Layout: How to use property in visual format

Question

I have two classes.

Parent class has a property:

@interface ParentVC

    @property (nonatomic, strong) UIImageView* logoImageView;
@end

So, now, I need to output this view with layout constraints in subclass.

@interface ChildVC : ParentVC
@end

How to correct format string of visual format?

@"V:|-(%i)-[self.logoImageView]" // wrong

I found solution:

UIView* selfLogoImageView = self.logoImageView;

@"V:|-(%i)-[selfLogoImageView]"

But is there any method without new variable?

Answer 1

Since you are using the visual format language, you are also using [NSLayoutConstraint constraintsWithVisualFormat:options:metrics:views]. That method expects a dictionary for the views: parameter. Are you using NSDictionaryOfVariableBindings? If so, you don’t have to. You can pass in any old dictionary, as long as it maps your views to names that you use in the visual format string. Incidentally, you can also pass a metrics dictionary, instead of what you are doing with the %i format specifier.

NSDictionary *views = @{ @"logo" : self.logoImageView, // use any name you want here
                         @"label" : self.someLabelView };

// kSomeSpacerConstant is an int or float primitive constant.
// We are wrapping it in @() because this dictionary needs NSNumbers as values.
NSDictionary *metrics = @{ @"spacer" : @(kSomeSpacerConstant) };

[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-(spacer)-[logo]-[label]"
                                                                  options:0
                                                                  metrics:metrics
                                                                    views:views]];

If you don’t want to make a new NSDictionary just to hold your metrics or views, just pass in the literal @{ ... } syntax for the metrics: or views: parameters. (But I advise against it for the sake of cleaner code, or if you are building more than one visual format string and you want to reuse the dictionaries.)