Kotlin 2.0.0 Android项目中显式幕后字段无法使用问题

场景与代码 #

在Android项目中使用Kotlin 2.0.0版本，编写如下代码：

class MainActivity : ComponentActivity() {

    val state: StateFlow<Int>
        field = MutableStateFlow(0)


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            AndroidTheme {
                SignInScreenRoute()
            }
        }
    }
}

错误提示 #

运行后出现如下编译错误：

Explicit backing field declarations are not supported in FE 1.0

项目配置 #

libs.versions.toml #

[versions]
androidGradlePlugin = "8.4.1"
kotlin = "2.0.0"
coreKtx = "1.13.1"
junit = "4.13.2"
junitVersion = "1.1.5"
espressoCore = "3.5.1"
lifecycleRuntimeKtx = "2.8.0"
activityCompose = "1.9.0"
composeBom = "2024.05.00"
navigationCompose = "2.7.7"
koinAndroid = "3.5.6"
koinAndroidxCompose = "3.5.6"
ktor = "2.3.10"
appcompat = "1.6.1"
material = "1.12.0"
serialization = "1.6.3"
googleFont = "1.6.7"

[libraries]
androidx-core-ktx = { group = "androidx.core", name = "core-ktx", version.ref = "coreKtx" }
junit = { group = "junit", name = "junit", version.ref = "junit" }
androidx-junit = { group = "androidx.test.ext", name = "junit", version.ref = "junitVersion" }
androidx-espresso-core = { group = "androidx.test.espresso", name = "espresso-core", version.ref = "espressoCore" }
androidx-lifecycle-runtime-ktx = { group = "androidx.lifecycle", name = "lifecycle-runtime-ktx", version.ref = "lifecycleRuntimeKtx" }
androidx-activity-compose = { group = "androidx.activity", name = "activity-compose", version.ref = "activityCompose" }
androidx-compose-bom = { group = "androidx.compose", name = "compose-bom", version.ref = "composeBom" }
androidx-ui = { group = "androidx.compose.ui", name = "ui" }
androidx-foundation = { group = "androidx.compose.foundation", name = "foundation" }
androidx-foundation-layout = { group = "androidx.compose.foundation", name = "foundation-layout" }
androidx-runtime = { group = "androidx.compose.runtime", name = "runtime" }
androidx-lifecycle-viewmodel-compose = { group = "androidx.lifecycle", name = "lifecycle-viewmodel-compose" }
androidx-navigation = { group = "androidx.navigation", name = "navigation-compose", version.ref = "navigationCompose" }
androidx-ui-graphics = { group = "androidx.compose.ui", name = "ui-graphics" }
androidx-ui-tooling = { group = "androidx.compose.ui", name = "ui-tooling" }
androidx-ui-tooling-preview = { group = "androidx.compose.ui", name = "ui-tooling-preview" }
androidx-ui-test-manifest = { group = "androidx.compose.ui", name = "ui-test-manifest" }
androidx-ui-test-junit4 = { group = "androidx.compose.ui", name = "ui-test-junit4" }
androidx-material3 = { group = "androidx.compose.material3", name = "material3" }
androidx-material_icons = { group = "androidx.compose.material", name = "material-icons-extended" }
koin-android = { group = "io.insert-koin", name = "koin-android", version.ref = "koinAndroid" }
koin-androidx-compose = { group = "io.insert-koin", name = "koin-androidx-compose", version.ref = "koinAndroidxCompose" }
ktor-client-okhttp = { group = "io.ktor", name = "ktor-client-okhttp", version.ref = "ktor" }
ktor-client-logging = { group = "io.ktor", name = "ktor-client-logging", version.ref = "ktor" }
ktor-client-content-negotiation = { group = "io.ktor", name = "ktor-client-content-negotiation", version.ref = "ktor" }
ktor-client-core = { group = "io.ktor", name = "ktor-client-core", version.ref = "ktor" }
ktor-serialization-kotlinx-json = { group = "io.ktor", name = "ktor-serialization-kotlinx-json", version.ref = "ktor" }
androidx-appcompat = { group = "androidx.appcompat", name = "appcompat", version.ref = "appcompat" }
material = { group = "com.google.android.material", name = "material", version.ref = "material" }
serialization-json = { group = "org.jetbrains.kotlinx", name = "kotlinx-serialization-json", version.ref = "serialization" }
google-font= { group = "androidx.compose.ui", name = "ui-text-google-fonts", version.ref = "googleFont" }

