在数字键盘上添加button

//定义一个消息中心
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(keyboardWillShow:) name:UIKeyboardWillShowNotification object:nil]; //addObserver:注册一个观察员 name:消息名称
- (void)keyboardWillShow:(NSNotification *)note {
    // create custom button
    UIButton *doneButton = [UIButton buttonWithType:UIButtonTypeCustom];
    doneButton.frame = CGRectMake(0, 163, 106, 53);
    [doneButton setImage:[UIImage imageNamed:@"5.png"] forState:UIControlStateNormal];
    [doneButton addTarget:self action:@selector(addRadixPoint) forControlEvents:UIControlEventTouchUpInside];
    
    // locate keyboard view
    UIWindow* tempWindow = [[[UIApplication sharedApplication] windows] objectAtIndex:1];//返回应用程序window
    UIView* keyboard;
    for(int i=0; i<[tempWindow.subviews count]; i++) //遍历window上的所有subview
    {
        keyboard = [tempWindow.subviews objectAtIndex:i];
        // keyboard view found; add the custom button to it
        if([[keyboard description] hasPrefix:@"<UIKeyboard"] == YES)
        [keyboard addSubview:doneButton];
    }
}