動態獲取鍵盤的高度

- (void)registerForKeyboardNotifications{
    
    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(keyboardWasShown:)
                                                 name:UIKeyboardWillChangeFrameNotification
                                               object:nil];
    
    [[NSNotificationCenter defaultCenter]  addObserver:self
                                              selector:@selector(keyboardWasHidden:)
                                                  name:UIKeyboardWillHideNotification
                                                object:nil];
    
}
- (void)keyboardWasShown:(NSNotification *)notif{
    
    CGRect keyboardEndFrameWindow;
    [[notif.userInfo valueForKey:UIKeyboardFrameEndUserInfoKey] getValue: &keyboardEndFrameWindow];
    
    double keyboardTransitionDuration;
    [[notif.userInfo valueForKey:UIKeyboardAnimationDurationUserInfoKey] getValue:&keyboardTransitionDuration];
    
    UIViewAnimationCurve keyboardTransitionAnimationCurve;
    [[notif.userInfo valueForKey:UIKeyboardAnimationCurveUserInfoKey] getValue:&keyboardTransitionAnimationCurve];
    
    CGRect keyboard = [self.view convertRect:keyboardEndFrameWindow fromView:nil];
    
    CGRect rect=CGRectMake(0, ScreenHeight-keyboard.size.height-40, ScreenWidth, 40);
    [UIView animateWithDuration:0.2 animations:^{
        _sendMsgView.frame=rect;
    }];
}

- (void) keyboardWasHidden:(NSNotification *) notif{
    CGRect rect=CGRectMake(0, ScreenHeight-90, ScreenWidth, 40);
    [UIView animateWithDuration:0.2 animations:^{
        _sendMsgView.frame=rect;
    }];
}