访问应用程序其他部分的构造函数中声明的类变量(React) [英] Accessing class variable declared in constructor in other parts of the app (React)

问题描述

我目前正在学习 React-native.我正在关注 Pluralsight 教程，但不幸的是他的一些代码已经过时了.我有以下代码:

I'm currently learning React-native. I am following a Pluralsight tutorial but unfortunately some of his code is out of date. I have the following code:

import Expo from 'expo';
import React from 'react';
import TaskList from './TaskList';
import {
  Component,
  View,
  Navigator,
  Text
} from 'react-native';

class todo extends React.Component {
  constructor(props,context){
    super(props,context);
    this.state ={
      todos: [
        {
          task: 'Task1',
        },
        {
          task: 'Task 2',
        },
      ]
    }
  }
  onAddStarted(){
    this.nav.push({
      name: 'taskform',
        })

  }

  renderScene(route,nav){
    switch(route.name){
      case 'taskform':
      return (
        <Text>Add form comes here!</Text>
      );
      default:
      return(
        <TaskList onAddStarted={this.onAddStarted}
        todos={this.state.todos}/>
      );

    }
  }
  render() {
    return (
    <Navigator initialRoute={{name: 'Tasklist'}}
                ref={((nav)=> {
                  this.nav=nav;
                })}
                renderScene={this.renderScene}/>
    );
  }

}




Expo.registerRootComponent(todo);

我的问题是:

    todos={this.state.todos}/>

如果我记录 this.state 它会出错并且状态未定义.如果我内联复制 todos 的内容，代码会编译，所以我知道它在范围界定方面存在一些问题，但我想我从根本上不明白如何正确地做到这一点.在我开始使用 Navigator 之前，我完全可以从构造函数中引用 this.state.

if i log this.state it errors and state is undefined. If i copy the content of todos inline the code compiles so I know its some problem with scoping but I guess I fundamentally don't understand how to do it properly. Before I started using Navigator I was able to reference this.state from the constructor absolutely fine.

如果有人可以帮助我理解，我会很高兴.

I would appericiate if someone could help me understand.

谢谢！

推荐答案

你需要给 renderScene 上下文，以便它可以访问 this 然后它的状态.为此，您可以在渲染函数中修改以下行:

You need to give renderScene context, so that it can access this and then its state. To do this you can amend the following line in the render function:

 renderScene={this.renderScene}

到

 renderScene={this.renderScene.bind(this)}

注意由于使用 bind 会生成一个新函数，这会降低性能(例如，如果您要渲染很多项目).所以一个常见的模式是在构造函数中进行绑定:

N.B. As using bind generates a new function this can degrade performance (for instance if you are rendering a lot of items). So a common pattern is to do the binding in the constructor:

this.renderScene = this.renderScene.bind(this);

如果您采用这种方法，您的渲染功能可以保持现在的状态.

If you take this approach your render function can stay like it is now.

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