主题调用 next 导致一个奇怪的错误 [英] Subject call to next causing a strange error

问题描述

这会导致以下错误:Cannot read property 'length' of undefined

const msg$ = new Subject<string>();
msg$.subscribe(console.log)
of("Hello").subscribe(msg$.next);

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但是，如果我将 msg$.next 包装在一个函数中，那么它可以正常工作而不会出现任何错误.

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If, however, I wrap msg$.next in a function, then it works without any errors.

• Lambda 函数

const msg$ = new Subject<string>();
msg$.subscribe(console.log)
of("Hello").subscribe(greeting => msg$.next(greeting));

• 匿名函数

const msg$ = new Subject<string>();
msg$.subscribe(console.log)
of("Hello").subscribe(function(greeting){
  msg$.next(greeting);
});

• 命名函数

function nextMsg(greeting){
  msg$.next(greeting);
}
const msg$ = new Subject<string>();
msg$.subscribe(console.log)
of("Hello").subscribe(nextMsg);

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它们都只是包装函数，看起来除了调用下一个函数什么都不做.这里发生了什么?在这里工作时似乎有一个我不知道的 JavaScript 问题.

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They're all just wrapper functions that seemingly do nothing but call the next function. What's going on here? Seems like there's a JavaScript gotcha I'm not aware of at work here.

推荐答案

为后人接受答案

我认为这个问题归结为这个"的价值是什么?将函数作为参数传递时?".您可能会在这里找到一些答案如何访问正确的 this回调?.

I think this question comes down to "What's the value of "this" when passing a function as parameter?". You might find some answers here How to access the correct this inside a callback?.

this 在您的第一个示例中具有错误的值.如果你把一个 console.log(this) 放在 nextMsg 里面，你会看到它是一个 SafeSubscriber ，它缺少属性 observers.length 被访问.rxjs6 中的 Subject#next 函数依赖于这是一个带有 observers.length 属性的 Subject 属性

this has the wrong value in your first example. If you put a console.log(this) inside nextMsg you will see that it's a SafeSubscriber which lacks the property observers.length that is accessed. The Subject#next function in rxjs6 relies on this to be a Subject with an observers.length property

是的，当然.看起来很傻，我没有注意到.msg$.next.bind(msg$) 有效.

Yup, of course. Seems silly that I didn't notice. msg$.next.bind(msg$) works.

obj.func 没有 obj 作为上下文，而 obj.func() 有.

obj.func doesn't have obj as a context, whereas obj.func() does.

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