How to use Coroutine inside a FloatingActionButton/Button Click event

Question

I have a FloatingActionButton, which when clicked will download few URL's from the web. I want to start this function from inside the setOnClickListener, and then let it does all the processing in the background.

I already did what I wanted to do using AsyncTask, but then someone suggested to use coroutines.

Inside the activity:

findViewById<FloatingActionButton>(R.id.fab).setOnClickListener { view ->
           
            val currentChapUrl = "https://www.novel.com/chap1.html"  
            GlobalScope.launch { downloadChapters(applicationContext,currentChapUrl) } 
        }

The downloadChapters function looks like this:

suspend fun downloadChapters(context : Context, startingChapUrl : String) : String {
   //download chapters.
   //show some toast messages

}

The app crashes on clicking the button.

Logcat:

2020-08-01 17:03:38.559 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:38.559 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.095 6937-6937/com.example.daoreader I/Choreographer: Skipped 32 frames!  The application may be doing too much work on its main thread.
2020-08-01 17:03:39.124 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.125 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.163 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.163 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.208 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.209 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
    
    
    --------- beginning of crash
2020-08-01 17:03:39.211 6937-7086/com.example.daoreader E/AndroidRuntime: FATAL EXCEPTION: DefaultDispatcher-worker-1
    Process: com.example.daoreader, PID: 6937
    java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
        at android.os.Handler.<init>(Handler.java:200)
        at android.os.Handler.<init>(Handler.java:114)
        at android.widget.Toast$TN$2.<init>(Toast.java:336)
        at android.widget.Toast$TN.<init>(Toast.java:336)
        at android.widget.Toast.<init>(Toast.java:103)
        at android.widget.Toast.makeText(Toast.java:256)
        at com.example.daoreader.WebviewActivity.downloadChapters(WebviewActivity.kt:65)
        at com.example.daoreader.WebviewActivity$onCreate$1$1.invokeSuspend(WebviewActivity.kt:32)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(Dispatched.kt:241)
        at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:594)
        at kotlinx.coroutines.scheduling.CoroutineScheduler.access$runSafely(CoroutineScheduler.kt:60)
        at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:740)
    
    
    --------- beginning of system
2020-08-01 17:03:39.257 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.257 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.307 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.307 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.394 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.394 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.487 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.487 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.565 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.566 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.581 6937-6985/com.example.daoreader D/EGL_emulation: eglMakeCurrent: 0xaa205000: ver 2 0 (tinfo 0xaa2034c0)
2020-08-01 17:03:39.595 6937-6985/com.example.daoreader D/EGL_emulation: eglMakeCurrent: 0xaa205000: ver 2 0 (tinfo 0xaa2034c0)
2020-08-01 17:03:39.658 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.658 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:39.714 6937-6985/com.example.daoreader D/EGL_emulation: eglMakeCurrent: 0xaa205000: ver 2 0 (tinfo 0xaa2034c0)
2020-08-01 17:03:39.727 6937-6985/com.example.daoreader D/OpenGLRenderer: endAllActiveAnimators on 0x9232d500 (RippleDrawable) with handle 0x92021280
2020-08-01 17:03:40.051 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:40.051 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:40.094 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5
2020-08-01 17:03:40.094 6937-6985/com.example.daoreader E/eglCodecCommon: glUtilsParamSize: unknow param 0x000085b5

Answer 1

It is crashing, because you are showing Toast not in the Main Thread.

GlobalScope.launch defaultly run code in another Thread which is Dispatchers.Default.

So what you need to do, before showing Toast, you should change the thread to Main Thread like this

suspend fun downloadChapters(context : Context, startingChapUrl : String) : String {
   //download chapters.
   
   withContext(Dispatchers.Main) {
       // Show Toast here
   }
}