The knockout.js subscribe function isn't activating during the initial set

Question

The knockout.js subscribe function isn't activating during the initial set

In my model class, I have defined observables and I am looking to subscribe to their sets. Below is the code snippet that showcases what I currently have:

var dto = function (data) {
    var self = this;
    self.Value1 = ko.observable(data.Value1);
    self.Value1.subscribe(function(){
         console.log('here');
    });   
};

The issue I am encountering is that the console.log statement is not triggered when Value1 is initially set using ko.observable(data.Value1).

Can someone guide me on how I can ensure that the subscribe function is called both during the initial setting of the value and also whenever it changes?

javascript knockout.js knockout-3.0

Answer 1

Answer №1

There is no direct support for triggering the subscribe function with initial values.

One workaround is to use the valueHasMutated function directly after subscribing:

self.Value1.subscribe(function(){
     console.log('here');
}); 
self.Value1.valueHasMutated();

Alternatively, you can set your initial values after subscribing like this:

var dto = function (data) {
    var self = this;
    self.Value1 = ko.observable(); // declaration only
    self.Value1.subscribe(function(){
         console.log('here');
    });   
    self.Value1(data.Value1); // assign initial value
};

Answer 2

There is no direct support for triggering the subscribe function with initial values.

One workaround is to use the valueHasMutated function directly after subscribing:

self.Value1.subscribe(function(){
     console.log('here');
}); 
self.Value1.valueHasMutated();

Alternatively, you can set your initial values after subscribing like this:

var dto = function (data) {
    var self = this;
    self.Value1 = ko.observable(); // declaration only
    self.Value1.subscribe(function(){
         console.log('here');
    });   
    self.Value1(data.Value1); // assign initial value
};

Answer 3

Answer №2

Here is a custom function I created for personal use:

customSubscribe = function(observer, action){
    observer.subscribe( action );

    if (observer()) {
        observer.valueHasMutated();
    }
}

This function was inspired by various sources but addresses the issue of "what if it has not been set yet?". It should be effective as long as the observable is established.

To utilize:

customSubscribe(dataPoint1, function() {…});

where dataPoint1 is a custom.observable.

Answer 4

Here is a custom function I created for personal use:

customSubscribe = function(observer, action){
    observer.subscribe( action );

    if (observer()) {
        observer.valueHasMutated();
    }
}

This function was inspired by various sources but addresses the issue of "what if it has not been set yet?". It should be effective as long as the observable is established.

To utilize:

customSubscribe(dataPoint1, function() {…});

where dataPoint1 is a custom.observable.

Answer 5

Answer №3

Expanding on Richard's response, if you find yourself in a recurring scenario where you need to check if an observable has a value before calling its subscriber function, you can enhance the KO subscribable with the following custom function:

ko.subscribable.fn.customSubscribe = function(subscriberFunction, context, eventType) {
  var observableValue = this();

  this.subscribe(subscriberFunction, context, eventType);

  if (observableValue !== undefined) {
    subscriberFunction.call(context, observableValue);
  }
};

This method also handles cases where the initial value of the observable is falsy without triggering any unintended subscribers.

Here's how you can use it based on the original question:

self.Value1.customSubscribe(function(){
     console.log('Subscriber called');
}, this);

Answer 6

Expanding on Richard's response, if you find yourself in a recurring scenario where you need to check if an observable has a value before calling its subscriber function, you can enhance the KO subscribable with the following custom function:

ko.subscribable.fn.customSubscribe = function(subscriberFunction, context, eventType) {
  var observableValue = this();

  this.subscribe(subscriberFunction, context, eventType);

  if (observableValue !== undefined) {
    subscriberFunction.call(context, observableValue);
  }
};

This method also handles cases where the initial value of the observable is falsy without triggering any unintended subscribers.

Here's how you can use it based on the original question:

self.Value1.customSubscribe(function(){
     console.log('Subscriber called');
}, this);

Answer 7

Answer №4

It seems that the subscribe function is not intended to be used in that manner. Essentially, you would utilize it to trigger a callback or logic when changes occur in your observable object.

According to an answer by @RP Niemeyer on Stack Overflow:

If you need to alert subscribers to re-evaluate, you can invoke valueHasMutated() on your observable object.

Answer 8

It seems that the subscribe function is not intended to be used in that manner. Essentially, you would utilize it to trigger a callback or logic when changes occur in your observable object.

According to an answer by @RP Niemeyer on Stack Overflow:

If you need to alert subscribers to re-evaluate, you can invoke valueHasMutated() on your observable object.

The knockout.js subscribe function isn't activating during the initial set

Answer №1

Answer №2

Answer №3

Answer №4

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