KnockoutJS - Dependency Tracking

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KnockoutJs automatically tracks the dependencies when the values get updated. It has a single object called dependency tracker (ko.dependencyDetection) which acts as an intermediate between the two parties for subscribing the dependencies.

Following is the algorithm for dependency tracking.

dependency tracking

Step 1 − Whenever you declare a computed observable, KO immediately invokes its evaluator function to get its initial value.

Step 2 − Subscription is set up to any observable that the evaluator reads. In an application, the old subscriptions which are no longer used are disposed.

Step 3 − KO finally notifies the updated computed observable.

Example

Live Demo 
<!DOCTYPE html>
<html>
   <head>
      <title>KnockoutJS How Dependency Tracking Works</title>
      <!-- CDN's-->
      <script src = "https://ajax.aspnetcdn.com/ajax/knockout/knockout-3.1.0.js"
         type = "text/javascript"></script>
   </head>
   
   <body>
      <div>
         <form data-bind = "submit: addFruits">
            <b>Add Fruits:</b>
            <input data-bind = 'value: fruitToAdd, valueUpdate: "afterkeydown"'/>
            <button type = "submit" data-bind = "enable: fruitToAdd().length > 0">Add</button>
            <p><b>Your fruits list:</b></p>
            <select multiple = "multiple" width = "50" data-bind = "options: fruits"> </select>
         </form>
      </div>
      
      <script>
         var Addfruit = function(fruits) {
            this.fruits = ko.observableArray(fruits);
            this.fruitToAdd = ko.observable("");
            
            this.addFruits = function() {
               
               if (this.fruitToAdd() != "") {
                  this.fruits.push(this.fruitToAdd());   // Adds a fruit
                  this.fruitToAdd("");                   // Clears the text box
               }
                
            }.bind(this);                                // "this" is the view model
         };

         ko.applyBindings(new Addfruit(["Apple", "Orange", "Banana"]));
      </script>
      
   </body>
</html>

Output

Let's carry out the following steps to see how the above code works −

  • Save the above code in dependency_tracking.htm file.

  • Open this HTML file in a browser.

  • Enter any fruit name and click the Add button.

Controlling Dependencies Using Peek

The Computed Observable can be accessed without creating a dependency, by using the peek function. It controls the Observable by updating the computed property.

Example

Live Demo 
<!DOCTYPE html>
<html>
   <head>
      <title>KnockoutJs Controlling Dependencies Using Peek</title>
      <!-- CDN's-->
      <script src = "https://ajax.aspnetcdn.com/ajax/knockout/knockout-3.1.0.js"
         type = "text/javascript"></script>
   </head>
   
   <body>
      <div class = "logblock">
         <h3>Computed Log</h3>
         <pre class = "log" data-bind = "html: computedLog"></pre>
      </div>

      <script>
         function AppData() {
            this.firstName = ko.observable('John');
            this.lastName = ko.observable('Burns');
            this.computedLog = ko.observable('Log: ');
            
            this.fullName = ko.computed(function () {
               var value = this.firstName() + " " + this.lastName();
               this.computedLog(this.computedLog.peek() + value + '; <br/>');
               return value;
            }, this);

            this.step = ko.observable(0);
            this.next = function () {
               this.step(this.step() === 2 ? 0 : this.step()+1);
            };
         };
         
         ko.applyBindings(new AppData());
      </script>
      
   </body>
</html>

Output

Let's carry out the following steps to see how the above code works −

  • Save the above code in dependency_tracking_peek.htm file.

  • Open this HTML file in a browser.

Observations

Ignoring Dependencies Within a Computed Dependency

The ko.ignoreDependencies function helps ignore those dependencies that you don't want to track within the computed dependencies. Following is its syntax.

ko.ignoreDependencies( callback, callbackTarget, callbackArgs );

Why Circular Dependencies Aren't Meaningful

If KO is evaluating a Computed Observable, then it will not restart an evaluation of the dependent Computed Observable. Hence, it doesn't make sense to include cycles in your dependency chains.

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