Controller-Directive Communication Part 3: Controller to Directive

The post closes a three-part Angular series by focusing on the case where a controller needs to tell a directive to act, such as scrolling a chat list to the bottom. It rejects events, services, and scope mangling as less clear or less appropriate for this kind of local communication.

Reading notes

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• The controller sometimes needs to tell the chat list directive to scroll to the bottom, for example when the user marked the chat as read from another device.
• Events work, but the post says they make component dependencies harder to find and worsen code quality.
• Services are described as better for global app-wide concerns, not for communication between two instances on screen.
• Scope mangling is presented as an abuse of bindings, because the directive adds functions onto an object passed from the controller.
• The preferred approach is an observer pattern with some boilerplate, so the controller manages handlers and notifies them when needed.
• In the example, the controller stores directive handlers and calls them when it needs to notify the directive.
• The directive receives a bound function used as a registration hook for change notifications.
• This approach is described as clearer and more robust, because the flow is explicit and failures happen at useful points.
• The note at the end warns that functions passed this way should be cleared when the bound scope is destroyed to avoid memory leaks and undefined behavior.