Kotlin Flow 冷流 StateFlow 热流 StateFlow 的应用

Flow是冷流。简单来说。如果Flow有了订阅者Collector以后，发射出来的值才会存在内存中，

这和懒加载的概念很像

与之相对的是热流，StateFlow和SharedFlow 是热流，在垃圾回收之前，都是存在内存之中，

并且处于活跃状态

StateFlow 是一个状态容器式可观察数据流，可以向其收集器发出当前状态更新和新状态更新，还可通过其value属性读取当前状态值。

和livedata很像。

package com.example.android_flow_practice.viewmodel

import androidx.lifecycle.AndroidViewModel
import androidx.lifecycle.ViewModel
import kotlinx.coroutines.flow.MutableStateFlow

class NumberViewModel : ViewModel() {
    val number = MutableStateFlow(0)
    fun increment() {
        number.value++
    }

    fun deincrement() {
        number.value--
    }
}

fragment

package com.example.android_flow_practice.fragment

import android.os.Bundle
import androidx.fragment.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.viewModels
import androidx.lifecycle.lifecycleScope
import com.example.android_flow_practice.R
import com.example.android_flow_practice.databinding.FragmentArticleBinding
import com.example.android_flow_practice.databinding.FragmentNumberBinding
import com.example.android_flow_practice.viewmodel.ArticleViewModel
import com.example.android_flow_practice.viewmodel.NumberViewModel
import kotlinx.coroutines.flow.collect

class NumberFragment : Fragment() {
    private val TAG = "NumberFragment"
    private val viewModel: NumberViewModel by viewModels()

    private val mBinding: FragmentNumberBinding by lazy {
        FragmentNumberBinding.inflate(layoutInflater)
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?
    ): View? {
        // Inflate the layout for this fragment
        return mBinding.root
    }


    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        mBinding.apply {
            btAdd.setOnClickListener { viewModel.increment() }
            btSubtraction.setOnClickListener { viewModel.deincrement() }
        }
        lifecycleScope.launchWhenCreated {
            viewModel.number.collect {
                mBinding.tvText.text = it.toString()
            }
        }
    }
}

布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".fragment.NumberFragment">

    <TextView
        android:id="@+id/tv_text"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingVertical="10dp"
        android:text="0"
        android:textAlignment="center" />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <Button
            android:id="@+id/bt_add"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:text="+" />

        <Button
            android:id="@+id/bt_subtraction"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:text="-" />
    </LinearLayout>

</LinearLayout>