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I have a button playButton and two UIViews, myView1 and myView2, whose positions may change during execution.
I want the top of playButton to be 10 units below the bottom of UIView1 OR the bottom of UIView2, whichever is the larger value (further down).
How would I express this using auto layout in code? I tried setting one constraint as greater or equal but it seemed to have no effect.
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Auto Layout defines margins for each view. These margins describe the preferred spacing between the edge of the view and its subviews. You can access the view's margins using either the layoutMargins or layoutMarginsGuide property. The layoutMargins property lets you get and set the margins as a UIEdgeInsets structure.
What is Auto Layout? Auto Layout is a constraint-based layout system designed for building dynamically sized user interfaces. It lets you create user interface layouts that dynamically adapt for all screen sizes without manually setting the frame of every view.
translatesAutoresizingMaskIntoConstraints. A Boolean value that determines whether the view's autoresizing mask is translated into Auto Layout constraints.
Here's one way to think about it: create a constraint that the top of playButton is greater than or equal to the bottom of myView1 plus 10, another constraint that the top of playButton is greater than or equal to the bottom of myView2 plus 10, and then a third constraint that the top of playButton be at the top of the shared superview at a low priority.
The two inequalities will make sure the button is below the two views. However, that leaves ambiguity. The button could be anywhere below both. The third constraint can't be satisfied as such, but the auto layout system will try to get as close as possible. This resolves the ambiguity. The button will be as close to the top as possible while still being below both views.
This can actually be simplified. You could sort of combine one of the inequalities with the low-priority equality. Have one constraint that the top of playButton is greater than or equal to the bottom of myView1 plus 10. Have a second constraint that the top of playButton is equal to the bottom of myView2 plus 10, but at a lower priority.
If myView1's bottom is lower than myView2's, then the first constraint requires that playButton be lower than it. The second constraint can't be satisfied, but the system tries to get as close as possible to the bottom of myView2. That keeps the button as high as possible while still being below myView1's bottom. If myView2's bottom is lower than myView1's, then the second constraint determines the position of the button directly. The first constraint is satisfied, too, because it's an inequality.
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Illustration of KenThomases answer :
view1-> Blue
view 2-> red
view 3-> pink
Constraint between view1 and view 3: Greater than or equal to 20 with priority 1000.
Constraint between view2 and view 3: Equal to 20 with priority 999.
The working is explained by Ken in his answer. Just have a look.
If view 1 is greater than view 2
If view 2 is greater than view 1
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