Why is this swift button setup crashing? - swift

Question

I'm trying to programmatically add a button to a view along with its action method. The key thing is that the action method should be in the same file with the button so I can drop the file into other apps. When the button is tapped I want the action method to be executed but it crashes instead. It just gives EXC_BAD_ACCESS with no log output.
Here's a simplified test app with only two classes: ViewController and BlueButton:
import UIKit
class ViewController: UIViewController {
override func viewDidAppear(animated: Bool) {
super.viewDidAppear(animated)
let button = BlueButton(mainView: view)
button.installButton()
}
}
class BlueButton: NSObject {
var mainView: UIView
init(mainView: UIView) {
self.mainView = mainView
}
func installButton() {
let button = UIButton(frame: CGRect(x: 25, y: 100, width: 150, height: 50))
button.setTitle("Tap Me", forState: UIControlState.Normal)
button.setTitleColor(UIColor.blueColor(), forState: UIControlState.Normal)
button.addTarget(self, action: "someAction", forControlEvents: .TouchUpInside)
mainView.addSubview(button)
}
func someAction() {
println("this is someAction")
}
}
Back trace shows nothing helpful. It ends with objc_msgSend. I tried many different ways of rearranging the code but nothing I tried worked. I added an 'All Exceptions' breakpoint but it doesn't get hit. I put a breakpoint on the someAction method just to be sure it isn't called -- it isn't. Can somebody tell me what's going on?

Answer 1

Your BlueButton instance is deallocated when viewDidAppear returns because there is no longer a strong reference to it.
When the button is tapped, it tries to reference the instance, but it's been deallocated, which causes the crash.
You could resolve the issue a number of ways. The simplest would be to create a property in the ViewController class and store the BlueButton in it as long as the button is visible.