Increase the height of iOS 8 custom keyboard without working

I tried to increase the height of the custom keyboard using an Apple document but it doesn't work. Here is my code:

-(void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    CGFloat _expandedHeight = 500;
    NSLayoutConstraint *_heightConstraint = [NSLayoutConstraint constraintWithItem:self.view attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute
                                                                        multiplier:0.0 constant: _expandedHeight];
    [self.view addConstraint: _heightConstraint];
    [super updateViewConstraints];

}