While loop keeps looping without intent going trough

Issue

What I need to happen is as follow:

1. Start loop
2. I need to have the intent go to the class to get a result
3. Result will be one of two things, were both are needed to exit the while loop.
4. Exit loop

Code:

        while ( plate == "0" ||  location == "0"){
            Toast.makeText(this, "Starting scan", Toast.LENGTH_SHORT).show()
            val intent = Intent(this, LocationScan::class.java)

            // 0 is request code
            startActivityForResult(intent, 0)}

What happens currently is the screen flashes and Toast keeps displaying. Intent does not load. Disabling loop takes me to intent and displays result.

plate and location is changed by the result of LocationScan. Result is received by :

    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)

        if (requestCode == 0) {
            if (resultCode == RESULT_OK) {
                // Get the result from intent
                val result = intent.getStringExtra("result")

                if (result!![0] in 'a'..'z' || result!![0] in 'A'..'Z'){
                    location = result

                    println("$result is an location.")}
                else{
                    plate = result

                    println("$result is a plate.")}

            } } }

Compile Sdk : 33 Gradle Version : 7.4 Android Gradle Version :7.3.1 Java : 1.8

Solution

Closing/suspending the current Activity to open a new one requires the main thread to do a bunch of work, including calling onPause in this Activity. So calling startActivityForResult won't create the new Activity immediately, it'll schedule the transition for the future.

And it can't carry that transition out while you're blocking the main thread spinning on a loop. Even if it did immediately interrupt execution, it wouldn't return to the same point in your code (you're even relying on it calling onActivityResult instead). Activities and Fragments just don't work like that.

---

What you probably want to do instead, is have a function that checks your conditions. If they're not met, call startActivityForResult and let execution continue. Then in onActivityResult, handle the result and then call the checking function again. If the conditions still aren't met, it'll start the Activity again, onActivityResult will be called again, the checking function will run again...

So this way, you're creating a cycle that runs through the Activity system and its callbacks - it's still looping behaviour, but it's not an actual execution loop, if you see what I mean. You're effectively putting the task back on the queue until everything's taken care of, instead of blocking the thread.

Answered By - cactustictacs