property-hooks-for-outlets.md

Use property hooks to configure outlets

Description

Instead of applying style to a UIView after the awakeFromNib() method, we do it in the didSet property of each outlet.

Rationale

In the traditional way, we would have something like this:

public final class MyView: UIView {

  @IBOutlet weak var myLabel: UILabel!
  @IBOutlet weak var myButton: UIButton!

  public override func awakeFromNib() {
    super.awakeFromNib()
    setMyLabelStyle()
    setMyButtonStyle()
    ...
  }

}

private extension MyView {

  private func setMyLabelStyle() {
    myLabel.backgroundColor = UIColor.whiteColor()
  }

  private func setMyButtonStyle() {
    myButton.backgroundColor = UIColor.whiteColor()
    myButton.setTitleColor(UIColor.whiteColor(), forState: .Normal)
  }
  ...
}

We face some problems with this approach:

1. We need to override the awakeFromNib() method.

This isn't really a problem, but it is usually not necessary.

2. We need to add a private function for each IBOutlet.

It often happens that we have to change that outlet, for example myLabel to a UITextField. Now, we should also change the function setMyLabelStyle() to setMyTextFieldStyle().

3. The code setting the outlet style is visually far from the outlet itself.

It forces us to first find which outlet we want to change, then look for the private function and then change that function.

4. The file gets bigger as we keep adding functions.

Example

public final class MyView: UIView {

  @IBOutlet weak var myLabel: UILabel! {
    didSet {
      myLabel.backgroundColor = UIColor.whiteColor()
    }
  }

  @IBOutlet weak var myButton: UIButton! {
    didSet {
      myButton.backgroundColor = UIColor.whiteColor()
      myButton.setTitleColor(UIColor.whiteColor(), forState: .Normal)
    }
  }
}