Kotlin flow realizes thread switching

flowOn Method to realize thread switching

 private fun flowOn() {
    
        lifecycleScope.launch {
    // The main thread 
            simpleFlowOn()
                .flowOn(Dispatchers.IO)// Sub thread execution , The long-running 
                .onStart {
    // Start first 
                    LogUtils.d("Thread is ${
      Thread.currentThread().name} onStart")
                }
                .onEach {
    
                    LogUtils.d("Thread is ${
      Thread.currentThread().name} onEach")
                }
                .catch {
     exception ->// Abnormal reception 
                    LogUtils.d("Thread is ${
      Thread.currentThread().name} exception $exception")
                }
                .onCompletion {
    // The whole flow is complete 
                    LogUtils.d("Thread is ${
      Thread.currentThread().name} onCompletion")
                }
                .collect {
    // Data reception 
                    LogUtils.d("Thread is ${
      Thread.currentThread().name} collect $it")
                }
        }
    }

 // Implement a stream , Send one data per second 
    private fun simpleFlowOn(): Flow<Int> = flow {
    
        for (i in 1..3) {
    
            delay(1000)
            LogUtils.d("Thread is ${
      Thread.currentThread().name} Emitting $i")
// if (i == 3) throw IllegalStateException(" Status error ")
            emit(i)
        }
    }

Log give the result as follows

	// At the beginning of the main thread onStart
    D/FlowFragment: Thread is main onStart
    // Send in sub thread 
    D/FlowFragment: Thread is DefaultDispatcher-worker-1 Emitting 1
    D/FlowFragment: Thread is main onEach
    // Received in the main thread 
    D/Collect: Thread is main collect 1
    D/FlowFragment: Thread is DefaultDispatcher-worker-1 Emitting 2
    D/FlowFragment: Thread is main onEach
    D/Collect: Thread is main collect 2
    D/FlowFragment: Thread is DefaultDispatcher-worker-1 Emitting 3
    D/FlowFragment: Thread is main onEach
    D/Collect: Thread is main collect 3
    // Finally, it is completed in the main thread 
    D/FlowFragment: Thread is main onCompletion

Be careful ：flowOn The method usually follows flow Behind the time-consuming stream , Otherwise, other methods may also be in the sub thread

Look at the following method , take flowOn Put it in onStart、onEach Back

private fun flowOn() {
    
        lifecycleScope.launch {
    
            simpleFlowOn()
                .onStart {
    
                    LogUtils.d("Thread is ${
      Thread.currentThread().name} onStart")
                }
                .onEach {
    
                    LogUtils.d("Thread is ${
      Thread.currentThread().name} onEach")
                }
                .flowOn(Dispatchers.IO)
                .catch {
     exception ->
                    LogUtils.d("Thread is ${
      Thread.currentThread().name} exception $exception")
                }
                .onCompletion {
    
                    LogUtils.d("Thread is ${
      Thread.currentThread().name} onCompletion")
                }
                .collect {
    
                    LogUtils.d("Thread is ${
      Thread.currentThread().name} collect $it")
                }
        }
    }

Print as follows ,onStart,onEach Method execution is in the sub thread ：

	//onStart,onEach Method execution is in the sub thread 
    D/FlowFragment: Thread is DefaultDispatcher-worker-1 onStart
    D/FlowFragment: Thread is DefaultDispatcher-worker-1 Emitting 1
    D/FlowFragment: Thread is DefaultDispatcher-worker-1 onEach
    D/Collect: Thread is main collect 1
    D/FlowFragment: Thread is DefaultDispatcher-worker-1 Emitting 2
    D/FlowFragment: Thread is DefaultDispatcher-worker-1 onEach
    D/Collect: Thread is main collect 2
    D/FlowFragment: Thread is DefaultDispatcher-worker-1 Emitting 3
    D/FlowFragment: Thread is DefaultDispatcher-worker-1 onEach
    D/Collect: Thread is main collect 3
    D/FlowFragment: Thread is main onCompletion