今天來了一個問題：軟鍵盤無法彈出。分析后是因為系統判斷當前有外接硬鍵盤，就會隱藏軟鍵盤。但實際情況并不是這么簡單，該問題只有在特定條件下偶現，具體分析過程就不說了，就是軟硬鍵盤支持上的邏輯問題。借著這個機會整理一下鍵盤檢測的過程。

Configuration

Android系統中通過讀取Configuration中keyboard的值來判斷是否存在外接鍵盤。Configuration中關于鍵盤類型的定義如下，

  public static final int KEYBOARD_UNDEFINED = 0; // 未定義的鍵盤  public static final int KEYBOARD_NOKEYS = 1; // 無鍵鍵盤，沒有外接鍵盤時為該類型  public static final int KEYBOARD_QWERTY = 2; // 標準外接鍵盤  public static final int KEYBOARD_12KEY = 3; // 12鍵小鍵盤

在最常見的情況下，外接鍵盤未連接時keyboard的值為KEYBOARD_NOKEYS，當檢測到鍵盤連接后會將keyboard的值更新為KEYBOARD_QWERTY 。應用就可以根據keyboard的值來判斷是否存在外接鍵盤，InputMethodService.java中有類似的判斷代碼。

  // 軟件盤是否可以顯示  public boolean onEvaluateInputViewShown() {    Configuration config = getResources().getConfiguration();    return config.keyboard == Configuration.KEYBOARD_NOKEYS        || config.hardKeyboardHidden == Configuration.HARDKEYBOARDHIDDEN_YES;  }  

現在的問題就轉向Configuration的keyboard是如何更新的。在WindowManagerService.java中，應用啟動時會更新Configuration，相關代碼如下。

  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;      // 默認值為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;      // 遍歷輸入設備      for (int i = 0; i < len; i++) {        InputDevice device = devices[i];        // 如果不是虛擬輸入設備，會根據輸入設備的flags來更新Configuration        if (!device.isVirtual()) {          ......          // 如果輸入設備的鍵盤類型為KEYBOARD_TYPE_ALPHABETIC，則將keyboard設置為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;      // 更新硬件鍵盤狀態      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被設置，將keyboard設置為KEYBOARD_NOKEYS，讓軟件盤可以顯示      if (mShowImeWithHardKeyboard) {        config.keyboard = Configuration.KEYBOARD_NOKEYS;      }      ......    }