Why isn't the parent view model subscribing to the updating observables in the Knockout component?

Question

Why isn't the parent view model subscribing to the updating observables in the Knockout component?

I created a component named Upload that enables users to upload files and receive a JSON object containing these files. In this specific case, the Upload component has an input from a parent view model:

<upload params="dropzoneId: 'uploadFilesDropzone', postLocation: '/create/upload', uploadedFiles: uploadedFiles"></upload>

The crucial parameter here is called uploadedFiles. By binding this parameter, I am able to access params.uploadedFiles in my component and use the .push() method to add new objects as they are uploaded. The data passed as uploadedFiles is actually an observableArray on my parent view model:

var UploadViewModel = function () {
     // Files waiting to be added to the queue.
     self.uploadedFiles = ko.observableArray([]);
};

I have verified that params.uploadedFiles in my component is indeed an observableArray, as it supports the push method. Despite changing this value in the component, when I console.log() it, there seems to be no update:

params.uploadedFiles.push(object);
console.log(params.uploadedFiles().length); // was 0, now it should be 1

The issue lies in the fact that this modification does not affect the array in my parent view model. The self.uploadedFiles() still displays a length of 0.

Even after adding a

self.uploadedFiles.subscribe(function(newValue) {});

subscription in the parent view model, the problem persists.

Similarly, implementing a

params.uploadedFiles.valueHasMutated()

method in the component post alteration did not resolve the issue.

How can I ensure that changes made to the array in my component are reflected in the array within the parent view model?

javascript knockout.js publish-subscribe

Answer 1

Answer №1

Why do you create a new observable array when the source is already one? It's important to remember that a new object will not have the same reference as another one: simply pass it to your component's viewModel like this this.uploads = params.uploads. In the simplified version below, when you click the Add button, both arrays (referenced in different contexts) stay synchronized.

ko.components.register('upload', {
  viewModel: function(params) {
    this.uploads = params.uploads;
    this.addUpload = function() { this.uploads.push('item'); }.bind(this);
  },
  template: [
    '<div><button type="button" data-bind="click: addUpload">Add upload</button>',
    '<span data-bind="text: uploads().length + \' - \' + $root.uploads().length"></span></div>'].join('')
  
});

var app = {
  uploads: ko.observableArray([])
};
ko.applyBindings(app);

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>
<div data-bind="component: {name: 'upload', params: {uploads: uploads}}"></div>

If your source array isn't observable, things become more complex and manual subscription is necessary to update the source. For example, you would insert the following code in the viewModel:

this.uploads.subscribe(function(newValue) { params.uploads = newValue; });

Furthermore, the output in the text binding wouldn't update for the source if it wasn't observable. If, for some reason, you need 2 different observableArrays (1 source & 1 component), you can still use the line above, but replace the function code with params.uploads(newValue)

Answer 2

Why do you create a new observable array when the source is already one? It's important to remember that a new object will not have the same reference as another one: simply pass it to your component's viewModel like this this.uploads = params.uploads. In the simplified version below, when you click the Add button, both arrays (referenced in different contexts) stay synchronized.

ko.components.register('upload', {
  viewModel: function(params) {
    this.uploads = params.uploads;
    this.addUpload = function() { this.uploads.push('item'); }.bind(this);
  },
  template: [
    '<div><button type="button" data-bind="click: addUpload">Add upload</button>',
    '<span data-bind="text: uploads().length + \' - \' + $root.uploads().length"></span></div>'].join('')
  
});

var app = {
  uploads: ko.observableArray([])
};
ko.applyBindings(app);

<script src="https://cdnjs.cloudflare.com/ajax/libs/knockout/3.2.0/knockout-min.js"></script>
<div data-bind="component: {name: 'upload', params: {uploads: uploads}}"></div>

If your source array isn't observable, things become more complex and manual subscription is necessary to update the source. For example, you would insert the following code in the viewModel:

this.uploads.subscribe(function(newValue) { params.uploads = newValue; });

Furthermore, the output in the text binding wouldn't update for the source if it wasn't observable. If, for some reason, you need 2 different observableArrays (1 source & 1 component), you can still use the line above, but replace the function code with params.uploads(newValue)

Answer 3

Answer №2

It appears that the issue at hand may be connected to a potential bug, although this has yet to be confirmed: https://github.com/knockout/knockout/issues/1863

Update 1: Upon further investigation, it seems that this is not actually a bug. To access the original observable, you need to unwrap the raw parameter. For example:

params.$raw.uploadedFiles() //this will grant access to the original observableArray allowing you to perform actions like "push", "remove", etc.

The key issue arises from the fact that parameters passed to a component are wrapped in computed observables, which results in the loss of access to the original observableArray upon unwrapping.

For more information, please refer to:

Answer 4

It appears that the issue at hand may be connected to a potential bug, although this has yet to be confirmed: https://github.com/knockout/knockout/issues/1863

Update 1: Upon further investigation, it seems that this is not actually a bug. To access the original observable, you need to unwrap the raw parameter. For example:

params.$raw.uploadedFiles() //this will grant access to the original observableArray allowing you to perform actions like "push", "remove", etc.

The key issue arises from the fact that parameters passed to a component are wrapped in computed observables, which results in the loss of access to the original observableArray upon unwrapping.

For more information, please refer to:

Answer 5

Answer №3

When dealing with binding properties that involve a parent-child relationship, it's important to use the correct method of binding.

If you need to bind data to a child property, make sure to use the following syntax: data-bind='BindingName: ParentViewModel.ChildViewModel.ObservableProperty'

If you want to subscribe to a function whenever new data is added to an array, you can achieve this by using the subscribe method on the length of the observable array. This will allow you to capture the event you're looking for.

By following these guidelines, you should be able to resolve any issues you may encounter.

Answer 6

When dealing with binding properties that involve a parent-child relationship, it's important to use the correct method of binding.

If you need to bind data to a child property, make sure to use the following syntax: data-bind='BindingName: ParentViewModel.ChildViewModel.ObservableProperty'

If you want to subscribe to a function whenever new data is added to an array, you can achieve this by using the subscribe method on the length of the observable array. This will allow you to capture the event you're looking for.

By following these guidelines, you should be able to resolve any issues you may encounter.

Why isn't the parent view model subscribing to the updating observables in the Knockout component?

Answer №1

Answer №2

Answer №3

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