backgroundFilters

Background filters affect the content behind the layer that shows through into the layer itself. Typically this content belongs to the superlayer that acts as the parent of the layer. These filters do not affect the content of the layer itself, including the layer’s background color and border.

The default value of this property is nil.

Changing the inputs of the CIFilter object directly after it is attached to the layer causes undefined behavior. In macOS, it is possible to modify filter parameters after attaching them to the layer but you must use the layer’s setValue(_:forKeyPath:) method to do so. In addition, you must assign a name to the filter so that you can identify it in the array. For example, to change the inputRadius parameter of the filter, you could use code similar to the following:

let layer = CALayer()
     
if let filter = CIFilter(name:"CIGaussianBlur") {
    filter.name = "myFilter"
    layer.backgroundFilters = [filter]
    layer.setValue(1,
                   forKeyPath: "backgroundFilters.myFilter.inputRadius")
}

You use the layer's masksToBounds to control the extent of its background filter's effect.

Listing 2 shows how to create two overlapping text layers, background and foreground. A Gaussian blur filter is added to the foreground layer's backgroundFilters array and its masksToBounds is set to true:

view.layer = CALayer()
view.layerUsesCoreImageFilters = true
     
let background = CATextLayer()
background.string = "background"
background.backgroundColor = NSColor.red.cgColor
background.alignmentMode = kCAAlignmentCenter
background.fontSize = 96
background.frame = CGRect(x: 10, y: 10, width: 640, height: 160)
     
let foreground = CATextLayer()
foreground.string = "foreground"
foreground.backgroundColor = NSColor.blue.cgColor
foreground.alignmentMode = kCAAlignmentCenter
foreground.fontSize = 48
foreground.opacity = 0.5
foreground.frame = CGRect(x: 20, y: 20, width: 600, height: 60)
foreground.masksToBounds = true
     
if let blurFilter = CIFilter(name: "CIGaussianBlur",
                             withInputParameters: [kCIInputRadiusKey: 2]) {
    foreground.backgroundFilters = [blurFilter]
}
     
view.layer?.addSublayer(background)
background.addSublayer(foreground)

Figure 1 shows the result: the background layer is only blurred where it is overlapped by the foreground layer.

However, if the foreground layer's masksToBounds is set to false, the entire background layer is blurred as illustrated in Figure 2.

This property is not supported on layers in iOS.