Using BindConstraint in Clutter to constrain the size of an actor

Question

I recently discovered constraints in Clutter, however, I can't find information on how to constrain the size of actor by proportion. For example, I want an actor to keep to 1/2 the width of another actor, say it's parent. It seems like it can only force the width to scale 100% with the source.

Answer 1

Clutter.BindConstraint matches the positional and/or dimensional attributes of the source actor. for fractional positioning, you can use Clutter.AlignConstraint, but there is no Clutter.Constraint class that allows you to set a fractional dimensional attribute. you can implement your own ClutterConstraint that does so by subclassing Clutter.Constraint and overriding the Clutter.Constraint.do_update_allocation() virtual function, which gets passed the allocation's of the actor that should be modified by the constraint. something similar to this (untested) code should work:

class MyConstraint (Clutter.Constraint):
    def __init__(self, source, width_fraction=1.0, height_fraction=1.0):
        Clutter.Constraint.__init__(self)
        self._source = source
        self._widthf = width_fraction
        self._heightf = height_fraction
    def do_update_allocation(self, actor, allocation):
        source_alloc = self._source.get_allocation()
        width = source_alloc.get_width() * self._widthf
        height = source_alloc.get_height() * self._heightf
        allocation.x2 = allocation.x1 + width
        allocation.y2 = allocation.y1 + height

this should illustrate the mechanism used by Clutter.Constraint to modify the allocation of an actor.