An object that provides a constraint-based layout manager.
You use the shared instance of this object by assigning it to the layoutManager property of any layer objects to which you have added constraints. During a layout update, Core Animation uses the layout manager to update the size and position of the sublayers based on the registered set of constraints.
Constraints let you define a set of geometric relationships between a layer and its sibling layers or between a layer and its superlayer. These relationships are expressed using constraint objects, which are instances of the CAConstraint class. When creating constraints, you can reference a layer by name using that object’s name property. You can also use the special name superlayer to refer to the layer's superlayer.