[plugins]
androidApplication = { id = "com.android.application", version.ref = "androidGradlePlugin" }
jetbrainsKotlinAndroid = { id = "org.jetbrains.kotlin.android", version.ref = "kotlin" }
androidLibrary = { id = "com.android.library", version.ref = "androidGradlePlugin" }
serialization = { id = "org.jetbrains.kotlin.plugin.serialization", version.ref = "kotlin" }
compose-compiler = { id = "org.jetbrains.kotlin.plugin.compose", version.ref = "kotlin" }

app模块build.gradle.kts #

plugins {
    alias(libs.plugins.androidApplication)
    alias(libs.plugins.jetbrainsKotlinAndroid)
    alias(libs.plugins.compose.compiler)
}

android {
    namespace = "com.hub.abc"
    compileSdk = 34

    defaultConfig {
        applicationId = "com.hub.abc"
        minSdk = 24
        targetSdk = 34
        versionCode = 1
        versionName = "1.0"

        testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
        vectorDrawables {
            useSupportLibrary = true
        }
    }

    buildTypes {
        release {
            isMinifyEnabled = false
            proguardFiles(
                getDefaultProguardFile("proguard-android-optimize.txt"),
                "proguard-rules.pro"
            )
        }
    }
    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_17
        targetCompatibility = JavaVersion.VERSION_17
    }
    kotlinOptions {
        jvmTarget = "17"
    }
    buildFeatures {
        compose = true
    }
    packaging {
        resources {
            excludes += "/META-INF/{AL2.0,LGPL2.1}"
        }
    }
}

dependencies {

    implementation(project(":network"))
    implementation(project(":core:coroutines"))
    implementation(project(":core:ui"))
    implementation(project(":feature:signup:ui"))
    implementation(project(":feature:signup:data"))

    implementation(platform(libs.androidx.compose.bom))
    implementation(libs.androidx.core.ktx)
    implementation(libs.androidx.lifecycle.runtime.ktx)
    implementation(libs.androidx.activity.compose)
    implementation(libs.androidx.ui)
    implementation(libs.androidx.foundation)
    implementation(libs.androidx.foundation.layout)
    implementation(libs.androidx.runtime)
    implementation(libs.androidx.lifecycle.viewmodel.compose)
    implementation(libs.androidx.navigation)
    implementation(libs.androidx.ui.graphics)
    implementation(libs.androidx.ui.tooling.preview)
    implementation(libs.androidx.material3)
    implementation(libs.koin.android)
    implementation(libs.koin.androidx.compose)

    implementation(libs.serialization.json)

    testImplementation(libs.junit)
    androidTestImplementation(libs.androidx.junit)
    androidTestImplementation(libs.androidx.espresso.core)
    androidTestImplementation(platform(libs.androidx.compose.bom))
    androidTestImplementation(libs.androidx.ui.test.junit4)
    debugImplementation(libs.androidx.ui.tooling)
    debugImplementation(libs.androidx.ui.test.manifest)
}

解决方案 #

错误原因是Kotlin 2.0.0中显式声明backing field（field = ...）的语法不再被FE 1.0前端支持。正确的写法有两种：

方式一：封装可变实现，暴露只读接口 #

这是最符合封装规范的写法，适合需要后续修改状态的场景：

class MainActivity : ComponentActivity() {

    private val _state = MutableStateFlow(0)
    val state: StateFlow<Int> = _state


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            AndroidTheme {
                SignInScreenRoute()
            }
        }
    }
}

方式二：直接赋值（无需后续修改时使用） #

如果不需要在类内部修改state的值，可以直接声明为只读属性并指定类型：

class MainActivity : ComponentActivity() {

    val state: StateFlow<Int> = MutableStateFlow(0)


    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            AndroidTheme {
                SignInScreenRoute()
            }
        }
    }
}

两种写法都能解决编译报错问题，可根据实际业务场景选择。

内容的提问来源于stack exchange，提问作者Compose Learner