用于angular2 + ngrx的滤波器数组 [英] Filter array for angular2 + ngrx

问题描述

我正在尝试在angular + ngrx应用中使用此滤镜:

I am trying to use this filter in an angular+ngrx app:

fetchById(id: number): Document {

return this.store.select( s => s.documents ).filter( obj => obj.id == id )[0]

 }

documents是Documents的数组，后者具有id属性.这个想法是过滤数组以选择一个文档.但是，我收到此错误消息:

documents is an array of Documents, and the latter have an id property. The idea is to filter the array to select one document. However, I get this error message:

属性id在类型Document[]上不存在.我不明白该消息，因为obj是数组的元素，因此具有ID属性的某个对象.对这里出什么问题有任何想法吗?谢谢！

Property id does not exist on type Document[]. I don't understand the message, since obj is an element of the array, hence a certain object which has an id property. Any ideas on what is wrong here? Thanks!

推荐答案

让我们分解一下操作以了解发生了什么...

Let's break down the operations to understand what's going on...

fetchById(id: number): Observable<Document> {
  const documents: Observable<Document[]> = this.store.select(s => s.documents);
  const filtered: Observable<Document[]> = documents.filter((obj: Document[]) => obj.id == id );
  const result: Observable<Document> = filtered.first();
  return result;
}

请注意，Observable#filter()的回调使用数组，而不是单个项.这与Array#filter()和错误消息的原因不同.另外，请参见我无法使用[0]从可观察对象中检索第一项，而应使用first().然后，我更改了返回类型以返回可观察的结果.

Notice that Observable#filter()'s callback takes an array, not an individual item. This is different from Array#filter() and the cause of the error message. Also, see that I can't retrieve the first item from an observable with [0], use first() instead. Then I changed the return type to return an observable.

但是，它不起作用.当我们考虑从可观察对象中过滤出一个数组时，可能我们想使用Observable#map()来生成具有单个元素的另一个数组.

Yet, it doesn't work. When we think about filtering an array from an observable, probably we want to use Observable#map() to produce another array with a single element.

fetchById(id: number): Observable<Document> {
  return this.store
    .select(s => s.documents)
    .map(list => obj.find(obj => obj.id === id));
    .first();
}

如果我们想返回可观察值的最后一个值，我们可以这样做:

If we want to return the last value of the observable, we can do this:

fetchById(id: number): Document | null {
  let result: Document | null = null;
  this.store
    .select(s => s.documents)
    .map(list => obj.find(obj => obj.id === id) || null);
    .first()
    .subscribe(item => result = item);
  return result;
}

我用| null键入了它，因为带有指定id的文档可能不存在.

I typed it with | null because the document with the specified id may not exist.

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