如何做条件承诺链

Question

I'm learning promises/typescript/angular and I want to chain promises conditionally.

This is the actual state of my method:

private executePromiseModificationEvenement<T>(edition: Models.CalendrierParametresModelEdition, modeCreation: boolean, deferred: ng.IDeferred<T>) {
    var promise: ng.IPromise<Object>;

    //Step1
    if (modeCreation) {
        promise = this.$calendrier.Actions.enregistrerEvenementOutlook(edition);
    } else {
        promise = this.$calendrier.Actions.modifierEvenementOutlook(edition);
    }

    if (this.$scope.outlook) {
        promise.then((x) => {
            if (x != '') edition.idOutlook = x.toString();;
            return deferred.resolve();
        }, (x) => {
            return deferred.reject();
        });
    } else {
        //Step2
        promise.then((x) => {
            if (x != '') edition.idOutlook = x.toString();
            return this.$calendrier.Actions.modifierEvenement(edition);
        }, (x) => {
            //Ajout MessageBox message error
            return this.$calendrier.Actions.modifierEvenement(edition);
        })
        //Step3
        .then((x) => {
            if (edition.opportunite != null) this.$rootScope.$broadcast("pushEchangeOpportunite", { idOpportunite: parseInt(edition.opportunite), action: 2, IdContact: edition.id, Libelle: edition.title, StartDate: moment(edition.start).toDate() });
            return deferred.resolve();
        }, (x) => {
            return deferred.reject();
        });
    }
}

I'm familiar of async/await of C#, neither of which gives a problem with conditional chaining, but I'm having trouble achieving the same with promises.

Is it to correct to put a .then not just after the creation of the promise but after an if ?

Is it possible that the .then may never be called because the promise is already finished?

Answer 1

It is fine to chain promises together in any order or use ifs, loops, whatever.

If you call .then on a resolved promise then it will execute instantly, so that's fine too.
The only way the then will not be called is if the promise chain is never resolved or is rejected.

A normal way of chaining might be to return the next object from within the function. This tends to be neater than calling deferred.resolve() .

Eg

var promise = this.$calendrier.Actions.enregistrerEvenementOutlook(edition);
promise = promise.then(function (x) {
    return 2 * x;
})
promise = promise.then(function (x) {
    return 2 * x;
})

or

var promise =
    this.$calendrier.Actions.enregistrerEvenementOutlook(edition)
    .then(function (x) {
        return 2 * x;
    })
    .then(function (x) {
        return 2 * x;
    })

Answer 2

executePromiseModificationEvenement() doesn't need a Deferred. There's absolutely no value in passing one in. Instead, you should be looking to return the promise returned by a promise chain formed within the function. The caller function(s) will need only a minor change.

Straightforwardly, your function can be rewritten with a series of (conditional) promise = promise.then(...) statements with a final return promise . Some of the code repetition can also be addressed.

private executePromiseModificationEvenement<T>(edition: Models.CalendrierParametresModelEdition, modeCreation: boolean<T>) {
    var promise: ng.IPromise<Object>;
    promise = modeCreation ?
        this.$calendrier.Actions.enregistrerEvenementOutlook(edition) :
        this.$calendrier.Actions.modifierEvenementOutlook(edition);

    promise = promise.then((x) => {
        if (x != '') {
            edition.idOutlook = x.toString();
        }
    });

    if (!this.$scope.outlook) {
        promise = promise.then(() => {
            return this.$calendrier.Actions.modifierEvenement(edition);
        }, () => {
            return this.$calendrier.Actions.modifierEvenement(edition);
        })
        .then((x) => {
            if (edition.opportunite != null) {
                this.$rootScope.$broadcast("pushEchangeOpportunite", {
                    idOpportunite: parseInt(edition.opportunite), 
                    action: 2, 
                    IdContact: edition.id, 
                    Libelle: edition.title, 
                    StartDate: moment(edition.start).toDate() 
                }); 
            }
        });
    }
    return promise;
}

However, that may not be the best solution.

It may be more appropriate to perform the if(this.$scope.outlook) test at the point where this.$calendrier.Actions.modifierEvenement()... is called, during chain settlement rather than during the chain building phase. The result will not necessarily be the same because this.$scope.outlook will have had an opportunity to change state.

Personally, I would guess that performing the test later is more appropriate (or inconsequential). If so, the promise chain can be built unconditionally and all the tests performed internally, which if nothing else, is much tidier.

private executePromiseModificationEvenement<T>(edition: Models.CalendrierParametresModelEdition, modeCreation: boolean<T>) {
    return (modeCreation ? 
        this.$calendrier.Actions.enregistrerEvenementOutlook(edition) :
        this.$calendrier.Actions.modifierEvenementOutlook(edition))
    .then((x) => {
        if (x != '') {
            edition.idOutlook = x.toString();
        }
    })
    .catch((x) => { return x; }) // this mid-chain-error-recovery line is rather odd but consistent with the original code. It may be better placed one step earlier.
    .then(() => {
        if (!this.$scope.outlook) {
            return this.$calendrier.Actions.modifierEvenement(edition)
            .then(() => {
                if (edition.opportunite != null) {
                    this.$rootScope.$broadcast("pushEchangeOpportunite", {
                        'idOpportunite': parseInt(edition.opportunite),
                        'action': 2,
                        'IdContact': edition.id,
                        'Libelle': edition.title,
                        'StartDate': moment(edition.start).toDate()
                    }); 
                }
            });
        }
    });
}