            } catch (IllegalAccessException unused) {
                // fall through
            }
        }
        editor.commit();
    }
}

这是google I/O 2010演讲Writing zippy Android apps演示项目里的一段代码，作者 Brad Fitzpatrick。
背景：
起先，Android的Editor只有commit一个提交方法：

/**
         * 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.
         *
         * <p>Note that when two editors are modifying preferences at the same
         * time, the last one to call commit wins.
         *
         * <p>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();

后来呢，应该是在开发Gingerbread的时候，这帮googlers折腾了一堆优化性能的方法，然后就出来一个appy接口：

/**
         * 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.
         *
         * <p>Note that when two editors are modifying preferences at the same
         * time, the last one to call apply wins.
         *
         * <p>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.
         *
         * <p>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.
         *
         * <p>You don't need to worry about Android component
         * lifecycles and their interaction with <code>apply()</code>
         * writing to disk.  The framework makes sure in-flight disk
         * writes from <code>apply()</code> complete before switching
         * states.
         *
         * <p class='note'>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 <code>apply()</code>, you can simply call
         * {@link #commit} from <code>apply()</code>.
         */
        void apply();

看完这两个方法的说明，你大概就明白了是神马意思了吧。反正是apply在某种程度上降低了main thread的负担，但还能保证数据修改的很及时。(注意这部分：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)

因此，出现这个SharedPreferencesCompat来兼容一下这两个api（compat就是compatible的缩写呀）。

ps:嗯。gingerbread为虾米快呢，这下有点了解了吧。。