for循环中的Javascript承诺角​​链

Question

I would like that the for loop will be executed and then the result will be sent over the next function:

FindIdsRequests =  function(){
    results = $scope.requests
    var deferred = $q.defer();
    var promise = deferred.promise;
    var array = []
    for (var i in results) {
        promise = promise.then(function(){
            array.push(results[i].uid)      
        })
        return promise
    }
    return promise.then(function(){
        return array
    })
}

$scope.ConfirmRequests = function(){
    //alert('req'+JSON.stringify($scope.requests))
    FindIdsRequests().then(function(array){
        alert('arr'+JSON.stringify(array))
    })
})

the FindIdsRequests function should return the result of the for loop however there is no return (the alert is not printed so does not arrive there). Any idea?

Answer 1

You can take advantages of $q insted of returning a promise like so:

$q.all([promise1, promise2]);

For example:

FindIdsRequests =  function(){

    var requests = $scope.requests;
    var results = [];    
    var array = [];

    for (var i in requests) {

        var req = $http.get('http://someUrl.com/' + requests[i].uid);

        array.push(req);

        req.then(function (response) {
            results.push(response.data);
        });
    }

    return $q.all(array).then(function(){
        return results;
    });
}

This code will return a promisse of all promisses in the array to be resolved.

Answer 2

Problems:

1. You are overwriting promise in each iteration of loop... so there is only ever one promise
2. There is no resolve of promise to ever trigger then() so the array will always be empty
3. You have a return in the loop so the loop will break on first iteration and never complete
4. If this were asynchronous i will not be what you think it is inside then() as it will have reached it's end before the promise resolves

I see no need to write this up in code since everything you are doing is synchronous and shows no need for promise in the first place

Answer 3

Ok thanks also to the comments of @charlietlf the following code works:

FindIdsRequests =  function(){

    results = $scope.requests
    var deferred = $q.defer();
    var promise = deferred.promise;
    var array = []
    for (var i in results) {
        alert('www'+JSON.stringify(results[i].uid))
        var obj = results[i].uid
        promise = promise.then(function(){
            array.push(results[i].uid)      
        })
        //return promise
    }
    deferred.resolve(array)
    return promise.then(function(){
            alert('ee'+JSON.stringify(array))
            return array
     })
}

essentially I forgot to put the resolve.

Answer 4

Creating a Chain of Promises from an Array of Promises

Use $.when to create an empty promise and use a for loop to create a chain:

function chainPromiseArray(promiseArray){
    var promise = $q.when();
    var array = [];
    for (var i=0;  i < promiseArray.length; i++) {
        promise = promise.then(function(){
            var derivedPromise = promiseArray[i].then(function(result) {
                //push data
                array.push(result.data);
            });
            //return promise to chain
            return derivedPromise;
        });
    };
    var finalPromise =  promise.then(function(){
        return array;
    });
    return finalPromise;
};

The above example starts with an empty promise, then chains from that empty promise. The final promise will resolve to the array created by the chain.

Chaining promises

Because calling the .then method of a promise returns a new derived promise, it is easily possible to create a chain of promises. It is possible to create chains of any length and since a promise can be resolved with another promise (which will defer its resolution further), it is possible to pause/defer resolution of the promises at any point in the chain. This makes it possible to implement powerful APIs.

-- AngularJS $q Service API Reference -- chaining promises

The example executes the promises sequentially. To execute the promises in parallel use the $q.all method .

Answer 5

Your FindIdsRequests() function has no async elements in the code as shown so it can just be a synchronous function and, in fact, trying to use promises in it is just complicating a very simple operation:

// get an array of uid from $scope.requests
function FindIdsRequests() {
    return $scope.requests.map(function(item) {
        return item.uid;
    });
});

$scope.ConfirmRequests = function() {
    var ids = FindIdsRequests();
    console.log('arr: '+ JSON.stringify(ids));
})