从可观察数组中获取一个对象 [英] Get one object from observable array

问题描述

如何编写接受Observable<T[]>并返回Observable<T>的函数?

How do you write a function that takes an Observable<T[]> and returns Observable<T>?

例如，我有两种方法:

getTransactions(): Observable<Transaction[]>{
        ...
    }
getTransaction(id: number): Observable<Transaction>{
    return this.getTransactions().pipe(find(txn => txn.id === id));
}

我收到错误消息，指出"Observable"类型无法分配给"Observable"类型.类型'Transaction []不可分配给类型'Transaction'.类型'Transaction []'中缺少属性'id'.

I get the error that type 'Observable' is not assignable to type 'Observable'. Type 'Transaction[] is not assignable to type 'Transaction'. Property 'id' is missing in type 'Transaction[]'.

据我了解，可观察管道函数(映射，单个，查找，最大值等)与数据流有关(即，随着时间的推移，可观察对象正在发射多个项目)，当可观察对象发射时，它就没有用单个项目(恰好是数组).

As I understand, the observable pipe functions (map, single, find, max, etc.) pertain to a stream of data (i.e., while the observable is emitting multiple items over time) and are not useful when the observable emits a single item (which happens to be an array) at once.

编写getTransaction(id)函数的正确方法是什么?我试过了:

What is the proper way to write the getTransaction(id) function? I tried:

let transactions : Transaction[] = [];
this.getTransactions().subscribe(txns => transactions=txns);
return of(transactions.find(txn => txn.id === id));

但这会导致错误，提示交易"未定义.我是Angular等人的新手，所以任何指导都对您有所帮助.谢谢！

But this causes an error saying 'transaction' is undefined. I'm very new to Angular etc. so any instruction is helpful. Thank you!

Stackblitz

我正在使用:
Angular CLI:6.0.3
角度:6.0.1
rxjs:6.1.0

I'm using:
Angular CLI: 6.0.3
Angular: 6.0.1
rxjs: 6.1.0

推荐答案

您需要将可观察的值视为是数组流，而不是像对待项目流一样对待.可观察值是单个数据点，每个数据点都包含自己的数组.

You need to be treating the observable value as though it is a stream of arrays, rather than attempting to treat it like a stream of streams of items. The values in your observable are single points of data that each contain their own array.

getTransaction(id: number): Observable<Transaction>{
    return this.getTransactions().pipe(
        map(txs => txs.find(txn => txn.id === id))
    );
}

您在这里所做的就是将流中的每个数组映射到具有与参数匹配的ID的任何项.

What you're doing here is mapping each array in your stream into whatever item has an ID that matches the parameter.

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