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# CAConstraintLayoutManager

An object that provides a constraint-based layout manager.

```
class CAConstraintLayoutManager
```

## Overview

You use the shared instance of this object by assigning it to the [`layoutManager`](/documentation/QuartzCore/CALayer/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`](/documentation/QuartzCore/CAConstraint) class. When creating constraints, you can reference a layer by name using that object’s [`name`](/documentation/QuartzCore/CALayer/name) property. You can also use the special name `superlayer` to refer to the layer’s superlayer.

The following example shows how you can use [`CAConstraintLayoutManager`](/documentation/QuartzCore/CAConstraintLayoutManager) to create a layer containing two constrained sublayers: `leftLayer` and `rightLayer`. A series of [`CAConstraint`](/documentation/QuartzCore/CAConstraint) objects are created so that the sublayers match their superlayer’s height and are half of its width. `leftConstraint` matches the [`CAConstraintAttribute.minX`](/documentation/QuartzCore/CAConstraintAttribute/minX) attribute and `rightConstraint` matches the [`CAConstraintAttribute.maxX`](/documentation/QuartzCore/CAConstraintAttribute/maxX) attribute.

The end result is that the two sublayers are always laid out so that `leftLayer` fills the left half of `layer` and `rightLayer` fills the right half of layer.

```swift
let layer = CALayer()
let heightConstraint = CAConstraint(attribute: .height,
                                    relativeTo: "superlayer",
                                    attribute: .height)
    
let widthConstraint = CAConstraint(attribute: .width,
                                   relativeTo: "superlayer",
                                   attribute: .width,
                                   scale: 0.5,
                                   offset: 0)
     
let leftConstraint = CAConstraint(attribute: .minX,
                                  relativeTo: "superlayer",
                                  attribute: .minX)
     
let rightConstraint = CAConstraint(attribute: .maxX,
                                   relativeTo: "superlayer",
                                   attribute: .maxX)
    
let bottomConstraint = CAConstraint(attribute: .minY,
                                    relativeTo: "superlayer",
                                    attribute: .minY)   
     
let leftLayer = CALayer()
leftLayer.frame = CGRect(x: 0, y: 0, width: 20, height: 20)
layer.addSublayer(leftLayer)
leftLayer.constraints = [heightConstraint, widthConstraint, 
                         leftConstraint, bottomConstraint]
leftLayer.backgroundColor = NSColor.red.cgColor
  
let rightLayer = CALayer()
layer.addSublayer(rightLayer)
rightLayer.constraints = [heightConstraint, widthConstraint, 
                          rightConstraint, bottomConstraint]
rightLayer.backgroundColor = NSColor.blue.cgColor
    
layer.layoutManager = CAConstraintLayoutManager()
```

> Important:
> It is possible to specify a set of constraints that will cause layout to fail. For example, layout may fail if your constraints contain a circular dependency. When that happens, the behavior of the layout manager is undefined.

This class is not meant to be subclassed.

## Topics

### Creating the layout manager

[`layoutManager`](/documentation/QuartzCore/CAConstraintLayoutManager/layoutManager)

Returns the shared layout manager object.


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