flow 终极系列

https://www.youtube.com/watch?v=sk3svS_fzZM&list=PLQkwcJG4YTCQHCppNAQmLsj_jW38rU9sC&index=52

1. flow 的创建

在viewmodel中创建一个计时倒数的flow

    val countDownFlow = flow<Int> {
        val startValue = 10
        var curValue = startValue
        while (curValue > 0) {
            delay(1000L)
            curValue--
            emit(curValue)
        }
    }

在compose中使用这个flow 

val time by flowDemoViewModel.countDownFlow.collectAsState(10)

Text(text = time.toString())

可以看到，页面启动后，text的数字有10变到0

抛开界面，单纯的收集流

   fun collectFlow() {
        viewModelScope.launch {
            countDownFlow.collect {time->
                println("The current time is $time")
            }

        }
    }

启动的时候开始收集

  LaunchedEffect(null) {
                        flowDemoViewModel.collectFlow()
                    }

流输出

2. collect和 collectLatest 有什么区别呢?

我们在发送和接收的地方都添加上打印

 //创建示例
    val countDownFlow = flow<Int> {
        val startValue = 10
        var curValue = startValue
        while (curValue > 0) {
            delay(1000L)
            curValue--
            println("The current emit $curValue")
            emit(curValue)
        }
    }

    fun collectFlow() {
        println("The current time collect")
        viewModelScope.launch {
            countDownFlow.collectLatest {time->
                println("The current time is start $time")
                delay(1500)
                println("The current time is end $time")
            }

        }
    }

先用collect，看下打印效果

countDownFlow.collect {}

数据发送端是一秒发送一个，但是接收地方需要1.5秒处理，但是使用collect的话，整个数据还是按顺序下来的<