How to set constraints programmatically for a varying number of UIView

Question

I'm new to auto layout constraints. In my application there are 3 buttons in one line. Now I want to set the constraints programatically as shown in the image below:

If there is one, then the button show in center horizontally. If two buttons are enable, then second line shown in same image as three image.

Answer 1

My approach is to setting all buttons centerX to superview's centerX at the beginning.

IBOutletCollection(UIButton) NSArray *allButtons;
//3 buttons' references
IBOutletCollection(NSLayoutConstraint) NSArray *allButtonCenterConstraints;
//buttons' centerX constraints' references

and then if in viewDidAppear:

int t = arc4random() %3;// one of the 3 cases you will need


    if (t == 0) {//if you want all 3 buttons appears
        //all buttons shown
        NSLayoutConstraint *c1 = allButtonCenterConstraints[0];//first button's centerX reference.
        NSLayoutConstraint *c2 = allButtonCenterConstraints[2];//third button's centerX reference.

        c1.constant = -self.view.frame.size.width*0.25;//push left
        c2.constant = +self.view.frame.size.width*0.25;//push right
    }
    else if (t == 1)
    {
        //two buttons shown;
        [allButtons[0] setHidden:YES];// close the one you dont need
        NSLayoutConstraint *c1 = allButtonCenterConstraints[1];
        NSLayoutConstraint *c2 = allButtonCenterConstraints[2];

        c1.constant = -self.view.frame.size.width*0.125;
        c2.constant = +self.view.frame.size.width*0.125;

    }
    else
    {
        //close 2 buttons
        [allButtons[0] setHidden:YES];
        [allButtons[1] setHidden:YES];
    }
    [self.view layoutIfNeeded];

if you need something depends on the width of buttons, you can set the constants according to the width of buttons.