Angular's change detection is a mechanism that detects and propagates changes in the application's data model to update the corresponding views. It ensures that the UI reflects the current state of the data. When there are changes in the application's data, Angular's change detection system automatically updates the affected components and their child components.
Here's how Angular's change detection works:
1. Initialization: When a component is created, Angular initializes its change detector. The change detector tracks the component's properties and listens for changes.
2. Change detection tree: Angular constructs a tree-like structure known as the change detection tree, which represents the component hierarchy. Each component has its own change detector, and child components are nested within their parent's change detector.
3. Detecting changes: Angular performs change detection by running a change detection cycle. This cycle is triggered by various events, such as user interactions, timers, or asynchronous operations. By default, Angular automatically triggers change detection for the entire application after these events.
4. Change detection cycle: During a change detection cycle, Angular starts from the root component's change detector and traverses the change detection tree in a top-down manner.
5. Checking for changes: In each component, Angular checks the properties that are bound to the component's template. It compares the current value of each property with its previous value.
6. Updating the view: If Angular detects a change in a component's property, it updates the corresponding view to reflect the new value. This includes updating the DOM, re-rendering the component's template, and triggering any necessary reflows.
7. Child component check: After updating the current component, Angular continues the change detection cycle by moving to the child components within the change detection tree. It recursively performs change detection on each child component.
8. Binding propagation: If changes occur in a parent component, Angular propagates these changes to its child components. This ensures that any affected child components are also updated accordingly.
9. Immutable data: Angular's change detection relies on object references to detect changes. If the reference to an object remains the same, Angular assumes that the object has not changed. Therefore, when working with immutable data patterns, it's important to ensure that new objects are created to represent changes.
10. Performance optimizations: Angular's change detection system includes several performance optimizations. It skips unnecessary change detection cycles for components that have not been affected by changes. Angular also supports the OnPush change detection strategy, which allows components to specify that they should only be checked for changes when their input properties change.
By efficiently detecting and propagating changes, Angular's change detection system helps keep the application's UI in sync with the underlying data model, ensuring a responsive and up-to-date user experience.