How can you test the contents of a UIAlertAction handler with OCMock

Question

I've got an app where I push a UIAlertController with several custom UIAlertActions. Each UIAlertAction performs unique tasks in the handler block of actionWithTitle:style:handler:.

I have several methods that I need to verify are executed within these blocks.

How can I execute the handler block so that I can verify that these methods are executed?

Orthostichy

Answer 1

After some playing around I finally figure it out. Turns out that the handler block can be cast as a function pointer and the function pointer can be executed.

Like so

UIAlertAction *action = myAlertController.actions[0];
void (^someBlock)(id obj) = [action valueForKey:@"handler"];
someBlock(action);

Here's an example of how it would be used.

-(void)test_verifyThatIfUserSelectsTheFirstActionOfMyAlertControllerSomeMethodIsCalled {

    //Setup expectations
    [[_partialMockViewController expect] someMethod];

    //When the UIAlertController is presented automatically simulate a "tap" of the first button
    [[_partialMockViewController stub] presentViewController:[OCMArg checkWithBlock:^BOOL(id obj) {

        XCTAssert([obj isKindOfClass:[UIAlertController class]]);

        UIAlertController *alert = (UIAlertController*)obj;

        //Get the first button
        UIAlertAction *action = alert.actions[0];

        //Cast the pointer of the handle block into a form that we can execute
        void (^someBlock)(id obj) = [action valueForKey:@"handler"];

        //Execute the code of the join button
        someBlock(action);
    }]
                                         animated:YES
                                       completion:nil];

   //Execute the method that displays the UIAlertController
   [_viewControllerUnderTest methodThatDisplaysAlertController];

   //Verify that |someMethod| was executed
   [_partialMockViewController verify];
}

Orthostichy

Any idea what the correct cast is in Swift? Simply casting to the expected function type gives an EXC_BAD_INSTRUCTION at runtime. – Cissie

I wish I could upvote this answer twice. Big help for mocking around alert action handlers. I understand Apple could change how this works and my tests will break, but I'm willing to live with it for now. – Dairying

Answer 2

With a bit of clever casting, I've found a way to do this in Swift (2.2):

extension UIAlertController {

    typealias AlertHandler = @convention(block) (UIAlertAction) -> Void

    func tapButtonAtIndex(index: Int) {
        let block = actions[index].valueForKey("handler")
        let handler = unsafeBitCast(block, AlertHandler.self)

        handler(actions[index])
    }

}

This allows you to call alert.tapButtonAtIndex(1) in your test and have the correct handler executed.

(I would only use this in my test target, btw)

Cissie

This fails in Swift 3.0.1 with fatal error: can't unsafeBitCast between types of different sizes, does anyone know how to fix this? – Cissie

Answer 3

The following code will work on Swift 5.5 I would suggest to create a private DSL in the test target

private extension UIAlertController {

typealias AlertHandler = @convention(block) (UIAlertAction) -> Void

func tapButtonAtIndex(index: Int) {
    let alertAction = actions[index]
    let block = alertAction.value(forKey: "handler")
    let handler = unsafeBitCast(block, to: AlertHandler.self)
    handler(alertAction)
  }
}

Brewery