函数是未定义的，如果它是之前声明的(Function is undefined althoug it is declared right before)

 我有一个ToDo List App，用来学习React Native。 当我尝试将clickhandler传递给子项时，它表示clickhandler函数未定义。 
 
 这是相关的代码。 
 
constructor(props) {
  super(props)

  this.handleTodoPress = this.handleTodoPress.bind(this)
}
handleTodoPress (event) {
  console.warn('Press handled')
}
renderItem ({section, item}) {
  return <TodoItem onItemPress={this.handleTodoPress} title={item.title} description={item.description} completed={item.completed} />
}
 
 如果我在renderItem记录handleTodoPress ，它显示为未定义。 这是为什么？ 

I have a ToDo List App I am creating to learn React Native. When I try to pass a clickhandler to the child item, it says that the clickhandler-function is undefined.
 
Here is the relevant code.
 
constructor(props) {
  super(props)

  this.handleTodoPress = this.handleTodoPress.bind(this)
}
handleTodoPress (event) {
  console.warn('Press handled')
}
renderItem ({section, item}) {
  return <TodoItem onItemPress={this.handleTodoPress} title={item.title} description={item.description} completed={item.completed} />
}
 
If I log handleTodoPress in renderItem it shows up as undefined. Why is that?

原文：https://stackoverflow.com/questions/49739350