Tips for accessing data in a child component that is not passed from a parent component (Angularjs versions 1.5 or 1.6)

To showcase the code snippets below:

 var app = angular.module('app', [])
  // navigation template
app.component('onChanges', {
      bindings: {
        name: '<'
      },
      template: '<h1>{{$ctrl.greeting}}</h1><br><h2>{{origin}}</h2> ',
      controller: function($scope) {
        $scope.origin = 'international';
        this.$onChanges = function(obj) {
          if (obj.name && this.name) {
            var prefix;
            (obj.name.currentValue.toLowerCase() === 'thomas') ?
            prefix = 'Howdy ': prefix = 'Hello ';
            this.greeting = prefix + this.name;
            (prefix === 'Hello ' || !prefix) ? $scope.origin = 'american': $scope.origin = 'australian';
          }

          // if the name is undefined clear greeting

          if (!this.name) {
            this.greeting = ''
          }
        };

        $scope.$watch('$scope.origin', function() {
          console.log("I am here");
        })

        }
      });

Adjusted Post: I have implemented $scope.$watch to monitor changes in $scope.origin. Can you spot any errors in my approach? Feel free to check out the Plunker link provided: https://plnkr.co/edit/DDZeTHQIji4FJnyhpQAJ?p=preview

New Version: My dilemma resembles a common challenge faced by users of Angular 1.5 or 1.6 components. Interestingly, I haven't stumbled upon a Stack Overflow query addressing this issue.

In this scenario, $ctrl.origin does not receive data from the parent component, unlike $ctrl.name which is passed from the parent to the child component.

Based on my understanding: The $onChanges function can exclusively monitor $ctrl.name - data inflowing into the component from an external source such as the parent component.

My inquiry: How can I track changes in local data of a component like $ctrl.origin - data not transmitted from a parent component? It would be beneficial to provide an example.

We appreciate your insights on the best practices for achieving this task.

Access our Plunker demonstration via this link: https://plnkr.co/edit/DDZeTHQIji4FJnyhpQAJ?p=info

Answer №1

Explaining the concept of AngularJS:

$onChanges(changesObj) - This function is called whenever one-way bindings are updated. The
changesObj contains a hash with keys representing the names of the bound properties that
have been altered, along with values in the format { currentValue,
previousValue, isFirstChange() }. Utilize this method to update components by 
cloning the bound value in order to prevent unintended changes 
to the original value.

It is important to ensure your bindings follow the one-way rule and actually exist (in cases where they do not exist, $onChanges won't be effective).

If you need to monitor variables from different scopes, consider using $watch:

$scope.$watch(
  () => { // Previous variable value; },
  '$ctrl.origin' => {
    console.log('something changed on variable', variable-name, 'blah');
  }
);

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