Is there a way for me to monitor the ngSrc attribute of an image element and retrieve the updated width and height within my personalized directive?

Question

Is there a way for me to monitor the ngSrc attribute of an image element and retrieve the updated width and height within my personalized directive?

Here is a snippet from index.html:

<img ng-src="{{ImageURL}}"  my-image/>

This is from app.js:

var app = angular.module('plunker', []);

app.controller('MyCtl', function($scope) {

  $scope.ImageURL = "";
  $scope.ImgWidth = 0;

  $scope.setImgSrc = function(imgURL) {
    $scope.ImageURL = imgURL;
  };

  $scope.setImgSrc('http://angularjs.org/img/AngularJS-large.png');

});

app.directive('myImage', [function() {

  return function(scope, elm, attrs) {

    scope.$watch(elm.width(), function(newValue, oldValue) {

      scope.ImgWidth = newValue; // always returns 0!

    });

  };

}]);

If you're curious or have a solution, check out the plunk. How can I retrieve the new dimensions of the image element in my custom directive when the ngSrc attribute changes? My suspicion is that there may be an issue with how I'm utilizing scope.$watch.

javascript angularjs

Answer 1

Answer №1

It appears that the watch on your plunk is accurate, unlike the example on SO which may not deliver the expected results.

In order for a watch expression to work properly, it should either be a string expression or a function. The example provided shows an attempt to watch the result of elm.width(), which likely returns 0. Essentially, this is akin to

scope.$watch(0, function() {...})

. If monitoring width changes is the goal, the proper approach would involve using

scope.$watch(function() { return elm.width(); }, function() {...})

. However, frequent access to the DOM through watch expressions is generally discouraged.

A more efficient alternative involves waiting until the image loads (using the load event) and updating measurements at that point. This way, DOM manipulation only occurs when necessary. For implementation details, refer to the updated plunk here.

Answer 2

It appears that the watch on your plunk is accurate, unlike the example on SO which may not deliver the expected results.

In order for a watch expression to work properly, it should either be a string expression or a function. The example provided shows an attempt to watch the result of elm.width(), which likely returns 0. Essentially, this is akin to

scope.$watch(0, function() {...})

. If monitoring width changes is the goal, the proper approach would involve using

scope.$watch(function() { return elm.width(); }, function() {...})

. However, frequent access to the DOM through watch expressions is generally discouraged.

A more efficient alternative involves waiting until the image loads (using the load event) and updating measurements at that point. This way, DOM manipulation only occurs when necessary. For implementation details, refer to the updated plunk here.

Answer 3

Answer №2

Due to the small size of the image, it is probably not noticeable, but the width is being obtained before the image has fully loaded. To address this issue, you can add an 'on load' event to the element.

app.directive('myImage', [function() {
    return function(scope, elm, attrs) {
      elm.on('load', function()
      {
        scope.ImgWidth = $(this).width();
        scope.$apply();
      });
    };
}]);

Answer 4

Due to the small size of the image, it is probably not noticeable, but the width is being obtained before the image has fully loaded. To address this issue, you can add an 'on load' event to the element.

app.directive('myImage', [function() {
    return function(scope, elm, attrs) {
      elm.on('load', function()
      {
        scope.ImgWidth = $(this).width();
        scope.$apply();
      });
    };
}]);

Answer 5

Answer №3

Your issue lies within the $watch function you are using. It requires the first argument to be a string for evaluation or a function that can be called to check values. You have passed an integer instead of meeting these requirements. Consider updating your code like this:

 scope.$watch(function() { return elm.width(); }, function(newValue, oldValue) {
     scope.ImageWidth = newValue;
 });

You can find the updated Plunk here: http://plnkr.co/edit/93SvAosQWkQzRq0DFXaK?p=preview

Additionally, make sure to include the complete jQuery library to access the width() function as demonstrated in my example.

Update - I have modified the Plunk based on @Andyrooger's suggestion to handle the load event. Ideally, fetching the width within the load event handler would be more efficient, but I have kept it as is to address the $watch method mentioned in the original question.

Answer 6

Your issue lies within the $watch function you are using. It requires the first argument to be a string for evaluation or a function that can be called to check values. You have passed an integer instead of meeting these requirements. Consider updating your code like this:

 scope.$watch(function() { return elm.width(); }, function(newValue, oldValue) {
     scope.ImageWidth = newValue;
 });

You can find the updated Plunk here: http://plnkr.co/edit/93SvAosQWkQzRq0DFXaK?p=preview

Additionally, make sure to include the complete jQuery library to access the width() function as demonstrated in my example.

Update - I have modified the Plunk based on @Andyrooger's suggestion to handle the load event. Ideally, fetching the width within the load event handler would be more efficient, but I have kept it as is to address the $watch method mentioned in the original question.

Is there a way for me to monitor the ngSrc attribute of an image element and retrieve the updated width and height within my personalized directive?

Answer №1

Answer №2

Answer №3

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