A straightforward C++ DSL to more concisely define layout constraints.

Constraint Expressions

The standard syntax for defining layout constraints is verbose. For example, to specify the x offset of one button relative to another with a 5 point gap between them, one would write:

self.view addConstraint:[NSLayoutConstraint constraintWithItem:_button2
                                                      attribute:NSLayoutAttributeLeft
                                                      relatedBy:NSLayoutRelationEqual
                                                         toItem:_button1
                                                      attribute:NSLayoutAttributeRight
                                                     multiplier:1.0
                                                       constant:5.0]];

With the layout DSL, this would simply be written as:

View(_button2).left == View(_button1).right + 5.0;

or more naturally as:

View(_button1).right + 5.0 == View(_button2).left;

That's it. When this expression is executed, the NSLayoutConstraint object will be created and automatically installed in the nearest common ancestor of _button1 and _button2.

Adding some sugar

If you're willing to compile your code as objective-C++, you can access an even more consise constraint definition. Just #import "UIView+AutoLayoutDSLSugar.h" and the constraint referred to above can be defined as:

_button1.right + 5.0 == _button2.left;

Now that's just about perfect.

Constraint installation

As mentioned before, just declaring a constraint expression is sufficient to install that constraint in the nearest common ancestor of the referenced views, but if you'd rather install the constraint yourself, you can simply use a constraint expression in place of an actual NSLayoutConstraint object in a call to addConstraint:. The act of passing the expression to the method will prevent the constraint from being automatically installed.

[self.view addConstraint:View(_button1).left == View(_button2).right + 5.0];

Basically, a constraint expression can be used anywhere an NSLayoutConstraint object is expected. For example, multiple constraints can be installed as follows:

[self.view addConstraints:@[View(_button1).right + 5.0 == View(_button2).left,
                            View(_button2).right + 5.0 == View(_button3).left]];

But why bother when this is so much more concise?

View(_button1).right + 5.0 == View(_button2).left;
View(_button2).right + 5.0 == View(_button3).left;

Constraints referencing superviews

To specify a constraint on a view relative to its superview, just omit the UIView* parameter from the View specification:

View().left + 5.0 == View(_button1).left;

Priorities

Priorities can be specified in a constraint expression by using the ^ operator, like so:

View(_button1).left == View(_button2).right + 5.0 ^ 999.0;

Constraint Identification

You can add an identifier to a single constraint or a group of constraints to more easily remove and replace them, when necessary. There are a few ways to accomplish this.

Using identifiers with single constraint

Just like adding priorities, identifiers can be added to a constraint expression by using the ^ operator, like so:

View(_image).width == View(_image).height * aspectRatio ^ @"maintainAspect";

Adding an identifier to a group of constraints

You can group constraints by assigning them the same identifier:

View(_avatar).left == View().left + 5.0 ^ @"A group of constraints";
View(_label).left == View(_avatar).right + StandardHorizontalGap ^ @"A group of constraints";

But a much cleaner way is by using the contraint grouping macros:

BeginConstraintGroup(@"A group of constraints")

View(_avatar).left == View().left + 5.0;
View(_label).left == View(_avatar).right + StandardHorizontalGap;

EndConstraintGroup;

Finding constraints

To find a single constraint given its identifier:

NSLayoutConstraint *constraint = [self.view constraintWithID:@"identifier"];

To find all constraints with a given identifier:

NSArray *constraint = [self.view constraintsWithID:@"identifier"];

To find all constraints referencing a given view:

NSArray *constraints = [self.view constraintsReferencingView:_button1];

Removing constraints

To remove a constraint, simply call remove

NSLayoutConstraint *constraint = [self.view constraintWithID:@"identifier"];
[constraint remove];

Caveats

Objective-C++ (C++11)

This DSL requires that your source file be compiled as Objective-C++ (just change its extension to .mm). This may be a concern to some. If you don't want to deal with any potential problems this may cause, you can still use the DSL by defining all of your constraints in a separate file using categories.

Auto vs. manual constraint installation

I used some unorthodox methods to acheive the results I was looking for. The ability to have a constraint auto-install itself is acheived by installing the newly built NSLayoutContraint object from the destructor of the internal ConstraintBuilder object. The ability to use a constraint expression in place of an NSLayoutConstraint object is acheived through an overloaded cast operator on the internal ConstraintBuilder class. In order to prevent multiple constraint installs, any casting the ConstraintBuilder object will transfer ownership of the newly built NSLayoutConstraint object to the caller, so when the ConstraintBuilder object is destroyed, there is no NSLayoutConstraint object to install. Unfortunately, this prevents directly obtaining a pointer to an auto-installed constraint:

// The following constraint has not been automatically installed
NSLayoutConstraint *constraint = View(_avatar).left == View().left + 5.0;

This is a small price to pay, since the auto-install behavior can easily be invoked:

[constraint install];

Regardless, this kind of magic behavior will be disliked by some.

Compiler warnings

Unfortunately, the compiler/static analyzer will most likely try to warn you that your free-standing constraint expressions are unused.

The only way to prevent this is to disable the warning. You can wrap your constraint expressions like so:

_Pragma( "clang diagnostic push" )
_Pragma( "clang diagnostic ignored \"-Wunused-value\" " )

View(_avatar).left == View().left + 5.0;

_Pragma( "clang diagnostic pop")

But that's pretty ugly. To clean this up a bit, I've added a couple of macros to wrap constraints with:

BeginConstraints

View(_avatar).left == View().left + 5.0;

EndConstraints;

The constraint grouping macros BeginConstraintGroup() and EndConstraintGroup also disable these warnings.

Syntax

The main expression syntax is pretty fixed, but the method of adding identifiers and priorities may change. The code currently allows for these to be added with the ^ operator, and also the , operator. I'm open to suggested improvements.

Alternatives

There are quite a few attempts to improve on the default constraint syntax. These are just a few that caught my attention:

• NSLayoutEquations
• NSLayoutConstraint+ExpressionFormat
• KeepLayout
• FLKAutoLayout
• Masonry
• Parus

Installation

AutoLayoutDSL is available with CocoaPods[http://cocoapods.org], to install simply add the following line to your Podfile:

pod AutoLayoutDSL

Author

David Whetstone [email protected]

Acknowledgements

The constraint expressions are my own design, but the ideas of constraint identification, grouping, and self-install are all thanks to Erica Sadun and her excellent book iOS Auto-Layout Demystified. Much of the code related to these features are massaged versions of the sample code included with that book, found here.

License

AutoLayoutDSL is available under the MIT license. See the LICENSE file for more info.