CODEWITHSUNDEEP
javascripthtmlrecursioncallbackjaydata
Prabhat
Prabhat

Reputation: 2263

Handling callback in recursive asynchronous calls

I am using Jaydata as API for HTML5 indexedDB. I have a table in indexedDB where I need to query recursively. I need a callback when entire process is completed. Following is the recursive function. I need to have a callback when everything is done. 

function getData(idValue) {
    myDB.MySplDB
        .filter( function(val) {
            return val.ParentId == this.parentId;
        }, {parentId: idvalue})
        .toArray( function(vals) {
            if(vals.length < 1) {
                // some operation to store the value
            } else {
                for (var j=0;j<vals.length;j++) {
                    getData(vals[j].Id);
                }
            }
        });
}

Adding .done(function(){...}); to .toArray doesn't work since it gets called before completion.

Upvotes: 1

Views: 624

Answers (2)

Gabor Dolla
Gabor Dolla

Reputation: 2736

(Disclaimer: I work for JayData)

To wait for the finish of the entire process you need to use promises. You always have to return a promise. In the loop it gets tricky, return a super promise. So the code should be something like this:

function getData(idValue) {
    return myDB.MySplDB
    .filter( function(val) {
        return val.ParentId == this.parentId;
    }, {parentId: idvalue})
    .toArray( function(vals) {
        if(vals.length < 1) {
            // some operation to store the value
            // important: return a promise from here, like:
            return myDB.saveChanges(); 
        } else {
            var promises = [];
            for (var j=0;j<vals.length;j++) {
                promises.push(getData(vals[j].Id));
            }
            return $.when.apply(this, promises);
        }
    });
}

getData(1)
.then(function() {
        // this will run after everything is finished
});

remarks:

  1. this example uses jQuery promises, so you'll need jQuery 1.8+ $.when uses varargs hence we need the apply

  2. this can work with q promise with a slightly different syntax

Upvotes: 1

phtrivier
phtrivier

Reputation: 13367

Would this pseudo-code make sense in your case ?

var helper = function (accu) {
 // Take an id from the accumulator
 // query the db for new ids, and in the db query callback : 
   // If there is child, do "some operation to store the value" (I'm not sure what you're trying to do here
   // Else add the child to the accumulator
   // if accu is empty, call the callback, it means you reached the end

getData() would call this helper with an accumulator containing the first id, and your final callback

Upvotes: 0

Related Questions