Auto Layout Shorthand

Auto Layout Shorthand (ALS) is an alternative system for creating and adding Auto Layout constraints.

It feels kind of like CSS, though more powerful and without HTML's frustrating default layout model.

Here's a simple example to introduce ALS:

[iconView als_addConstraints:@{
 @"left ==": als_superview,
 @"width ==": @(kIconWidth),
 @"top ==": als_superview,
 @"height ==": @(kIconHeight),
 }];

Here's mostly¹ the same example using the Visual Format Language (VFL):

[iconView.superview addConstraints:@[
 [NSLayoutConstraint constraintsWithVisualFormat:@"H:|[iconView(kIconWidth)]"
                                         options:0
                                         metrics:NSDictionaryOfVariableBindings(kIconWidth)
                                           views:NSDictionaryOfVariableBindings(iconView)],
 [NSLayoutConstraint constraintsWithVisualFormat:@"V:|[iconView(kIconHeight)]"
                                         options:0
                                         metrics:NSDictionaryOfVariableBindings(kIconHeight)
                                           views:NSDictionaryOfVariableBindings(iconView)],
 ]];

And mostly² the same example using +[NSLayoutConstraint constraintWithItem:…]:

[iconView.superview addConstraints:@[
 [NSLayoutConstraint constraintWithItem:iconView
							  attribute:NSLayoutAttributeLeft
							  relatedBy:NSLayoutRelationEqual
								 toItem:iconView.superview
							  attribute:NSLayoutAttributeLeft
							 multiplier:1.0
							   constant:0.0],
 [NSLayoutConstraint constraintWithItem:iconView
							  attribute:NSLayoutAttributeWidth
							  relatedBy:NSLayoutRelationEqual
								 toItem:nil
							  attribute:NSLayoutAttributeNotAnAttribute
							 multiplier:1.0
							   constant:kIconWidth],
 [NSLayoutConstraint constraintWithItem:iconView
							  attribute:NSLayoutAttributeTop
							  relatedBy:NSLayoutRelationEqual
								 toItem:iconView.superview
							  attribute:NSLayoutAttributeTop
							 multiplier:1.0
							   constant:0.0],
 [NSLayoutConstraint constraintWithItem:iconView
							  attribute:NSLayoutAttributeHeight
							  relatedBy:NSLayoutRelationEqual
								 toItem:nil
							  attribute:NSLayoutAttributeNotAnAttribute
							 multiplier:1.0
							   constant:kIconHeight],
 ]];

Auto Layout Shorthand Reference

Auto Layout Shorthand is a poor man's DSL wedged into a normal Objective-C dictionary literal.

Each key-value pair contains enough information to create one NSLayoutConstraint. This stands in contrast to VFL, where one string can be used to generate multiple constraints.

The dictionary key encodes two pieces of information: the NSLayoutConstraint's firstAttribute and its relation. Here's some examples:

@"width >="
@"height <="
@"centerX =="

For the first part of the key, the attribute, every NSLayoutAttribute is supported except NSLayoutAttributeNotAnAttribute. See the ALS Dictionary Key Name column below (we'll get to ALS Dictionary Value Name in a moment):

NSLayoutAttribute constant	ALS Dictionary Key Name	ALS Dictionary Value Name
NSLayoutAttributeLeft	left	als_left
NSLayoutAttributeRight	right	als_right
NSLayoutAttributeTop	top	als_top
NSLayoutAttributeBottom	bottom	als_bottom
NSLayoutAttributeLeading	leading	als_leading
NSLayoutAttributeTrailing	trailing	als_trailing
NSLayoutAttributeWidth	width	als_width
NSLayoutAttributeHeight	height	als_height
NSLayoutAttributeCenterX	centerX	als_centerX
NSLayoutAttributeCenterY	centerY	als_centerY
NSLayoutAttributeBaseline	baseline	als_baseline

The second part of the dictionary key encodes the NSLayoutRelation in the obvious manner:

NSLayoutRelation constant	Auto Layout Shorthand equivalent
NSLayoutRelationLessThanOrEqual	<=
NSLayoutRelationEqual	==
NSLayoutRelationGreaterThanOrEqual	>=

The dictionary value encodes either a relation or a constant. Simple relations and simple constants are directly assigned:

@"top ==": headerView.als_bottom
@"width ==": @(42)

Let's talk about headerView.als_bottom some more. als_bottom is a method added as a category to UIView. Auto Layout Shorthand adds a suite of methods, one for each NSLayoutAttribute. That's the ALS Dictionary Value Name column above.

These categories enable you to refer to both a view and an attribute in one expression. The result is a simple class that just packages up both of them into one object that's later consumed by -[UIView(AutoLayoutShorthand) als_addConstraints:].

