RxSwift toArray does not compile with generic parameter

Question

RxSwift toArray is not working for me when using generics:

struct SaveModelsCommand<M> where M:Model {

    let models:[M]

    func create() -> Observable<[M]> {

        let cloudKitRecords:[CKRecord] = models.map({ 
            // convert models to CKRecords
            ... 
        })

        return SaveRecordsCommand(cloudKitRecords)
            .createObservable()
            .flatMap({ savedRecords in
                // convert array to multiple emissions so we can iterate it
                return Observable.from(savedRecords)
            })
            .flatMap({ (record:CKRecord) -> M in
                // convert CKRecord back to a model (aka M)
                ... create model (e.g. Member) ...
                return model
            })
            // convert back to a single emission (array)
            .toArray() // <<<<< ERROR
    }
}

Here is the error:

Cannot convert return expression of type 'Observable<[M.E]>' (aka 'Observable>') to return type 'Observable<[M]>' (aka 'Observable>')

The only difference that I can see in the return type is M.E vs M.

Any ideas?

solidcell · Accepted Answer · 2016-09-20 13:41:10Z

2

flatMap expects the closure to return an Observable<M>, not just a plain M:

.flatMap({ (record:CKRecord) -> Observable<M> in
    // convert CKRecord back to a model (aka M)
    //... create model (e.g. Member) ...
    return Observable.just(model)
})

Alternatively, you can use map and just return an M:

.map({ (record:CKRecord) -> M in
    // convert CKRecord back to a model (aka M)
    //... create model (e.g. Member) ...
    return model
})

answered Sep 20, 2016 at 13:41

solidcell

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1 Comment

David James Over a year ago

Thank you so much! I went with map since that more closely represents the intent.

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