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Preloading Images In AngularJS With Promises
<!doctype html>
<html ng-app="Demo">
<head>
<meta charset="utf-8" />
<title>
Preloading Images In AngularJS With Promises
</title>
</head>
<body ng-controller="AppController">
<h1>
Preloading Images In AngularJS With Promises
</h1>
<div ng-switch="isLoading">
<!-- BEGIN: Loading View. -->
<p ng-switch-when="true">
Your images are being loaded... {{ percentLoaded }}%
</p>
<!-- END: Loading View. -->
<!-- BEGIN: Results View. -->
<div
ng-switch-when="false"
ng-switch="isSuccessful">
<p ng-switch-when="true">
<img
ng-repeat="src in imageLocations"
ng-src="{{ src }}"
style="width: 100px ; margin-right: 10px ;"
/>
</p>
<p ng-switch-when="false">
<strong>Oops</strong>: One of your images failed to load :(
</p>
</div>
<!-- END: Results View. -->
</div>
<!-- Load scripts. -->
<script type="text/javascript" src="../../vendor/jquery/jquery-2.0.3.min.js"></script>
<script type="text/javascript" src="../../vendor/angularjs/angular-1.2.min.js"></script>
<script type="text/javascript">
// Create an application module for our demo.
var app = angular.module( "Demo", [] );
// -------------------------------------------------- //
// -------------------------------------------------- //
// I control the root of the application.
app.controller(
"AppController",
function( $scope, preloader ) {
// I keep track of the state of the loading images.
$scope.isLoading = true;
$scope.isSuccessful = false;
$scope.percentLoaded = 0;
// I am the image SRC values to preload and display./
// --
// NOTE: "cache" attribute is to prevent images from caching in the
// browser (for the sake of the demo).
$scope.imageLocations = [
( "./ahhh.jpg?v=1&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=2&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=3&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=4&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=5&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=6&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=7&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=8&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=9&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=10&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=11&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=12&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=13&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=14&cache=" + ( new Date() ).getTime() ),
( "./ahhh.jpg?v=15&cache=" + ( new Date() ).getTime() ),
];
// Preload the images; then, update display when returned.
preloader.preloadImages( $scope.imageLocations ).then(
function handleResolve( imageLocations ) {
// Loading was successful.
$scope.isLoading = false;
$scope.isSuccessful = true;
console.info( "Preload Successful" );
},
function handleReject( imageLocation ) {
// Loading failed on at least one image.
$scope.isLoading = false;
$scope.isSuccessful = false;
console.error( "Image Failed", imageLocation );
console.info( "Preload Failure" );
},
function handleNotify( event ) {
$scope.percentLoaded = event.percent;
console.info( "Percent loaded:", event.percent );
}
);
}
);
// -------------------------------------------------- //
// -------------------------------------------------- //
// I provide a utility class for preloading image objects.
app.factory(
"preloader",
function( $q, $rootScope ) {
// I manage the preloading of image objects. Accepts an array of image URLs.
function Preloader( imageLocations ) {
// I am the image SRC values to preload.
this.imageLocations = imageLocations;
// As the images load, we'll need to keep track of the load/error
// counts when announing the progress on the loading.
this.imageCount = this.imageLocations.length;
this.loadCount = 0;
this.errorCount = 0;
// I am the possible states that the preloader can be in.
this.states = {
PENDING: 1,
LOADING: 2,
RESOLVED: 3,
REJECTED: 4
};
// I keep track of the current state of the preloader.
this.state = this.states.PENDING;
// When loading the images, a promise will be returned to indicate
// when the loading has completed (and / or progressed).
this.deferred = $q.defer();
this.promise = this.deferred.promise;
}
// ---
// STATIC METHODS.
// ---
// I reload the given images [Array] and return a promise. The promise
// will be resolved with the array of image locations.
Preloader.preloadImages = function( imageLocations ) {
var preloader = new Preloader( imageLocations );
return( preloader.load() );
};
// ---
// INSTANCE METHODS.
// ---
Preloader.prototype = {
// Best practice for "instnceof" operator.
constructor: Preloader,
// ---
// PUBLIC METHODS.
// ---
// I determine if the preloader has started loading images yet.
isInitiated: function isInitiated() {
return( this.state !== this.states.PENDING );
},
// I determine if the preloader has failed to load all of the images.
isRejected: function isRejected() {
return( this.state === this.states.REJECTED );
},
// I determine if the preloader has successfully loaded all of the images.
isResolved: function isResolved() {
return( this.state === this.states.RESOLVED );
},
// I initiate the preload of the images. Returns a promise.
load: function load() {
// If the images are already loading, return the existing promise.
if ( this.isInitiated() ) {
return( this.promise );
}
this.state = this.states.LOADING;
for ( var i = 0 ; i < this.imageCount ; i++ ) {
this.loadImageLocation( this.imageLocations[ i ] );
}
// Return the deferred promise for the load event.
return( this.promise );
},
// ---
// PRIVATE METHODS.
// ---
// I handle the load-failure of the given image location.
handleImageError: function handleImageError( imageLocation ) {
this.errorCount++;
// If the preload action has already failed, ignore further action.
if ( this.isRejected() ) {
return;
}
this.state = this.states.REJECTED;
this.deferred.reject( imageLocation );
},
// I handle the load-success of the given image location.
handleImageLoad: function handleImageLoad( imageLocation ) {
this.loadCount++;
// If the preload action has already failed, ignore further action.
if ( this.isRejected() ) {
return;
}
// Notify the progress of the overall deferred. This is different
// than Resolving the deferred - you can call notify many times
// before the ultimate resolution (or rejection) of the deferred.
this.deferred.notify({
percent: Math.ceil( this.loadCount / this.imageCount * 100 ),
imageLocation: imageLocation
});
// If all of the images have loaded, we can resolve the deferred
// value that we returned to the calling context.
if ( this.loadCount === this.imageCount ) {
this.state = this.states.RESOLVED;
this.deferred.resolve( this.imageLocations );
}
},
// I load the given image location and then wire the load / error
// events back into the preloader instance.
// --
// NOTE: The load/error events trigger a $digest.
loadImageLocation: function loadImageLocation( imageLocation ) {
var preloader = this;
// When it comes to creating the image object, it is critical that
// we bind the event handlers BEFORE we actually set the image
// source. Failure to do so will prevent the events from proper
// triggering in some browsers.
var image = $( new Image() )
.load(
function( event ) {
// Since the load event is asynchronous, we have to
// tell AngularJS that something changed.
$rootScope.$apply(
function() {
preloader.handleImageLoad( event.target.src );
// Clean up object reference to help with the
// garbage collection in the closure.
preloader = image = event = null;
}
);
}
)
.error(
function( event ) {
// Since the load event is asynchronous, we have to
// tell AngularJS that something changed.
$rootScope.$apply(
function() {
preloader.handleImageError( event.target.src );
// Clean up object reference to help with the
// garbage collection in the closure.
preloader = image = event = null;
}
);
}
)
.prop( "src", imageLocation )
;
}
};
// Return the factory instance.
return( Preloader );
}
);
</script>
</body>
</html>
@alpual

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alpual commented Sep 16, 2016

Thanks! This is very helpful. I've been using this, however, this throws some errors with jQuery 3+. Either change .load and .error functions to .on("load", and .on("error", or see my fork: https://gist.github.com/alpual/f67e1a97fa1d1c964fe4be0f25c31a01

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