Best way to preload images with Angular.js

问题:

Angular's ng-src keeps previous model until it preloads image internally. I am using different image for the banner on each page, when I switch routes, i change main view, leaving header view as it is, just changing bannerUrl model when I have it.

This is resulting in seeing previous banner image while new one is loading.

I was surprised that there's no directive for it yet, but I wanted to make a discussion before trying to build one.

What I want to do I think is have banner model on custom attribute. like:

<img preload-src="{{bannerUrl}}" ng-src="{{preloadedUrl}}">

Then $scope.watch for bannerUrl change, and as soon as it changes, replace ng-src with loader spinner first, and then create temproary img dom element, preload image from preload-src and then assing it to preloadUrl.

Need to think how to handle multiple images too for galleries for example.

Does anyone have any input on it? or maybe someone can point me to existing code?

I've seen existing code on github that uses background-image - but that doesn't work for me as I need dynamic height/width as my app is responsive, and I cannot do it with background-image.

Thank you

回答1:

Having the 2 urls on the directive seems a touch overcomplicated. What I think is better is to write a directive that works like:

<img ng-src="{{bannerUrl}}" spinner-on-load />

And the directive can watch ng-src and (for example) set visibility:false with a spinner until the image has loaded. So something like:

scope: {
  ngSrc: '='
},
link: function(scope, element) {
  element.on('load', function() {
    // Set visibility: true + remove spinner overlay
  });
  scope.$watch('ngSrc', function() {
    // Set visibility: false + inject temporary spinner overlay
  });
}

This way the element behaves very much like a standard img with an ng-src attribute, just with a bit of extra behaviour bolted on.

http://jsfiddle.net/2CsfZ/47/

回答2:

If anyone is interested this is my final solution: I use twitter bootstrap. So added class of "fade" to all images and just toggling class "in" with directive to fade in and out when image is loaded

angular.module('myApp').directive('imgPreload', ['$rootScope', function($rootScope) {
    return {
      restrict: 'A',
      scope: {
        ngSrc: '@'
      },
      link: function(scope, element, attrs) {
        element.on('load', function() {
          element.addClass('in');
        }).on('error', function() {
          //
        });

        scope.$watch('ngSrc', function(newVal) {
          element.removeClass('in');
        });
      }
    };
}]);

<img img-preload class="fade" ng-src="{{imgSrc}}">

Working example: http://ishq.org

回答3:

If you want you can pass the image fail and image loader as attributes for the directive....

myApp.directive("mySrc", function() {
    return {
      link: function(scope, element, attrs) {
        var img, loadImage;
        var IMAGE_LOAD="123.jpg";
        var IMAGE_FAIL="123.jpg";
        img = null;

        loadImage = function() {

          element[0].src = IMAGE_LOAD;

          img  = new Image();
          img.src = attrs.mySrc;

          img.onload = function() {
            element[0].src = attrs.mySrc;
          };
          img.onerror=function ()
          {
              element[0].src = IMAGE_FAIL;
          }
        };

        loadImage();


      }
    };
  });

回答4:

I think this is perhaps the most elegant solution because the directive actually creates the spinner and removes it automatically:

app.directive('spinnerLoad', [function spinnerLoad() {
    return {
        restrict: 'A',
        link: function spinnerLoadLink(scope, elem, attrs) {
            scope.$watch('ngSrc', function watchNgSrc() {
                elem.hide();
                elem.after('<i class="fa fa-spinner fa-lg fa-spin"></i>');  // add spinner
            });
            elem.on('load', function onLoad() {
                elem.show();
                elem.next('i.fa-spinner').remove(); // remove spinner
            });
        }
    };
}]);

Here is the html:

<img ng-src='{{imgUrl}}' spinner-load />

Note: you'll need to be using font-awesome for this to work as described here

回答5:

Just to share ^^

 //css
.media-box{
                position: relative;
                width:220px;
                height: 220px;
                overflow: hidden;
            }
            .media-box div{
                position: absolute;
                left: 0;
                top: 0;
            }
            .spinner{
                position: absolute;
                left: 0;
                top: 0;
                background: #CCC url(./spinner.gif) no-repeat center center;
                display: block;
                width:220px;
                height: 220px;
            }
            .feed img.spinner-show{
                visibility: visible;
            }
            .feed img.spinner-hide{
                visibility: hidden;
            }

//html
<div class="media-box">
  <div>
    <img data-ng-src="{{item.media}}" alt="" title="" data-spinner-on-load>
  </div>
</div>

//js
.directive('spinnerOnLoad', function() {
            return {
                restrict: 'A',
                link: function(scope,element){
                    element.on('load', function() {
                        element.removeClass('spinner-hide');
                        element.addClass('spinner-show');
                        element.parent().find('span').remove();
                    });
                    scope.$watch('ngSrc', function() {
                        element.addClass('spinner-hide');
                        element.parent().append('<span class="spinner"></span>');
                    });      
                }
            }
        });

回答6:

Instead of using

element.on('load', function() {});

use imagesLoaded plugin. It will speed up dramatically your images.

So the final code would be:

link: function(scope, element) {
  imagesLoaded(element, function() {

  });
  scope.$watch('ngSrc', function() {

  });
}

回答7:

I have this directive which shows a spinner when img-src changes:

<img-with-loading
      img-src="{{src}}"
      spinner-class="{{spinnerClass}}"
/>

Code here: http://jsfiddle.net/ffabreti/yw74upyr/

回答8:

images can be preloaded on route change by using image-preloader factory and resolve:

// call REST
                    return getContent.get().$promise.then(function(response) {

                                //return response;

                                // preload images from response
                                var imageLocations = [
                                  // put image(s) from response to array
                                  response.PostImage.big[0],
                                ];

                                // check do we have (all) image(s) in array
                                console.log(imageLocations);

                                // return when all images are preloaded
                                return preloader.preloadImages( imageLocations )
                                .then(function() {

                                    //if it was success 
                                    return response;
                                },
                                function() {

                                        //if it failed 
                                    return response;
                                });

                            });

complete tutorial here: https://www.coditty.com/code/angular-preload-images-on-route-change-by-using-resolve

回答9:

A simple solution I've found is to change the url to '//:0' before assigning it's new value

$scope.bannerUrl = 'initial value'; 

// When we want to change it
$scope.bannerUrl = '//:0';  // remove the previous img so it's not visible while the new one loads
$scope.bannerUrl = scope.preloadedUrl