如何在协程中制作敬酒消息？

Question

Setting

Let's say I have a function that creates a toast message

fun makeToast(success: Boolean){
    if (success){
        Toast.makeText(someContext, "Success", Toast.LENGHT_SHORT).show()
    }
}

This function is used in another function that is suspended eg

suspend fun makeRequest(){
    success = doSomeHTTPRequest()
    makeToast(success)
}

When I'm using this function to execute the request I will do it in a coroutine for IO for example ie

CoroutineScope(IO).launch{
    makeRequest()
}

Issue

Doing it like proposed above ends in:

java.lang.NullPointerException: Can't toast on a thread that has not called Looper.prepare()

Question

What is the best practice to fix this? The straight forward way would be to change makeToast to

fun makeToast(success: Boolean){
    if (success){
        Looper.prepare()
        Toast.makeText(someContext, "Success", Toast.LENGHT_SHORT).show()
        Looper.loop()
    }
}

However this feels akward since this means that the function makeToast already knows that it might not be called in the main thread which creates a weird coupling in my opinion.

Answer 1

You need to specify using the Main dispatcher when running code that is only allowed to be called from the main thread. I suppose from the error message it is suggesting you can use other threads if they have a prepared Looper, but it's a lot easier if you simply make Toasts exclusively from the main thread.

Also, a properly composed suspend function should be safe to call from any dispatcher. You should never have to specify a Dispatcher to call a suspend function. A dispatcher should be specified when you are calling blocking code and therefore need to specify an appropriate background dispatcher, or when calling a function that is required to be called on the main thread.

So, you can remove (IO) from your launch call. Specify your dispatchers at the point where the blocking or main-exclusive functions are being called, which is in your makeRequest function:

suspend fun makeRequest(){
    val success = withContext(Dispatchers.IO) { 
        doSomeHTTPRequest()
    }
    withContext(Dispatchers.Main) {
        makeToast(success)
    }
}

Answer 2

best way to show Toast message in app Step 1 create a kotlin class ToastMessage.kt and write code

import android.content.Context
import android.widget.Toast

fun Context.tost(message: String) = Toast.makeText(this, message, Toast.LENGTH_SHORT).show()

/*call this method to any class*/

this.tost("successfuly show message")