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 [[email protected]] 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 ascii_strtod <number> // A real number parseable by 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
// 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)] ```
| this |
a `GtkConstraintLayout` |
| lines |
an array of Visual Format Language lines defining a set of constraints |
| hspacing |
default horizontal spacing value, or -1 for the fallback value |
| vspacing |
default vertical spacing value, or -1 for the fallback value |
| views |
a dictionary of `[ name, target ]` pairs; the `name` keys map to the view names in the VFL lines, while the `target` values map to children of the widget using a `GtkConstraintLayout`, or guides |
| n_lines |
the number of lines |
|
the list of [[email protected]] instances that were added to the layout |