AngularJS : Asynchronously initialize filter

Here is an example:

app.filter("testf", function($timeout) {    var data = null, // DATA RECEIVED ASYNCHRONOUSLY AND CACHED HERE        serviceInvoked = false;    function realFilter(value) { // REAL FILTER LOGIC        return ...;    }    return function(value) { // FILTER WRAPPER TO COPE WITH ASYNCHRONICITY        if( data === null ) {            if( !serviceInvoked ) {                serviceInvoked = true;                // CALL THE SERVICE THAT FETCHES THE DATA HERE                callService.then(function(result) {                    data = result;                });            }            return "-"; // PLACEHOLDER WHILE LOADING, COULD BE EMPTY        }        else return realFilter(value);    }});

This fiddle is a demonstration using timeouts instead of services.

EDIT: As per the comment of sgimeno, extra care must be taken for not calling the service more than once. See the serviceInvoked changes in the code above and the fiddles. See also forked fiddle with Angular 1.2.1 and a button to change the value and trigger digest cycles: forked fiddle

EDIT 2: As per the comment of Miha Eržen, this solution does no logner work for Angular 1.3. The solution is almost trivial though, using the $stateful filter flag, documented here under "Stateful filters", and the necessary forked fiddle.

Do note that this solution would hurt performance, as the filter is called each digest cycle. The performance degradation could be negligible or not, depending on the specific case.

Let's start with understanding why the original code doesn't work. I've simplified the original question a bit to make it more clear:

angular.module('angularApp').filter('pathToName', function(Service) {    return function(input) {        return Service.getCorresp().then(function(response) {            return response;        });    });}

Basically, the filter calls an async function that returns the promise, then returns its value. A filter in angular expects you to return a value that can be easily printed, e.g string or number. However, in this case, even though it seems like we're returning the response of getCorresp, we are actually returning a new promise - The return value of any then() or catch() function is a promise.

Angular is trying to convert a promise object to a string via casting, getting nothing sensible in return and displays an empty string.

So what we need to do is, return a temporary string value and change it asynchroniously, like so:

JSFiddle

HTML:

<div ng-app="app" ng-controller="TestCtrl">    <div>{{'WelcomeTo' | translate}}</div>    <div>{{'GoodBye' | translate}}</div></div>

Javascript:

app.filter("translate", function($timeout, translationService) {    var isWaiting = false;    var translations = null;    function myFilter(input) {        var translationValue = "Loading...";        if(translations)        {            translationValue = translations[input];        } else {            if(isWaiting === false) {                isWaiting = true;                translationService.getTranslation(input).then(function(translationData) {                    console.log("GetTranslation done");                    translations = translationData;                    isWaiting = false;                });            }        }        return translationValue;    };    return myFilter;});

Everytime Angular tries to execute the filter, it would check if the translations were fetched already and if they weren't, it would return the "Loading..." value. We also use the isWaiting value to prevent calling the service more than once.

The example above works fine for Angular 1.2, however, among the changes in Angular 1.3, there is a performance improvement that changes the behavior of filters. Previously the filter function was called every digest cycle. Since 1.3, however, it only calls the filter if the value was changed, in our last sample, it would never call the filter again - 'WelcomeTo' would never change.

Luckily the fix is very simple, you'd just need to add to the filter the following:

JSFiddle

myFilter.$stateful = true;

Finally, while dealing with this issue, I had another problem - I needed to use a filter to get async values that could change - Specifically, I needed to fetch translations for a single language, but once the user changed the language, I needed to fetch a new language set. Doing that, proved a bit more tricky, though the concept is the same. This is that code:

JSFiddle

var app = angular.module("app",[]);debugger;app.controller("TestCtrl", function($scope, translationService) {    $scope.changeLanguage = function() {        translationService.currentLanguage = "ru";    }});app.service("translationService", function($timeout) {    var self = this;    var translations = {"en": {"WelcomeTo": "Welcome!!", "GoodBye": "BYE"},                         "ru": {"WelcomeTo": "POZHALUSTA!!", "GoodBye": "DOSVIDANYA"} };    this.currentLanguage = "en";    this.getTranslation = function(placeholder) {        return $timeout(function() {            return translations[self.currentLanguage][placeholder];        }, 2000);    }})app.filter("translate", function($timeout, translationService) {    // Sample object: {"en": {"WelcomeTo": {translation: "Welcome!!", processing: false } } }    var translated = {};    var isWaiting = false;    myFilter.$stateful = true;    function myFilter(input) {        if(!translated[translationService.currentLanguage]) {            translated[translationService.currentLanguage] = {}        }        var currentLanguageData = translated[translationService.currentLanguage];        if(!currentLanguageData[input]) {            currentLanguageData[input] = { translation: "", processing: false };        }        var translationData = currentLanguageData[input];        if(!translationData.translation && translationData.processing === false)        {            translationData.processing = true;            translationService.getTranslation(input).then(function(translation) {                console.log("GetTranslation done");                translationData.translation = translation;                translationData.processing = false;            });        }        var translation = translationData.translation;        console.log("Translation for language: '" + translationService.currentLanguage + "'. translation = " + translation);        return translation;    };    return myFilter;});