How to change text of button while function is running

I have the following function that runs whenever a button is pressed:

func deletekeyPressed(sender: UIButton!) {

    while(textDocumentProxy.hasText()) {
        (textDocumentProxy as UIKeyInput).deleteBackward()
    }

    for _ in 1..<2000 {
        (textDocumentProxy as UIKeyInput).deleteBackward()
    }

}

To prevent user from thinking the program is crashing I want the text of the delete button which normally says "Delete All" to "Deleting..." or even better have the title alternate between the following states: "Deleting", "Deleting.", "Deleting..", "Deleting..."

So far I have tried putting a setTitle() for UIControlState.Normal at the beginning and end of the function in order to make the text say "Deleting" when the function is running.

This makes my function look like this:

    func deletekeyPressed(sender: UIButton!) {
    sender.setTitle("Deleting...", forState: .Normal)

    while(textDocumentProxy.hasText()) {
        (textDocumentProxy as UIKeyInput).deleteBackward()
    }

    for _ in 1..<2000 {
        (textDocumentProxy as UIKeyInput).deleteBackward()
    }

    sender.setTitle("Deleting...", forState: .Normal)
}

However, nothing is happening. How do I implement this solution?

Update One: I implemented Aaron's solution like this:

 func deletekeyPressed(sender: UIButton!) {

    NSLog("-------------------")
    NSLog("Pressing delete key")

    sender.setTitle("Deleting...", forState: .Normal)
    sender.userInteractionEnabled = false



    dispatch_async(dispatch_get_main_queue()) {
    NSLog("Starting delete function")

        while(self.textDocumentProxy.hasText()) {
            NSLog("Deleting 3 times")
            (self.textDocumentProxy as UIKeyInput).deleteBackward()
            (self.textDocumentProxy as UIKeyInput).deleteBackward()
            (self.textDocumentProxy as UIKeyInput).deleteBackward()
        }

        for _ in 1..<2000 {
            (self.textDocumentProxy as UIKeyInput).deleteBackward()
        }

        sender.setTitle("Clear", forState: .Normal)
        sender.userInteractionEnabled = true

        NSLog("Finishing delete function")
    }

However I run into the following problem. The solution partially works in the sense that the button text will say "Deleting..." until the code inside dispatch_async finishes running. But the problem is the UI of textfield in which the text is being deleted is not updated as soon as the code inside dispatch_async finishes running. This leads the delete button's text to say "Deleting..." for a short amount of time (as long as it takes for the code to finish running) even if the UITextfield has not updated itself to show the change from the code.