Android. How to correct launch coroutine without blocking UI?

Question

I'm new in coroutines. And I'm trying add it to my project. Also I'm using MVVM. In the documentation I read viewModelScope.launch { }:

Launches a new coroutine without blocking the current thread

But at the same time, I often see code constructs like this:

viewModelScope.launch {
   launch {
      // call some suspend fun here
  }
}

Why is another launch{} here if the documentation says that viewModelScope.launch { } launches a new coroutine without blocking the current thread.

Wouldn't it be enough to write like this:

 viewModelScope.launch {
    // call some suspend fun here
  }

Could such a construction (launch inside viewModelScope.launch) be useful in some cases? Maybe I don't understand something, please help me.

Answer 1

Writing

viewModelScope.launch {
  // call some suspend fun here
}

is enough to launch a coroutine and execute a suspend function without blocking UI.

The launch within launch is used to launch tasks in parallel, for example:

viewModelScope.launch {
   launch {
      // call task 1
      task1()
   }
   launch {
      // call task 2
      task2()
   }
}

task1() and task2() are suspend functions and will execute in parallel.

But if we write like the following:

viewModelScope.launch {
   task1()
   task2()
}

task2() will wait until task1() is completed and then will start execution.