Question

In language Cocoa uses to define view constraints, examples are always named “button1”, “button2”, etc. But what do this names actually mean? Are they names of the nub files, the class names of controllers or views? If I instantiate several views of the same nib file and class, what names do they get, and how can I set them?

Answer 1

They are placeholders that will be associated with actual buttons in your nib file.

Update

If you’re adding constraints in code using addConstraints:options:metrics:views: method on view you’re trying to constraint (whether that instance of UIView was created in code or loaded from nib), the last parameter (views:) accepts dictionary who’s keys are the ones you provide in UIConstraint‘s ascii art (button1, button2, etc.) and values are actual views that you create in code or get the instances from somewhere else (IBOutlet maybe).

You can create the dictionary quickly by using NSDictionaryOfVariableBindings macro which will accept variable number of your instances of views and associate them with same placeholder names like names of those instances. E.g.

// This is a ASCII constraint:
NSString *constraintAscii = @"|-[button1]-12-[button2]-|";

// Bound views for constraints:
UIButton *button2 = ... // code to create button2 variable
// let's say that button1 is actually an IBOutlet for instance variable
// now we create the dictionary of views for our new constraint
NSDictionary *views = NSDictionaryOfVariableBindings(button1, button2);

// now we create the constraint using these variables
NSArray *constraints = [NSLayoutConstraint constraintsWithVisualFormat: constraintAscii
                                                               options: 0
                                                               metrics: nil
                                                                 views: views];