Android ViewModel 与没有 Context 的 View 的关系

Question

我很难理解如何迁移到 Google 批准的 Android 现代开发。在 MVVM 中，“业务逻辑”应该位于 ViewModel 中，并且建议 ViewModel 的子类应该不感知 Context，但大多数 Android 类需要 Context 才能提供任何功能。例如，如果没有上下文（如 WifiManager 和 ConnectivityManager），就无法获取对系统服务的引用。

作为示例，我正在尝试为具有单个按钮的应用程序构建一个简单的架构，该按钮根据 ConnectivityManager 文档中概述的活动默认网络链接更改其图像。这个系统级操作应该在代码的“业务逻辑”区域中处理，因为它与 UI 无关。它检测网络状态，更改 ViewModel 中的一些状态数据，这应该“神奇地”更改按钮图像，因为某些 Observable 通知 UI 数据已更改并触发 Compose“重组”，从而更改按钮图像。我已经开始构建这个应用程序，我将发布到目前为止我所拥有的内容。如果 ViewModel 是应用程序状态的仲裁者，如何在没有 Context 的情况下创建 ViewModel？这看起来很矛盾。 ViewModel 摘录：

enum class ConnectivityState {
    WIFI,
    CELLULAR,
    DISCONNECTED
}

class MainActivityViewModel(context: Context) : ViewModel() {
    private val logger: Logger = LoggerFactory.getLogger(this::class.java)

    private val _uiState = MutableStateFlow(MainUIState())
    val uiState: StateFlow<MainUIState> = _uiState.asStateFlow()

    //TODO: how to perform system level business logic in ViewModel without Context reference?
    //DESIRED: keep this 'business logic' away from UI.  ViewModel should be appropriate but Context should not be used here?
    private val wifiManager =
        context.getSystemService(WIFI_SERVICE) as WifiManager
    private val connectivityManager =
        context.getSystemService(CONNECTIVITY_SERVICE) as ConnectivityManager

    /**
     * a NetworkCallback which will be triggered when a change in network state is sent from the system
     */
    private val networkCallback = object : ConnectivityManager.NetworkCallback() {
        override fun onAvailable(network: Network) {
            super.onAvailable(network)
            _uiState.value.connectivityState = getConnectivityState()
        }

        override fun onCapabilitiesChanged(
            network: Network,
            networkCapabilities: NetworkCapabilities
        ) {
            super.onCapabilitiesChanged(network, networkCapabilities)
            _uiState.value.connectivityState = getConnectivityState()
        }

        override fun onLost(network: Network) {
            super.onLost(network)
            _uiState.value.connectivityState = getConnectivityState()
        }
    }

    init {
        connectivityManager.registerDefaultNetworkCallback(networkCallback)
        _uiState.value.connectivityState = getConnectivityState()
    }

    fun getConnectivityState(): ConnectivityState {
        //these objects represent information about the currently active network
        val currentNetwork = connectivityManager.getActiveNetwork()
        val capabilities = connectivityManager.getNetworkCapabilities(currentNetwork)
        val linkProperties = connectivityManager.getLinkProperties(currentNetwork)

        if(currentNetwork == null) {
            return ConnectivityState.DISCONNECTED
        }

        if(wifiManager.isWifiEnabled) {
            //if wifi is enabled, it should be the current active network
            val isWifi = capabilities?.hasTransport(NetworkCapabilities.TRANSPORT_WIFI)
            if(isWifi != null) {
                if(isWifi) {
                    //the current active link is a WiFi connection
                    logger.debug("Current active default network is WiFi.")
                    return ConnectivityState.WIFI
                }
            }
        }

        val isCellular = capabilities?.hasTransport(NetworkCapabilities.TRANSPORT_CELLULAR)
        if(isCellular != null) {
            if(isCellular) {
                logger.info("Current active default network is cellular data.")
                return ConnectivityState.CELLULAR
            }
        }

        return ConnectivityState.DISCONNECTED
    }
}

data class MainUIState(
    var connectivityState: ConnectivityState = ConnectivityState.DISCONNECTED
)

活动摘录：

    @Composable
    fun wifiComposable() {
        //WiFi button
        Button(
            onClick = {
                MainActivity.openWifiSettings(LocalContext.current)
            }
        ) {
            //TODO: this image changes between DOWN, CELLULAR, WIFI images depending on connectivity state / source
            Image(
                //TODO: 'StateFlow.value should not be called within composition' why?
                //DESIRED BEHAVIOR: when this state value changes, recomp and change image accordingly
                painter = when(viewModel.uiState.value.connectivityState) {
                    ConnectivityState.WIFI -> painterResource(id = R.drawable.wifi_indicator)
                    ConnectivityState.CELLULAR -> painterResource(id = R.drawable.cellular_indicator)
                    ConnectivityState.DISCONNECTED -> painterResource(id = R.drawable.down_indicator)
                },
                modifier = Modifier.size(40.dp),
                contentDescription = "WiFi Settings")
        }
    }

Answer 1

演示 - UI 层

在屏幕上显示应用程序数据的UI层。

UI的作用是在屏幕上显示应用数据 也作为用户交互的主要点。每当 数据发生变化，或者由于用户交互（例如按 按钮）或外部输入（如网络响应），UI 应该 更新以反映这些更改。实际上，UI 是一种视觉效果 从数据中检索到的应用程序状态的表示 层。

ViewModel 被视为状态持有者。

数据层

数据层包含应用程序的业务逻辑并公开应用程序数据。

根据您的情况，将网络状态设置为数据源。我建议使用 Dagger Hilt 注入 Context。这毕竟是业务逻辑。对数据执行操作。