[英]How do I subscribe in sequence and return only the value from the last observable in RxJS?
I'm in an Angular route resolver and want to return an observable.我在一个 Angular 路由解析器中,想要返回一个可观察的。
I need to subscribe to multiple async processes in order:我需要按顺序订阅多个异步进程:
A => B(a) => C(b) A => B(a) => C(b)
C depends on B and B depends on A. A must complete, then B, then C, but I only want the value from C to be used to cause the route to resolve. C 依赖于 B,B 依赖于 A。A 必须完成,然后 B,然后 C,但我只想使用 C 中的值来解决路由。
I've tried two different approaches:我尝试了两种不同的方法:
return A.pipe(
concatMap(a =>
B(a).pipe(
concatMap(b =>
C(b).pipe((c) => c); // also tried of(c)
)
)
)
);
I've also tried我也试过
return A.pipe(
concatMap(a => {
return B(a).pipe(map(b => {a: a, b: b});
),
concatMap({a, b} => {
return C(b);
)
);
How do I subscribe to A, then B, then C..., and then only get the value from the innermost observable?我如何订阅 A,然后是 B,然后是 C...,然后只从最里面的 observable 中获取值?
If I put a tap after my last concatMap I get my expected return value.如果我在最后一个 concatMap 之后点击,我会得到预期的返回值。 But my resolver never resolves?
但是我的解析器永远不会解析? (or the wrong thing is getting emitted? I cannot really tell.)
(或者发出了错误的东西?我真的不能说。)
If one of the observables in the chain never completes (like route params or query params), it'll halt the whole train.如果链中的一个 observables 永远不会完成(如路由参数或查询参数),它将停止整个火车。
switchMap
should do: switchMap
应该这样做:
A.pipe(
switchMap(B), // which is more or less the same as `switchMap(a => B(a))`
switchMap(C),
).subscribe(//...
Route resolvers need to complete before the Angular Router will continue.路由解析器需要在 Angular 路由器继续运行之前完成。 This is the same for route guards also.
路线守卫也是如此。 Adding a operator that takes an emission, such as take(1), or first() and completes will solve the problem.
添加一个接受发射的运算符,例如 take(1) 或 first() 并完成将解决问题。
return A.pipe(
concatMap(a => {
return B(a).pipe(map(b => {a: a, b: b});
),
concatMap({a, b} => {
return C(b);
),
take(1)
);
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