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How to Test UIAlertController

Introduced in iOS 9, UIAlertController is the quickest way to present alerts and action sheets to users on iOS. Powered by a simple, straightforward API, you can get something on the screen in just a few lines of code. However, overly hiding internal architecture, namely action handlers, makes testing this class quite difficult. Let’s learn how to test UIAlertController with protocols, mocks, and dependency injection. No swizzling required.

The API

A brief reminder of what we are working with.

let alert = UIAlertController(title: "Alert Title", message: "Alert message.",
                              preferredStyle: .alert)

let confirmAction = UIAlertAction(title: "Confirm", style: .default) { (_) in
    self.confirm()
}
alert.addAction(confirmAction)

let cancelAction = UIAlertAction(title: "Cancel", style: .cancel, handler: nil)
alert.addAction(cancelAction)

controller.present(alert, animated: true, completion: nil)

In summary, you set the title and message, add some actions, then present the alert on a UIViewController.

What to Test

Even in this tiny example I see four things that should be tested:

  1. The alert's title and message
  2. The alert's button titles
  3. The action performed when a button is tapped
  4. The controller the alert was presented on

Let's walk through these step by step.

1. The alert's title and message

To verify these we will need a reference to the assigned values under test. An initial scan of the docs looks promising as we can access the alert's title and message directly.

expect(alert.title).to(equal("Alert Title"))
expect(alert.messge)to(equal("Alert message."))

2. The alert's button titles

There is also an actions() method that returns a list of all the UIAlertAction instances we created. Those docs deliver again, providing title.

expect(alert.actions.count).to(equal(2))
expect(alert.actions.first?.title).to(equal("Confirm"))
expect(alert.actions.last?.title).to(equal("Cancel"))
// or
expect(alert.actions.map({ $0.title })).to(equal(["Confirm", "Cancel"]))

3. The action performed when a button is tapped

Ideally, UIAlertAction would expose the attached handler. Then we could execute it under test and verify the correct behvaiour. But it doesn't, so we need to get creative.

4. The controller the alert was presented on

How to Test

Our first step is to keep track of the added actions and handlers.

One approach is to swizzle the calls to addAction() and save the parameters via associated objects. Quality Coding has an excellent article (and plug-and-play library) on how and why (not) to do this.

A second take "wraps" UIAlertController in a new class responsible for building and presenting the alert. This abstracts the existing Apple API, enabling us to store and reference parameters for later use (hint: in the tests!).

AlertBuilder

At a minimum, the class needs to do three things: add actions, present the alert, and retrive stored action handlers. Each can be accomplished with their own method and saving a reference to each UIAlertAction in a dictionary.

class AlertPresenter {
    private var actionHandlers = [UIAlertAction: AlertHandler]()

    func addAction(titled title: String, style: UIAlertActionStyle, handler: AlertHandler?) {
        let action = UIAlertAction(title: title, style: style, handler: handler)
        actionHandlers[action] = handler
    }

    func present(title: String, message: String, on controller: ViewControllerPresentable) {
        let alert = UIAlertController(title: title, message: message, preferredStyle: .alert)
        actionHandlers.keys.forEach({ alert.addAction($0) })
        controller.present(alert, animated: false, completion: nil)
    }

    func handler(for action: UIAlertAction) -> AlertHandler? {
        return actionHandlers[action]
    }
}
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