不能在 render() 中使用 this.state 来反应本机答案

【问题标题】：Can not use this.state in render() in react native不能在 render() 中使用 this.state 来反应本机
【发布时间】：2019-09-10 14:02:18
【问题描述】：

在我的 componentDidMount 函数中，我正在执行 fetch()，我将响应存储在我的状态中。

我还添加了一些其他对象，例如 isLoaded: true。

如果我在 render() 中执行console.log(this.state.isLoaded);，我会收到以下错误。

TypeError: null is not an object (evaluating 'this.state.isLoaded')

Stack trace:
  App.js:38:27 in render
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:11581:21 in finishClassComponent
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:11509:4 in updateClassComponent
  ...
ERROR
09:58
Warning: %s: Error boundaries should implement getDerivedStateFromError(). In that method, return a state update to display an error message or fallback UI., RootErrorBoundary

Stack trace:
  node_modules/react-native/Libraries/YellowBox/YellowBox.js:59:8 in error
  node_modules/expo/build/environment/muteWarnings.fx.js:26:24 in error
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:638:36 in warningWithoutStack
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:15918:16 in callback
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:13857:16 in callCallback
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:13901:19 in commitUpdateEffects
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:13892:22 in commitUpdateQueue
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:15123:10 in commitLifeCycles
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:16636:8 in commitAllLifeCycles
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:93:15 in invokeGuardedCallbackImpl
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:318:36 in invokeGuardedCallback
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:16876:8 in commitRoot
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:18501:17 in <unknown>
  node_modules/scheduler/cjs/scheduler.development.js:255:23 in unstable_runWithPriority
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:18499:4 in completeRoot
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:18427:21 in performWorkOnRoot
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:18324:24 in performWork
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:18285:14 in performSyncWork
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:18169:19 in requestWork
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:17969:16 in scheduleWork
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:18642:15 in scheduleRootUpdate
  node_modules/react-native/Libraries/Renderer/oss/ReactNativeRenderer-dev.js:19512:20 in render
  node_modules/react-native/Libraries/ReactNative/renderApplication.js:61:52 in renderApplication
  node_modules/react-native/Libraries/ReactNative/AppRegistry.js:104:10 in run
  node_modules/react-native/Libraries/ReactNative/AppRegistry.js:198:26 in runApplication
  node_modules/react-native/Libraries/BatchedBridge/MessageQueue.js:366:47 in __callFunction
  node_modules/react-native/Libraries/BatchedBridge/MessageQueue.js:106:26 in <unknown>
  node_modules/react-native/Libraries/BatchedBridge/MessageQueue.js:314:10 in __guard
  node_modules/react-native/Libraries/BatchedBridge/MessageQueue.js:105:17 in callFunctionReturnFlushedQueue

否则，如果我这样做，console.log(this.state); 将打印状态，我将在 Expo 的日志中看到 isLoaded: true。

什么？为什么？为什么看不到this.state.isLoaded

【问题讨论】：

标签： react-native

【解决方案1】：

也许你没有定义你的状态。应该是这样的：

constructor(props){
    super(props);
    this.state = {
        isLoaded: false
    }
}

【讨论】：

为什么和 react 不一样？在我没有定义的反应应用程序中，这很好。不管怎样，谢谢。