ConstraintLayout.addConstraintsFromDescription

Creates a list of constraints from a VFL description.

The Visual Format Language, VFL, is based on Apple's AutoLayout VFL.

The views dictionary is used to match gtk.constraint_target.ConstraintTarget instances to the symbolic view name inside the VFL.

The VFL grammar is:

     <visualFormatString> = (<orientation>)?
                            (<superview><connection>)?
                            <view>(<connection><view>)*
                            (<connection><superview>)?
            <orientation> = 'H' | 'V'
              <superview> = '|'
             <connection> = '' | '-' <predicateList> '-' | '-'
          <predicateList> = <simplePredicate> | <predicateListWithParens>
        <simplePredicate> = <metricName> | <positiveNumber>
<predicateListWithParens> = '(' <predicate> (',' <predicate>)* ')'
              <predicate> = (<relation>)? <objectOfPredicate> (<operatorList>)? ('@' <priority>)?
               <relation> = '==' | '<=' | '>='
      <objectOfPredicate> = <constant> | <viewName> | ('.' <attributeName>)?
               <priority> = <positiveNumber> | 'required' | 'strong' | 'medium' | 'weak'
               <constant> = <number>
           <operatorList> = (<multiplyOperator>)? (<addOperator>)?
       <multiplyOperator> = [ '*' | '/' ] <positiveNumber>
            <addOperator> = [ '+' | '-' ] <positiveNumber>
               <viewName> = [A-Za-z_]([A-Za-z0-9_]*) // A C identifier
             <metricName> = [A-Za-z_]([A-Za-z0-9_]*) // A C identifier
          <attributeName> = 'top' | 'bottom' | 'left' | 'right' | 'width' | 'height' |
                            'start' | 'end' | 'centerX' | 'centerY' | 'baseline'
         <positiveNumber> // A positive real number parseable by g_ascii_strtod()
                 <number> // A real number parseable by g_ascii_strtod()

**Note**: The VFL grammar used by GTK is slightly different than the one defined by Apple, as it can use symbolic values for the constraint's strength instead of numeric values; additionally, GTK allows adding simple arithmetic operations inside predicates.

Examples of VFL descriptions are:

// Default spacing
[button]-[textField]

// Width constraint
[button(>=50)]

// Connection to super view
|-50-[purpleBox]-50-|

// Vertical layout
V:[topField]-10-[bottomField]

// Flush views
[maroonView][blueView]

// Priority
[button(100strong)]

// Equal widths
[button1(==button2)]

// Multiple predicates
[flexibleButton(>=70,<=100)]

// A complete line of layout
|-[find]-[findNext]-[findField(>=20)]-|

// Operators
[button1(button2 / 3 + 50)]

// Named attributes
[button1(==button2.height)]