topic Re: Call back return value Scopr problem in ArcGIS JavaScript Maps SDK Questions

Call back return value Scope problem

NadirHussain — Mon, 28 Mar 2016 09:47:46 GMT

var dist1;

getDistance(clickedPt,gp.geometry,function(dist){

dist1=dist;

alert("inside"+dist1);

});

alert("outside"+dist1);

result inside block shows.but outside undefined.how to get dist outside from get distance block.dist1 undefined.Please help what can be the reason.

Thanks

Re: Call back return value Scopr problem

TyroneBiggums — Mon, 28 Mar 2016 11:19:47 GMT

Demystifying JavaScript Closures, Callbacks and IIFEs

Re: Call back return value Scopr problem

RobertScheitlin__GISP — Sat, 11 Dec 2021 06:20:30 GMT

Nadir,

You use dojo lang.hitch to keep your function in scope:

var dist1;
getDistance(clickedPt, gp.geometry, lang.hitch(this, function(dist){
  dist1=dist;
  alert("inside"+dist1);
}));
alert("outside"+dist1);

Re: Call back return value Scopr problem

NadirHussain — Sun, 03 Apr 2016 07:20:19 GMT

Dear Robert,

I call my function same like you explain above

statment

alert("outside"+dist1);not giving any result.

alert("inside"+dist1);giving result.

so i am still stuck for scope variable.

Re: Call back return value Scopr problem

RobertScheitlin__GISP — Sun, 03 Apr 2016 12:42:47 GMT

Nadir,

I will have to see more of your code then as this should work.

Re: Call back return value Scopr problem

NadirHussain — Mon, 04 Apr 2016 04:32:28 GMT

Dear Robert,

My function defination:

function getDistance (p1,p2,callback) {

var distParams = new esri.tasks.DistanceParameters();

distParams.distanceUnit = esri.tasks.GeometryService.UNIT_STATUTE_MILE;

distParams.geometry1 = p1;

distParams.geometry2 = p2;

distParams.geodesic = true;

var gsvc= new esri.tasks.GeometryService("http://myserver/arcgis/rest/services/Utilities/Geometry/GeometryServer");

gsvc.distance(distParams,function(response){

callback(response);

});

}

function calling:

var dist1;

getDistance(clickedPt,gp.geometry, dojo._base.lang.hitch(this, function(dist){

dist1=dist;

alert("inside"+dist1);

}));

alert("outside"+dist1);

outside nothing show.inside alert show values.i want this outside callback block.

Thanks.

Re: Call back return value Scope problem

TyroneBiggums — Sat, 11 Dec 2021 06:20:33 GMT

I don't think you can do things like that. First, the lang.hitch maintains the context in your objects. I don't think context and scope are the same. I think hitch only keeps up with what "this" is.

Second, you're calling an asynchronous function (your getDistance) that calls an asynchronous function (geo svc function). Your outside alert is going to run synchronously and won't care about the asynchronous timing of the other objects.

Please see my code snippet below for how I show that dist1 remains in scope if you move things around. I have a map click to start the get distance. I have a second test button click event that alerts dist1 for the 'outside'.

var dist1;

var getDistanceCallback = function(x) {
    dist1 = x
    alert('inside callback ' + dist1);
}

map.on("click", function(evt) {
    var sReference = new SpatialReference(102100);
    var newGraphic = new Point(0, 0, sReference);
    var newGraphic2 = new Point(75, Math.floor((Math.random() * 50) + 1), sReference);
    
    getDistance(newGraphic, newGraphic2, getDistanceCallback);
});

on(dom.byId('btn'), 'click', function() {
    alert('button: ' + dist1);
});