Suspend function 'callGetApi' should be called only from a coroutine or another suspend function

Suspend function should be called only from a coroutine.

That means to call a suspend function you need to use a coroutine builder, e.g. launch, async or runBlocking(recommended to use only in unit tests). For example:

class Activity : AppCompatActivity(), CoroutineScope {
    private var job: Job = Job()

    override val coroutineContext: CoroutineContext
        get() = Dispatchers.Main + job

    override fun onDestroy() {
        super.onDestroy()
        job.cancel()
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        launch {
            val result =  callGetApi()
            onResult(result) // onResult is called on the main thread
        }
    }

    suspend fun callGetApi(): String {...}

    fun onResult(result: String) {...}
}

To use Dispatchers.Main in Android add dependency to the app's build.gradle file:

implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:1.0.1'

The MOST RECENT APPROACH would be to use extension properties in ViewModel and Activity/Fragment:

• In ViewModel we can use viewModelScope to launch a coroutine:

  viewModelScope.launch { ... }
  

It attached to the lifecycle of Activity/Fragment and cancels launched coroutines when they destroyed.

• Similar in Activity/Fragment we can use the following extension properties to launch a coroutine: lifecycleScope.launch {}, lifecycle.coroutineScope.launch {}, viewLifecycleOwner.lifecycleScope.launch {}(applicable in Fragments).

Looks like the most elegant way to do it as of July 2019, is the one described here:

import androidx.lifecycle.lifecycleScope
import kotlinx.coroutines.launch

class Activity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super...

        lifecycleScope.launch {
            val result =  callGetApi()
            onResult(result) 
        }
    }
}

Don't forget to add the correponding lib:

implementation "androidx.lifecycle:lifecycle-runtime-ktx:2.2.0-alpha02"

The above answer worked , but i solved it without inheriting CoroutineScope class by just using .... gradle.build

  dependencies {
      implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.3.3'
  }

Activity.kt

  import kotlinx.coroutines.GlobalScope
  import kotlinx.coroutines.Dispatchers

  GlobalScope.launch (Dispatchers.Main) { callGetApi() }

Dispatchers.Main is important cause you cannot update the UI in any other thread than main.

But its recommended to inherit CoroutineScope to maintain the lifecycle of the activity and onDestroy of the activity to kill the job