Android中外接键盘的检测的实现

今天来了一个问题：软键盘无法弹出。分析后是因为系统判断当前有外接硬键盘，就会隐藏软键盘。但实际情况并不是这么简单，该问题只有在特定条件下偶现，具体分析过程就不说了，就是软硬键盘支持上的逻辑问题。借着这个机会整理一下键盘检测的过程。

Configuration

Android系统中通过读取Configuration中keyboard的值来判断是否存在外接键盘。Configuration中关于键盘类型的定义如下，

  public static final int KEYBOARD_UNDEFINED = 0; // 未定义的键盘  public static final int KEYBOARD_NOKEYS = 1; // 无键键盘，没有外接键盘时为该类型  public static final int KEYBOARD_QWERTY = 2; // 标准外接键盘  public static final int KEYBOARD_12KEY = 3; // 12键小键盘

在最常见的情况下，外接键盘未连接时keyboard的值为KEYBOARD_NOKEYS，当检测到键盘连接后会将keyboard的值更新为KEYBOARD_QWERTY 。应用就可以根据keyboard的值来判断是否存在外接键盘，InputMethodService.java中有类似的判断代码。

  // 软件盘是否可以显示  public boolean onEvaluateInputViewShown() {    Configuration config = getResources().getConfiguration();    return config.keyboard == Configuration.KEYBOARD_NOKEYS        || config.hardKeyboardHidden == Configuration.HARDKEYBOARDHIDDEN_YES;  }  

现在的问题就转向Configuration的keyboard是如何更新的。在WindowManagerService.java中，应用启动时会更新Configuration，相关代码如下。

  boolean computeScreenConfigurationLocked(Configuration config) {    ......    if (config != null) {      // Update the configuration based on available input devices, lid switch,      // and platform configuration.      config.touchscreen = Configuration.TOUCHSCREEN_NOTOUCH;      // 默认值为KEYBOARD_NOKEYS      config.keyboard = Configuration.KEYBOARD_NOKEYS;      config.navigation = Configuration.NAVIGATION_NONAV;            int keyboardPresence = 0;      int navigationPresence = 0;      final InputDevice[] devices = mInputManager.getInputDevices();      final int len = devices.length;      // 遍历输入设备      for (int i = 0; i < len; i++) {        InputDevice device = devices[i];        // 如果不是虚拟输入设备，会根据输入设备的flags来更新Configuration        if (!device.isVirtual()) {          ......          // 如果输入设备的键盘类型为KEYBOARD_TYPE_ALPHABETIC，则将keyboard设置为KEYBOARD_QWERTY          if (device.getKeyboardType() == InputDevice.KEYBOARD_TYPE_ALPHABETIC) {            config.keyboard = Configuration.KEYBOARD_QWERTY;            keyboardPresence |= presenceFlag;          }        }      }      ......      // Determine whether a hard keyboard is available and enabled.      boolean hardKeyboardAvailable = config.keyboard != Configuration.KEYBOARD_NOKEYS;      // 更新硬件键盘状态      if (hardKeyboardAvailable != mHardKeyboardAvailable) {        mHardKeyboardAvailable = hardKeyboardAvailable;        mH.removeMessages(H.REPORT_HARD_KEYBOARD_STATUS_CHANGE);        mH.sendEmptyMessage(H.REPORT_HARD_KEYBOARD_STATUS_CHANGE);      }      // 如果Setting中SHOW_IME_WITH_HARD_KEYBOARD被设置，将keyboard设置为KEYBOARD_NOKEYS，让软件盘可以显示      if (mShowImeWithHardKeyboard) {        config.keyboard = Configuration.KEYBOARD_NOKEYS;      }      ......    }