StateFlow当值发生变化，就会将值发送出去，下流就可以接收到新值。在某些场景下，StateFlow比LiveData更适用

效果：

1.定义ViewModel

StateFlow需要初始值

package com.aruba.flowapplyapplication.viewmodel

import android.view.View
import androidx.lifecycle.ViewModel
import kotlinx.coroutines.flow.MutableStateFlow
import kotlinx.coroutines.flow.StateFlow

/**
 * Created by aruba on 2021/9/21.
 */
class StateFlowViewModel : ViewModel() {
    val stateFlow = MutableStateFlow<Int>(0)

    fun add(v: View) {
        stateFlow.value++
    }

    fun reduce(v: View) {
        stateFlow.value--
    }
}

2.Fragment的布局文件

DataBinding也支持在xml中直接使用StateFlow

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".StateFlowFragment">

        <TextView
            android:id="@+id/textView2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="24dp"
            android:text="@{String.valueOf(stateFlowViewModel.stateFlow)}"
            app:layout_constraintBottom_toTopOf="@+id/guideline2"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.498"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.13" />

        <androidx.constraintlayout.widget.Guideline
            android:id="@+id/guideline2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            app:layout_constraintGuide_percent="0.09" />

        <Button
            android:id="@+id/button6"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="24dp"
            android:onClick="@{stateFlowViewModel.add}"
            android:text="add"
            app:layout_constraintEnd_toStartOf="@+id/guideline3"
            app:layout_constraintHorizontal_bias="0.5"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="@+id/guideline2" />

        <Button
            android:id="@+id/button7"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="24dp"
            android:text="reduce"
            android:onClick="@{stateFlowViewModel.reduce}"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.5"
            app:layout_constraintStart_toStartOf="@+id/guideline3"
            app:layout_constraintTop_toTopOf="@+id/guideline2" />

        <androidx.constraintlayout.widget.Guideline
            android:id="@+id/guideline3"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            app:layout_constraintGuide_percent="0.5" />

    </androidx.constraintlayout.widget.ConstraintLayout>

    <data>

        <variable
            name="stateFlowViewModel"
            type="com.aruba.flowapplyapplication.viewmodel.StateFlowViewModel" />
    </data>
</layout>

3.Fragment中绑定ViewModel

package com.aruba.flowapplyapplication

import android.os.Bundle
import androidx.fragment.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.databinding.DataBindingUtil
import androidx.lifecycle.ViewModelProvider
import androidx.lifecycle.lifecycleScope
import com.aruba.flowapplyapplication.databinding.FragmentStateFlowBinding
import com.aruba.flowapplyapplication.viewmodel.StateFlowViewModel
import kotlinx.coroutines.flow.collect
import kotlinx.coroutines.launch

class StateFlowFragment : Fragment() {
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val binding: FragmentStateFlowBinding = DataBindingUtil.inflate(
            inflater,
            R.layout.fragment_state_flow,
            container,
            false
        )

        val stateFlowViewModel = ViewModelProvider(
            this,
            ViewModelProvider.AndroidViewModelFactory(requireActivity().application)
        ).get(StateFlowViewModel::class.java)
//        lifecycleScope.launch {
//            stateFlowViewModel.stateFlow.collect {
//                binding.textView2.text = it.toString()
//            }
//        }
        binding.stateFlowViewModel = stateFlowViewModel
        binding.lifecycleOwner = viewLifecycleOwner

        return binding.root
    }

}

Demo地址：https://gitee.com/aruba/flow-apply-application.git