Monitoring and tracking every alteration to a property in a designated observable area

Question

Monitoring and tracking every alteration to a property in a designated observable area

I'm currently working with KnockoutJS version 3.4.

In TypeScript, my view model is structured as follows:

export class ItemsViewModel {
    public currentItem: KnockoutObservable<ItemViewModel>;
    public items: KnockoutObservableArray<KnockoutObservable<ItemViewModel>>;
    /* snip */
}

The ItemViewModel object consists of multiple observable properties such as Id, Name, and Description.

I am looking to subscribe to specific property changes within the currentItem. Using currentItem.subscribe allows me to monitor changes in the overall value of the currentItem observable, but I need a way to identify when individual properties are modified within that item. The other question referenced as a duplicate only tracks changes at the object level, whereas I require detection at the property level.

What would be the most efficient approach to achieve this?

Edit: Taking into account the feedback and associated question, it does not provide details on which property was altered. Therefore, I have made amendments to clarify this inquiry.

typescript knockout.js knockout-3.0

Answer 1

Answer №1

To pinpoint the modified property, a more specific approach is necessary. This can be achieved by adding an extender to each property that requires tracking. The extender should accept a subscribable at the view-model level to notify and the name of the extended property. (unfortunately JavaScript lacks reflection for this purpose)

ko.extenders.trackChange = function (target, options) {
    target.subscribe(function (newValue) {
        if (ko.isSubscribable(options.notifier)) options.notifier.notifySubscribers(options.propertyName);
    });
    return target;
};

Subsequently, in your view-model, include the subscribable and subscription.

self.propertyChanged = new ko.subscribable();
self.propertyChanged.subscribe(function(propertyName){
  console.log(propertyName + " was updated");
});

Lastly, apply the extender to your properties.

self.property1 = ko.observable(0).extend({trackChange: { propertyName: 'property1', notifier: self.propertyChanged }});
self.property2 = ko.observable(0).extend({trackChange: { propertyName: 'property2', notifier: self.propertyChanged }});
self.property3 = ko.observable(0).extend({trackChange: { propertyName: 'property3', notifier: self.propertyChanged }});

If you wish to apply this to every property automatically, you could use a loop within the view-model:

self.property1 = ko.observable();
self.property2 = ko.observable();
self.property3 = ko.observable();

for (key in self) {
    if (self.hasOwnProperty(key) && ko.isObservable(self[key])) {
        self[key].extend({trackChange: { propertyName: key, notifier: self.propertyChanged }})
    }
}

Answer 2

To pinpoint the modified property, a more specific approach is necessary. This can be achieved by adding an extender to each property that requires tracking. The extender should accept a subscribable at the view-model level to notify and the name of the extended property. (unfortunately JavaScript lacks reflection for this purpose)

ko.extenders.trackChange = function (target, options) {
    target.subscribe(function (newValue) {
        if (ko.isSubscribable(options.notifier)) options.notifier.notifySubscribers(options.propertyName);
    });
    return target;
};

Subsequently, in your view-model, include the subscribable and subscription.

self.propertyChanged = new ko.subscribable();
self.propertyChanged.subscribe(function(propertyName){
  console.log(propertyName + " was updated");
});

Lastly, apply the extender to your properties.

self.property1 = ko.observable(0).extend({trackChange: { propertyName: 'property1', notifier: self.propertyChanged }});
self.property2 = ko.observable(0).extend({trackChange: { propertyName: 'property2', notifier: self.propertyChanged }});
self.property3 = ko.observable(0).extend({trackChange: { propertyName: 'property3', notifier: self.propertyChanged }});

If you wish to apply this to every property automatically, you could use a loop within the view-model:

self.property1 = ko.observable();
self.property2 = ko.observable();
self.property3 = ko.observable();

for (key in self) {
    if (self.hasOwnProperty(key) && ko.isObservable(self[key])) {
        self[key].extend({trackChange: { propertyName: key, notifier: self.propertyChanged }})
    }
}

Monitoring and tracking every alteration to a property in a designated observable area

Answer №1

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