1. 外接键盘

iOS9 及以上 UIKeyboard[Will|Did][Show|Hide]Notification 这些通知照样有用，键盘 frame 改变时，如果计算键盘的位置超出 keyboardWindow 就是用了外接键盘。

1.1. 通过键盘高度位置判断

参考 https://github.com/shimohq/react-native-keyboard-view/blob/master/ios/YYKeyboardManager.m#L521-L533

在 UIKeyboardDidChangeFrameNotification 的时候通过下面逻辑判断

- (void)updateInHardwareKeyboardMode:(CGRect)keyboardFrame {
    CGRect keyboardWindowFrame = [self keyboardWindow].frame;
    if (CGRectGetMaxY(keyboardFrame) == CGRectGetMaxY(keyboardWindowFrame)) {
        // >= iOS11, CGRectGetHeight(after) < 200 when in hardware keyboard mode
        [self setInHardwareKeyboardMode:CGRectGetHeight(keyboardFrame) < 200];
    } else if (CGRectGetMinY(keyboardFrame) == CGRectGetMaxY(keyboardWindowFrame) && CGRectGetHeight(keyboardFrame) < 200) {
        // iPhone in hardware keyboard mode
        [self setInHardwareKeyboardMode:YES];
    } else if (CGRectGetMinY(keyboardFrame) < CGRectGetMaxY(keyboardWindowFrame) && CGRectGetMaxY(keyboardFrame) > CGRectGetMaxY(keyboardWindowFrame)) {
        // iPad in hardware keyboard mode
        [self setInHardwareKeyboardMode:YES];
    }
}

1.2. UIKeyboardImpl

https://gist.github.com/myell0w/d8dfabde43f8da543f9c

使用私有 API，可能被苹果封

1.3. References

• How can I detect if an external keyboard is present on an iPad?