Having issues with FileReader and $scope.$watch in AngularJS?

Question

Having issues with FileReader and $scope.$watch in AngularJS?

Seeking assistance with a new challenge - working with files in JavaScript is uncharted territory for me. I have a directive with a template that includes an input file and upload button.

<input type="file" ng-file-select ng-model="files">
<button ng-click="onFileSelect()">Upload</button>

In the directive's controller, my code looks like this:

$scope.onFileSelect = function() {
  reader = new FileReader();
  reader.onload = function() {
    $scope.file_contents = this.result;
  };
  reader.readAsArrayBuffer($scope.files[0]);

//debugger;
  $scope.$watch('file_contents',function(file_contents) {
    console.log(file_contents);
    if (angular.isDefined(file_contents)) {
      ... lots of coding to handle file_contents ...
    }
  });
};

After selecting a file and clicking Upload, the console displays undefined for (file_contents) on the first click. It only shows a value after clicking the button a second time.

When I uncomment the debugger, $scope.file_contents has a value when checked.

So, it seems like file_contents takes some time to be set but the $watch doesn't detect it. Could there be a unique reason for this delay? Does $watch not work properly with FileReader objects?

Edit 1:

Following advice provided, here's an updated version of the code:

$scope.onFileSelect = function() {
  reader = new FileReader();
  reader.onload = function() {
    file_contents = this.result;
    upload_file($scope.files[0],file_contents);
  };
   reader.readAsArrayBuffer($scope.files[0]);
};

upload_file = function(file,file_contents) {
  <performing operations related to file_contents>
};

Despite removing all $watches, why do $digest errors still occur? There are no interactions with $scope in upload_file. The error message only states:

Error: [$rootScope:inprog] $digest already in progress
http://errors.angularjs.org/1.3.0-beta.10/$rootScope/inprog?p0=%24digest

What might be causing this issue?

javascript angularjs filereader

Answer 1

Answer №1

It appears that the watch is not detecting changes because you are updating the scope outside of the angular context. To resolve this, you should manually trigger the digest cycle using scope.$apply(), or another method such as using a $timeout within the onload async function. Additionally, it would be recommended to move the watch outside of the onFileSelect method to prevent adding multiple watches with each upload click.

Consider implementing the following:

$scope.onFileSelect = function() {
  reader = new FileReader();
  reader.onload = function() {
    $scope.file_contents = this.result;
    $scope.$apply(); // <-- Here
  };
  reader.readAsArrayBuffer($scope.files[0]);
};

$scope.$watch('file_contents',function(file_contents) {
    console.log(file_contents);
    if (angular.isDefined(file_contents)) {
      ... code for handling file contents ...
    }
});

In some cases, creating a watch may not be necessary. Instead, encapsulate the file processing logic within a separate method and call this method from the onload event while also triggering a scope.$apply(). The reason why your watch only executes once initially is because it runs upon setup to initiate dirty checking, but at that point in time the asynchronous onload event has not yet assigned any value to the scope. Even if a value is eventually set, the digest cycle remains unaware of it.

Answer 2

It appears that the watch is not detecting changes because you are updating the scope outside of the angular context. To resolve this, you should manually trigger the digest cycle using scope.$apply(), or another method such as using a $timeout within the onload async function. Additionally, it would be recommended to move the watch outside of the onFileSelect method to prevent adding multiple watches with each upload click.

Consider implementing the following:

$scope.onFileSelect = function() {
  reader = new FileReader();
  reader.onload = function() {
    $scope.file_contents = this.result;
    $scope.$apply(); // <-- Here
  };
  reader.readAsArrayBuffer($scope.files[0]);
};

$scope.$watch('file_contents',function(file_contents) {
    console.log(file_contents);
    if (angular.isDefined(file_contents)) {
      ... code for handling file contents ...
    }
});

In some cases, creating a watch may not be necessary. Instead, encapsulate the file processing logic within a separate method and call this method from the onload event while also triggering a scope.$apply(). The reason why your watch only executes once initially is because it runs upon setup to initiate dirty checking, but at that point in time the asynchronous onload event has not yet assigned any value to the scope. Even if a value is eventually set, the digest cycle remains unaware of it.

Answer 3

Answer №2

There is a more elegant solution for this problem. Here's how you can define a directive:

//directive.js
(function() {
    angular.module('<yourApp>').
    directive('fileread', ['$parse', function($parse){
        // Executes during compile time
        return {
            restrict: 'A',
            link: function($scope, iElm, iAttrs, controller) {

                var model = $parse(iAttrs.fileread);
                var modelSetter = model.assign;

                iElm.bind('change', function(){
                    $scope.$apply(function(){
                        modelSetter($scope, iElm[0].files[0]);

                    });
                });     
            }
        };
    }]);
})();

Next, in your controller (or another directive), create a model like this:

//controller.js
...
$scope.photo.data = null;
...

$scope.photo.upload = function() {
    fd = new FormData();
    fd.append('<name_of_form_field_the_photo_should_be_sent_as>', $scope.photo.data);

$http.post('<url>', fd, {
     transformRequest: angular.identity,
     headers: {'Content-Type': undefined}
})

Finally, add the following code to your HTML file:

<input type='file' fileread="photo.data">
<button ng-click="photo.upload"> </button>

I hope this explanation is helpful.

Answer 4

There is a more elegant solution for this problem. Here's how you can define a directive:

//directive.js
(function() {
    angular.module('<yourApp>').
    directive('fileread', ['$parse', function($parse){
        // Executes during compile time
        return {
            restrict: 'A',
            link: function($scope, iElm, iAttrs, controller) {

                var model = $parse(iAttrs.fileread);
                var modelSetter = model.assign;

                iElm.bind('change', function(){
                    $scope.$apply(function(){
                        modelSetter($scope, iElm[0].files[0]);

                    });
                });     
            }
        };
    }]);
})();

Next, in your controller (or another directive), create a model like this:

//controller.js
...
$scope.photo.data = null;
...

$scope.photo.upload = function() {
    fd = new FormData();
    fd.append('<name_of_form_field_the_photo_should_be_sent_as>', $scope.photo.data);

$http.post('<url>', fd, {
     transformRequest: angular.identity,
     headers: {'Content-Type': undefined}
})

Finally, add the following code to your HTML file:

<input type='file' fileread="photo.data">
<button ng-click="photo.upload"> </button>

I hope this explanation is helpful.

Having issues with FileReader and $scope.$watch in AngularJS?

Answer №1

Answer №2

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