iOS 6 Auto Layout Visual Format Language

Visual Format Language

Visual Format Syntax

The following are examples of constraints you can specify using the visual format. Note how the text visually matches the image.

Standard Space

[button]-[textField]

Width Constraint

[button(>=50)]

Connection to Superview

|-50-[orchidBox]-50-|

Vertical Layout

V:[topField]-10-[bottomField]

Flush Views

[maroonView][oceanView]

Priority

[button([email protected])]

Equal Widths

[button1(==button2)]

Multiple Predicates

[flexibleButton(>=70,<=100)]

A Complete Line of Layout

|-[find]-[findNext]-[findField(>=20)]-|

The notation prefers good visualization over completeness of expressibility. There are constraints that cannot be expressed in visual format syntax, although most of the constraints that are useful in real user interfaces can be. One useful constraint that cannot be expressed is a fixed aspect ratio (for example, 

imageView.width = 2 * imageView.height

). To create such a constraint, you must use

constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:

.

Visual Format String Grammar

The visual format string grammar is defined as follows (literals are shown in 

code font

; e denotes the empty string).

Symbol	Replacement rule
<visualFormatString>	(<orientation>:)? (<superview><connection>)? <view>(<connection><view>)* (<connection><superview>)?
<orientation>	H | V
<superview>	|
<view>	[ <viewName>(<predicateListWithParens>)? ]
<connection>	e| - <predicateList> - | -
<predicateList>	<simplePredicate>|<predicateListWithParens>
<simplePredicate>	<metricName>|<positiveNumber>
<predicateListWithParens>	( <predicate>( , <predicate>)* )
<predicate>	(<relation>)?(<objectOfPredicate>)( @ <priority>)?
<relation>	== | <= | >=
<objectOfPredicate>	<constant>|<viewName> (see note)
<priority>	<metricName>|<number>
<constant>	<metricName>|<number>
<viewName>	Parsed as a C identifier. This must be a key mapping to an instance of  NSView  in the passed views dictionary.
<metricName>	Parsed as a C identifier. This must be a key mapping to an instance of  NSNumber  in the passed metrics dictionary.
<number>	As parsed by  strtod_l , with the C locale.

Note: For the 

objectOfPredicate

 production, a 

viewName

 is only acceptable if the subject of the predicate is the width or height of a view. That is, you can use 

[view1(==view2)]

 to specify that 

view1

 and 

view2

 have the same width.

If you make a syntactic mistake, an exception is thrown with a diagnostic message. For example:

Expected ':' after 'V' to specify vertical arrangement

V|[backgroundBox]|

^

A predicate on a view's thickness must end with ')' and the view must end with ']'

|[whiteBox1][blackBox4(blackWidth][redBox]|

^

Unable to find view with name blackBox

|[whiteBox2][blackBox]

^

Unknown relation. Must be ==, >=, or <=

V:|[blackBox4(>30)]|

^

Some sample code to create views and then specify multiple constraints using a visual format languagestring, would, therefore, read as follows:

NSDictionary *viewsDictionary = NSDictionaryOfVariableBindings (mybutton1,mybutton2);

// Get a reference to the superview UIView *superview = self.view; //Create a label UILabel *mylabel = [[UILabel alloc]init]; [mylabel setTranslatesAutoresizingMaskIntoConstraints:NO];mylabel.text = @"My Label"; //Create a button UIButton *mybutton = [UIButton buttonWithType:UIButtonTypeRoundedRect];[mybutton setTitle:@"My Button" forState:UIControlStateNormal];[mybutton setTranslatesAutoresizingMaskIntoConstraints:NO];

//Add the button and label to the superview [superview addSubview:mylabel]; [superview addSubview:mybutton]; // Get the views dictionary NSDictionary *viewsDictionary = NSDictionaryOfVariableBindings(mylabel, mybutton);

Understanding the iOS 6 Auto Layout Visual Format Language

//Create the constraints using the visual language formatNSArray *constraintsArray = [NSLayoutConstraint

constraintsWithVisualFormat:@"|-[mybutton]-[mylabel(==mybutton)]-|"options:NSLayoutFormatAlignAllBaseline metrics:nilviews:viewsDictionary];

//Work through the array of constraints, applying each to the superviewfor (int i = 0; i<constraintsArray.count; i++) {

}