You can use a dictionary to specify more complex constraints:

@"top ==": @{als_view:headerView.als_bottom, als_constant:@(10)},
@"width ==": @{als_constant:@(42), als_priority:@(UILayoutPriorityDefaultHigh)]

Supported keys are:

Auto Layout Shorthand Key	Corresponding NSLayoutConstraint property
als_view	secondItem
als_multiplier	multiplier
als_constant	constant
als_priority	priority

Finally, VFL has @"|", which represents the superview. als_superview is ALS's version of the same thing.

Auto Layout Shorthand Advantages

More concise than +[NSLayoutConstraint constraintWithItem:…] but just as powerful. Actually, just a smidge more powerful since you can specify the constraint's priority at creation time.
Easier to read+understand than +[NSLayoutConstraint constraintWithItem:…].
Often more concise than even Visual Format Language, yet more powerful (Auto Layout Shorthand can specify centerX and centerY).
Refactoring-friendly. Visual Format Language strings are opaque to Xcode's refactoring support. This results in nasty runtime exceptions if you use refactoring to rename a variable or property and forget to update any corresponding VFL string.

View Property-friendly. This code doesn't work:

  [NSLayoutConstraint constraintsWithVisualFormat:@"H:|[self.iconView(32)]"
                                          options:0
                                          metrics:nil
                                            views:NSDictionaryOfVariableBindings(self.iconView)];

apparently because of the self.. One work-around is to access the ivar directly:

  [NSLayoutConstraint constraintsWithVisualFormat:@"H:|[_iconView(32)]"
                                          options:0
                                          metrics:nil
                                            views:NSDictionaryOfVariableBindings(_iconView)];

which I don't much care for since it bypasses the accessor. I recommend creating the dictionary yourself:

  [NSLayoutConstraint constraintsWithVisualFormat:@"H:|[iconView(32)]"
  	                                    options:0
                                          metrics:nil
                                            views:@{@"iconView": self.iconView}];

or using ALS.

Built-in Closest-Common-Superview Discovery. When relating attributes across views, Auto Layout Shorthand automatically calculates the views' closest common superview and adds the generated constraints there. This is the Apple-recommended place to put your constraints.

Auto Layout Shorthand Disadvantages

Another Dependancy. Not that bad though, since it's a self-contained .h/.m pair.
Another thing to learn. You probably have to learn VFL anyway to understand Auto Layout's debugging logs.
Encourages you to think of constraints being applied to the target view itself instead of the superview they're actually added to. I think removing the level of indirection is worth it.
Like Only God Can Create A Tree, Only VFL Can Create NSSpacers. Fortunately ALS plays nicely with VFL, so use VFL if you want to take advantage of spacers.

TODO

Write an example apps to showcase common scenarios.
Either upgrade als_superview to a UIView or remove it altogether. Probably the former -- it's not strictly needed (you can always just myview.superview, but the conceptual clarity is worth a more-complicated implementation AFAICS.

Version History

v1.0: Apr 06 2014

[CHANGE] Allow either view to be the lowest common ancestor. (Andrew Hershberger)
[FIX] Implicit conversion warnings. (Andrew Hershberger)
[FIX] Priority doesn't work. (Joe Szymanski)

v0.4: Jun 24 2013

[NEW] Improved handling for groups of constraints (the common case):
- als_addConstraints: now returns an array of the constraints its created.
- als_activateConstraints and als_deactivateConstraints category methods on NSArray allow enabling and disabling groups of NSLayoutConstraints.
  
  Coupled with als_addConstraints: above, this allows you to create groups of constraints, easily switching them on or off based on user interaction and/or application state.
- als_hostView and als_setHostView: category methods on NSLayoutConstraint handle the to-one nature ((UI|NS)View <->> NSLayoutConstraint) of views and their constraints and to keep track of which host view a constraint has been assigned to so it can be activated (added) and deactivated (removed) easily at runtime.
- als_isActive and als_setActive: category methods on NSLayoutConstraint to provide individual constraint activation control. Used by als_activateConstraints & als_deactivateConstraints.

This replaces the idea that I'd add a way to get/set constraints by their ALS keys previously mentioned in the TODO section. Also the idea to allow overwriting of previously-set constraints.

v0.3: Jun 18 2013

[DEV] Re-unify UIView and NSView implementations.

v0.2: May 22 2013

[NEW] Add NSView support. (Tony Arnold)

v0.1.1: Apr 24 2013

[FIX] Make Closest-Common-Superview a little more forgiving.

v0.1: Apr 22 2013

Initial version.

As its Semantic Version suggests, its interface may still change in client-breaking ways.

Actually +[NSLayoutConstraint constraintsWithVisualFormat:…] will create a constraint with an attribute of NSLayoutAttributeLeading where the example above uses NSLayoutAttributeLeft. ALS does support @"leading ==" in addition to @"left ==", I just wanted to make the example straightforward to folks who haven't learned about Auto Layout's Right-To-Left text system support yet.
Same deal as ¹.

rentzsch/AutoLayoutShorthand