How do I wait for another class's functions to complete calling next function

Question

I'm writing an app in Swift and I've run into a problem I can't find much help for.

So I've got a ViewController and a class named CameraHandler that uses calls the an ActionSheet which then presents camera or gallery picker and saves resulting images.

What I want to do is: call camera, save the image, and then store relevant information to an object.

     CameraHandler.shared.showActionSheet(vc: self, reg: car.registration) 

        self.car.imagePath = CameraHandler.shared.returnFilePath()
        self.storeCar() // completes before CameraHandler can get imagepath

My workaround is using a simple DispatchQueue to wait 10 secs hoping it completes in time.

You don't wait. You don't stall. You don't use a timer. You don't guess. You don't hope. You pass _in_ a completion handler, and the other class _calls_ that completion handler when everything is over. — matt, Jun 03 '18 at 20:34
@matt I think you're missing a "You don't pass Go!" in there. :) — rmaddy, Jun 03 '18 at 20:37

score 0 · Answer 1 · answered Jun 04 '18 at 00:07

As matt says in his comment, don't wait. Don't expect the answer to be returned from your function. You need to rewrite your showActionSheet() function to take a completion handler. (That is a block of code {more specifically, a closure} that you pass as a parameter.) When your function gets an answer from the async process it needs to complete (fetching a value from a remote server, prompting the user for something, etc.) then it calls the completion handler.

See my answer to the thread below. I include a sample project that uses a function with a completion handler to fetch data from a remote server.

Swift: Wait for Firebase to load before return a function

score 0 · Answer 2 · answered Jun 04 '18 at 05:13

Using a closure or block in other programing languages as a completion handler, let take an example.

func returnFilePath(completionHandler: ((String) -> ()) {
     // after get file path
     completionHandler(path)
}

then you can use it like:

returnFilePath { path in
     // do some thing with the path
}

How do I wait for another class's functions to complete calling next function

2 Answers2