访问变量从RxJS subscribe（）函数声明了一个组件 [英] Access variables declared a component from a RxJS subscribe() function

问题描述

我可以使用 this.variable 来访问组件任何部分的变量，但在RxJS函数中除外，例如 subscribe（）或 catch（）。

I am able to use this.variable to access variables in any part of the component, except inside RxJS functions like subscribe() or catch().

在下面的示例中，我想在运行流程后打印一条消息：

In the example below, I want to print a message after running a process:

import {Component, View} from 'angular2/core';

@Component({
    selector: 'navigator'
})
@View({
    template: './app.component.html',
    styles: ['./app.component.css']
})
export class AppComponent {
    message: string;

    constructor() {
        this.message = 'success';
    }

    doSomething() {
        runTheProcess()
        .subscribe(function(location) {
            console.log(this.message);
        });
    }
}

当我运行 doSomething（ ），我得到未定义。这个场景可以使用局部变量来解决：

When I run doSomething(), I get undefined. This scenario can be solved using a local variable:

import {Component, View} from 'angular2/core';

@Component({
    selector: 'navigator'
})
@View({
    template: './app.component.html',
    styles: ['./app.component.css']
})
export class AppComponent {
    message: string;

    constructor() {
        this.message = 'success';
    }

    doSomething() {

        // assign it to a local variable
        let message = this.message;

        runTheProcess()
        .subscribe(function(location) {
            console.log(message);
        });
    }
}

我想这与<$ c $有关c>这个，但是，为什么我无法访问 subscribe（） this.message c $ c>？

I suppose this is related to the this, however, why I can't access the this.message inside the subscribe()?

推荐答案

这与rx或angular无关，而且与Javascript和Typescript有关。

This has nothing to do with rx or angular, and everything to do with Javascript and Typescript.

我假设您在Javascript函数调用的上下文中熟悉这个的语义（如果没有） ，有不缺在线解释） - 这些语义适用当然，在第一个片段中，这是 this.message 在 subscribe（）中未定义的唯一原因。那只是Javascript。

I assume you're familiar with the semantics of this in the context of function invocations in Javascript (if not, there are no shortage of explanations online) - those semantics apply in the first snippet, of course, and that's the only reason this.message is undefined inside subscribe()there. That's just Javascript.

因为我们在谈论的是Typescript：
箭头函数是一个Typescript构造，旨在（部分）通过词汇捕捉这个的含义来回避这些语义的尴尬，这意味着此在箭头函数内=== 来自外部上下文的。

Since we're talking about Typescript: Arrow functions are a Typescript construct intended (in part) to sidestep the awkwardness of these semantics by lexically capturing the meaning of this, meaning that this inside an arrow function === this from the outer context.

所以，如果你替换：

.subscribe(function(location) {
        //this != this from outer context 
        console.log(this.message); //prints 'undefined'
    });

by：

.subscribe((location) => {
     //this == this from the outer context 
        console.log(this.message); //prints 'success'
    });

您将获得预期的结果。

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