为什么 getSystemService(string) 方法只是 Activity class 中的存根？

Question

我刚刚开始学习 Android，所以我在 Android Studio 中四处闲逛并学习了一些新的键盘快捷键。我对 getSystemService(string) 方法进行了快速定义查找，并将其提取出来：

此方法的来源是否以某种方式隐藏了？它不能真的只是抛出一个 RuntimeException，对吧......?

Answer 1

它不是存根，但源代码是隐藏的，当您实际调用此方法时可以看到这一点的证据，并且效果理想。

此外，您会在属于 Android Framework 的各种类中找到许多此类方法，它们不会公开处理框架的实际逻辑。

Answer 2

如果您想真正了解 getSystemService 的作用，请查看 Github 上的 android 源代码。

这是继承树。

Activity -> ContextThemeWrapper -> Context

在 Context.java 中您可以找到它的实际作用。

/**
 * Return the handle to a system-level service by class.
 * <p>
 * Currently available classes are:
 * {@link android.view.WindowManager}, {@link android.view.LayoutInflater},
 * {@link android.app.ActivityManager}, {@link android.os.PowerManager},
 * {@link android.app.AlarmManager}, {@link android.app.NotificationManager},
 * {@link android.app.KeyguardManager}, {@link android.location.LocationManager},
 * {@link android.app.SearchManager}, {@link android.os.Vibrator},
 * {@link android.net.ConnectivityManager},
 * {@link android.net.wifi.WifiManager},
 * {@link android.media.AudioManager}, {@link android.media.MediaRouter},
 * {@link android.telephony.TelephonyManager}, {@link android.telephony.SubscriptionManager},
 * {@link android.view.inputmethod.InputMethodManager},
 * {@link android.app.UiModeManager}, {@link android.app.DownloadManager},
 * {@link android.os.BatteryManager}, {@link android.app.job.JobScheduler},
 * {@link android.app.usage.NetworkStatsManager}.
 * </p><p>
 * Note: System services obtained via this API may be closely associated with
 * the Context in which they are obtained from.  In general, do not share the
 * service objects between various different contexts (Activities, Applications,
 * Services, Providers, etc.)
 * </p>
 *
 * @param serviceClass The class of the desired service.
 * @return The service or null if the class is not a supported system service.
 */
@SuppressWarnings("unchecked")
public final <T> T getSystemService(Class<T> serviceClass) {
    // Because subclasses may override getSystemService(String) we cannot
    // perform a lookup by class alone.  We must first map the class to its
    // service name then invoke the string-based method.
    String serviceName = getSystemServiceName(serviceClass);
    return serviceName != null ? (T)getSystemService(serviceName) : null;
}

为什么 getSystemService(string) 方法只是 Activity class 中的存根？

Why is the getSystemService(string) method just a stub in the Activity class?

java

android

android-service

android-studio