解决moko:biometry-compose桌面不兼容及@Composable调用报错问题

问题场景 #

使用dev.icerock.moko:biometry-compose库时，仅需在Android和iOS平台提供生物认证支持，桌面平台无需实现。通过expect/actual+接口的方案封装后，在LoginScreen的ViewModel工厂内调用createBiometryAuthenticator()，以及在BindBiometryAuthenticatorEffect中均触发错误：@composable invocations can only happen from the context of a @composable function。

错误原因 #

1. ViewModel工厂无Composable上下文：viewModelFactory的lambda不属于@Composable函数，无法直接调用带@Composable注解的createBiometryAuthenticator()方法。
2. 实例持有逻辑错位：试图让ViewModel持有需要Compose上下文创建的BiometryAuthenticator，违反了ViewModel与UI层解耦的设计原则。

解决方案 #

1. 调整实例创建位置，将Authenticator移至Composable层 #

在LoginScreen（本身是@Composable函数）中直接创建并持有BiometryAuthenticator实例：

@Composable
fun LoginScreen() {
    val biometryAuthenticatorFactory = BiometryAuthenticatorFactory()
    // 在Composable上下文内调用@Composable方法创建实例
    val biometryAuthenticator = biometryAuthenticatorFactory.createBiometryAuthenticator()
    
    val viewModel = getViewModel<AuthViewModel>(key = "biometry-screen")
    
    // 用LaunchedEffect监听ViewModel的认证请求事件
    LaunchedEffect(viewModel.authRequest) {
        viewModel.authRequest.collect { request ->
            val authResult = biometryAuthenticator.checkBiometryAuthentication(
                requestTitle = request.title,
                requestReason = request.reason,
                failureButtonText = request.failureText,
                allowDeviceCredentials = request.allowDeviceCreds
            )
            viewModel.onAuthResult(authResult)
        }
    }

    Column {
        // 界面元素示例
        Button(onClick = {
            viewModel.triggerBiometryAuth(
                title = "登录验证",
                reason = "请验证生物信息以登录",
                failureText = "使用密码登录",
                allowDeviceCreds = true
            )
        }) {
            Text(text = "生物识别登录")
        }
    }
}

2. 重构ViewModel，通过事件流触发认证 #

让ViewModel仅负责发送认证请求事件，不直接持有BiometryAuthenticator：

class AuthViewModel : ViewModel() {
    private val _authRequest = MutableSharedFlow<AuthRequest>()
    val authRequest = _authRequest.asSharedFlow()

    private val _authResult = MutableStateFlow<Boolean?>(null)
    val authResult = _authResult.asStateFlow()

    // 对外暴露触发认证的方法
    fun triggerBiometryAuth(
        title: String,
        reason: String,
        failureText: String,
        allowDeviceCreds: Boolean
    ) {
        viewModelScope.launch {
            _authRequest.emit(AuthRequest(title, reason, failureText, allowDeviceCreds))
        }
    }

    // 接收认证结果并更新状态
    fun onAuthResult(result: Boolean) {
        _authResult.value = result
    }

    data class AuthRequest(
        val title: String,
        val reason: String,
        val failureText: String,
        val allowDeviceCreds: Boolean
    )
}

3. 桌面平台提供空实现 #

在desktopMain目录下实现BiometryAuthenticatorFactory的空逻辑，避免编译错误：

actual class BiometryAuthenticatorFactory {
    @Composable
    actual fun createBiometryAuthenticator(): BiometryAuthenticator {
        return DesktopBiometryAuthenticator()
    }

    private class DesktopBiometryAuthenticator : BiometryAuthenticator {
        override suspend fun checkBiometryAuthentication(
            requestTitle: String,
            requestReason: String,
            failureButtonText: String,
            allowDeviceCredentials: Boolean
        ): Boolean {
            // 桌面平台不支持生物认证，返回默认结果或抛出异常（根据业务需求调整）
            return false
        }
    }
}

核心思路 #

• 遵循ViewModel与UI层解耦原则：ViewModel不依赖Compose上下文，仅处理业务逻辑和事件传递。
• 利用Composable上下文创建依赖UI环境的实例：BiometryAuthenticator需要rememberBiometryAuthenticatorFactory()这类Composable API，必须在Composable函数内创建。
• 通过事件驱动实现ViewModel与UI层的交互：ViewModel发送请求，UI层执行具体操作并返回结果。

内容的提问来源于stack exchange，提问作者sgireddy