Fetching data as reaction to observable array change in MobX - mobx

Question

Suppose we have an observable main object array, and observable data about that array (e.g. suppose we have selectedReports and reportParameters) . Now suppose we emit action to either add report to the array or remove report from that array. How do we run an action to fetch the data for reportParameters, as reaction?
Thus far, my attempt, which isn't working, looks like this:
// report parameters stuff
async fetchAllReportParameters() {
reaction(
() => this.selectedReports,
async (reports) => {
// reset the report parameters
this.reportParameters = {}
// fetch the parameters for all the reports
await reports
.forEach((report) => {
this.fetchReportParameters(report.Id)
})
}
)
}
/**
* fetches report parameters for a reportId
* #param {number} reportId
*/
fetchReportParameters = (reportId) => {
this.reportParameters[reportId] = []
const onSuccess = (reportParameters) => {
this.reportParameters[reportId] = reportParameters
}
this.api.GetReportParameters(reportId)
.then(onSuccess, this.fetchReportParametersError)
}
fetchReportParametersError = (error) => {
// TODO: output some error here
}

Answer 1

Are you ever actually calling fetchAllReportParameters? If you don't, the reaction will never be created. You may instead like to create the reaction from the constructor, assuming you always want it to be run. One example:
class SomeStore {
constructor() {
this.disposeReportsReaction = reaction(
() => this.selectedReports.slice(),
reports => {
// ...
}
)
}
}
Call storeInstanceName.disposeReaction() whenever you're done with the reaction.
Notice that I've used .slice() here. This is because if you simply pass the array reference, the reaction will never be called. See reaction docs: you have to actually use the value in some way.
You also need to tweak the async code a bit. This:
async (reports) => {
await reports.forEach((report) => {
// ...
})
}
won't do what you hope, because forEach returns undefined. Even if you shift the async keyword to the forEach callback, all the API requests will be sent in quick succession. Consider using something like this instead, depending on whether you want to wait for the preceding request before sending the next one:
try {
for (const report of reports) {
await this.fetchReportParameters(report.id)
}
} catch (e) {
// handle error
}
This isn't always the right answer: sometimes it's fine to send a bunch of requests in quick succession (perhaps especially if it's a small batch, and/or in the context of HTTP/2). If that's ok with you, you could use:
reports => {
// ...
reports.forEach(report => this.fetchReportParameters(report.id))
}