The components are structured to be most easily modified if you want to change something or apply your use case. You can explore the difference between the default change detection strategy and the OnPush strategy in the demo below. I’ve prepared a simple demo where you can explore the effect of the OnPush change detection yourself. By default, the and the async pipe mark the component to be checked if the object changes. Some notifications are built into the system – that is one reason for the immutable objects. How does Angular know where to traverse the tree and where to stop?Īngular must be notified about changes when the OnPush strategy is used. Afterward, the object’s identity is directly compared, the view is updated if necessary, and the method returns if a view update must be made or not while updating the binding with the new value. It accesses the bound value directly through a map and index, no need to iterate through all bindings. It is on us to optimize the number of changes that are being checked.įollowing, you find the Angular source code that is used to determine if an update is necessary. Many events can fire rapidly, resulting in the change detection (CD) being run at some point for all components in view.īut how can we define the performance of our application in this context? Let’s try this simplified approach: The measurement indicator results from times we have to check for a change and the time it takes to perform the check.Ĭhange detection performance = (time to check a change) * (number of changes) or short P = N * CĪs mentioned above, the time to check a change is optimized by Angular internally. The Angular team spent a lot of effort on highly optimizing the change check internally. It does not matter how often a component is marked for a check between two cycles. You can also mark a component as changed manually by calling the markForCheck() function on the ChangeDetectorRef to check the next cycle. These occasions include Network requests, mouse clicks, mouse scrolls, mouse moves, keyboard events, component events, timeouts, intervals, and more. How Does Angular’s Default Change Detection Work?Īngular checks all components for changes on different occasions, even if some components did not change at all. Other changes are registered internally by using directives, binding to DOM events, and You can configure the observed events, set the corresponding flags in a zone-flags.js file, and import it into your polyfills.ts before importing zone.js itself. Then on different occasions, it compares the bound value to the displayed value and updates it if necessary.Īngular catches some events by monkey patching the browser APIs with the zone.js framework, effectively creating the NgZone. It registers when an object or value is bound in the template and observes its changes. Angular’s change detection keeps the HTML view and our internal state in sync. Since HTML is a static markup language, we need Angular to replace or display the values we want to change. Once fetched, data is often not available when rendering the page for the first time, or it changes over time. The last part is where the change detection kicks in. At first, we perform a network request to fetch such data, store it in some variable or state, and display it somehow to the user. Imagine displaying some user data – it does not matter what kind of data exactly. Angular, or any other SPA framework, must have a way to synchronize its internal model state to the view.
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