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;
      }
      ......
    }