如何通过间隔获得先前的结果？

Question

我需要在 Pipe observable 中以间隔获取 observable 的前一个值。例子是：

const imageStatus = interval(5000)
this.statusSubscription = imageStatus.pipe(
  startWith(0),
  pairwise(),
  switchMap(() => this.settingsService.getImageStatus()),
  tap(res => {
    this.statusMessage = '';
    this.statusError = '';
    if (res.data.message) {
      this.statusError = ''
      this.statusMessage = res.data.message
    }
    if (res.data.error) {
      this.statusMessage = '';
      this.statusError = res.data.error
    }
  }),
  switchMap(res => {
    // here I need to cmpare prevRes and NextRes
    if (res.data.status !== 0) {
      return this.settingsService.getImageList()
    } else {
      return of({})
    }
  })
).subscribe(res => {
  if (res.data) {
    this.imageTableList = new MatTableDataSource(res.data)
  }
})

所以这里我需要知道之前区间迭代的getImageStatus() 的结果。我需要在第二个 switchMap 中比较以前的结果和新的结果

Answer 1

“pairWise”需要放在switchMap之后，有些像

interval(5000).pipe(
   startWith(0),
   switchMap(_=>this.settingsService.getImageStatus()),
   tap(res=>{..if you want to do something with the response of getImageStatus...}),
   pairwise(),
   switchMap(([old,value])=>{
      ..here you has in old the before value
      ..and in value the response actual..
      return ....
   })
).subscribe(res=>{
   ...
})