AngularJS: Why and when people declare controller inside custom directives

In AngularJS, people sometimes declare controllers inside custom directives to encapsulate the behavior and logic specific to that directive. This practice is commonly known as "directive controller" or "isolated scope controller."

Here are some reasons why and when people declare controllers inside custom directives:

1. Encapsulation and Reusability: Placing the controller inside the directive allows you to encapsulate the directive's logic, making it more reusable and self-contained. This is especially useful when you have complex or custom UI components that require specific functionality.

2. Isolated Scope: Controllers declared inside directives often use an isolated scope (scope: {}) by default. This means that the directive's controller and template can work with their own isolated data without affecting the parent scope, which helps prevent unintended side effects or scope pollution.

3. Modularity: Declaring the controller inside the directive promotes a modular approach, as it keeps the controller code closer to the directive's template and behavior. This can improve code organization and maintainability, making it easier to understand and maintain complex directives.

4. Directive-Specific Behavior: Sometimes, a directive might need behavior that doesn't fit naturally into a standard AngularJS controller outside the directive. By declaring a controller inside the directive, you can define functions and properties that are specifically tailored to the directive's purpose.

5. Interacting with Other Directives: A directive controller allows you to establish communication and interactions between multiple directives. Directives can communicate with each other by accessing their respective controllers and sharing data through the isolated scope.

Here's an example of declaring a controller inside a custom directive:

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angular.module('myApp').directive('myCustomDirective', function() {
  return {
    restrict: 'E',
    template: '<div>{{ directiveData }}</div>',
    scope: {},
    controller: function($scope) {
      $scope.directiveData = 'Data specific to myCustomDirective';
      $scope.doSomething = function() {
        // Directive-specific functionality
      };
    }
  };
});

In this example, we define a custom directive called myCustomDirective with an isolated scope and a controller inside it. The controller sets directiveData to a specific value and defines the doSomething function, which is specific to the behavior of this directive.

It's important to note that with the release of AngularJS 1.5 and later, the preferred way of defining component-based directives is by using the component API, which combines the directive definition with a controller and is more in line with Angular's component architecture. However, the approach of declaring controllers inside custom directives is still commonly used and can be suitable for certain scenarios.