How to Communicate with server in a Jetpack Compose App? - android-jetpack-compose

Question

I have made a simple Jetpack compose app coupled with a server. When I try to communicate with the server from the app, the app crashes without any error messages.
The app code:
import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.material.Text
import androidx.compose.runtime.*
import java.io.BufferedReader
import java.io.InputStreamReader
import java.io.PrintWriter
import java.net.Socket
fun getServerMessage(onServerMessage: (msg: String)-> Unit) {
val client = Socket("127.0.0.1", 9999)
val output = PrintWriter(client.getOutputStream(), true)
val input = BufferedReader(InputStreamReader(client.inputStream))
output.println("msg")
onServerMessage(input.readLine())
}
#Composable
fun GreetingFromServer(name: String) {
var text by remember { mutableStateOf("waiting for message") }
thread { getServerMessage { msg -> text = msg } }
Text(text)
}
class MainActivity : ComponentActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContent {
GreetingFromServer("Android")
}
}
}
The server code:
import java.io.BufferedReader
import java.io.InputStreamReader
import java.io.PrintWriter
import java.net.ServerSocket
fun main(args: Array<String>) {
val server = ServerSocket(9999)
println("printing")
while (true) {
val client = server.accept()
println("a client connected")
val output = PrintWriter(client.getOutputStream(), true)
val input = BufferedReader(InputStreamReader(client.inputStream))
output.println("${input.readLine()} back!")
}
}
Why does the app crash when initiating the client object and when running the thread? And how should I go about fixing that?
(The app still crashes when removing the "thread {}" wrapping.)

Answer 1

Ignoring the backend code, since the question was on Jetpack compose Android aspects, a simplified version of a network call would look something like this.
API call
suspend fun getServerMessage(): String {
delay(5000)
return "Success"
}
ViewModel
class GreetingFromServerViewModel : ViewModel() {
var text by mutableStateOf("waiting for message")
init {
getMessage()
}
private fun getMessage() {
viewModelScope.launch(Dispatchers.IO) {
text = getServerMessage()
}
}
}
View Composable
#Composable
fun GreetingFromServer(
viewmodel: GreetingFromServerViewModel = viewModel(),
) {
Text(viewmodel.text)
}
Actual product apps would have a lot more things like using Repositories to manage data fetching, Retrofit if using REST APIs, etc.
Refer: ViewModel, Coroutines