Android UEventObserver

UEventObserver是Android Java層利用uevent與獲取Kernel層狀態變化的機制。

通過grep發現framework有如下模塊使用UEventObserver的功能來提供服務：
電池狀態：services/java/com/android/server/BatteryService.java
耳機狀態：services/java/com/android/server/HeadsetObserver.java
DOCK狀態：services/java/com/android/server/DockObserver.java
USB狀態：services/java/com/android/server/usb/UsbService.java

它們全部繼承自UEventObserver，先看看這個類的構造和原理：
./core/java/android/os/UEventObserver.java
                              |
          [ native_setup(), next_event() ]
                             \|/
./core/jni/android_os_UEventObserver.cpp
                              |
          [ uevent_init(),uevent_next_event() ]
                             \|/
/hardware/libhardware_legacy/uevent/uevent.c
                              |                                                                                         [userspace]
---------------------[socket]-----------------------------------------------------------------------------
                              |
                             \|/                                                                                           [kernel]
     socket(PF_NETLINK, SOCK_DGRAM, NETLINK_KOBJECT_UEVENT)

下面用HeadsetObserver作為例子說明如何使用UEventObserver來監聽kernel的uevent。
繼承UEventObserver的類必須實現自己的public abstract void onUEvent(UEvent event)：

1. @Override  
2. public void onUEvent(UEventObserver.UEvent event) {  
3.     if (LOG) Slog.v(TAG, "Headset UEVENT: " + event.toString());  
4.   
5.     try {  
6.         update(event.get("SWITCH_NAME"), Integer.parseInt(event.get("SWITCH_STATE"))); // update中處理事務   
7.     } catch (NumberFormatException e) {  
8.         Slog.e(TAG, "Could not parse switch state from event " + event);  
9.     }  
10. }  

這個函數會在UEventObserver接收到event的時候由UEventObserver來回調，HeadsetObserver使用startObserving("DEVPATH=/devices/virtual/switch/h2w")來開始監聽，這個API會確保sThread已經運行並且以字串參數作為匹配參數增加一個observer:
    public final synchronized void startObserving(String match) {
        ensureThreadStarted();
        sThread.addObserver(match, this);
    }