#Run function as soon as element exists update

#Run function as soon as element exists update

Only after rendering of all components is complete will Blazor compare the new and previous render trees and then update the DOM with as few changes as possible. Note: Do not use element references too soon!Īs we saw in the Render Trees section, during its rendering phase Blazor does not update the browser DOM at all. This solution results in an error element.focus is not a function, because the ElementReference was invalid at the point its value was passed to our AutoFocus component.

The problem with this solution as it stands is that component parameters’ values are passed during the render tree building process, whereas Element References are not valid until after the render tree has been built and the result has been rendered to HTML in the browser. Protected override async Task OnAfterRenderAsync(bool firstRender)Īwait JSRuntime.InvokeVoidAsync('tFocus', Control) ĭefines a parameter for the component named Control which accepts an ElementReference to identify which control should receive focus.Įxecutes our JavaScript in order to set focus to the specified control. Var BlazorUniversity = BlazorUniversity || Within that folder create a new file named AutoFocus.js and enter the following script.In the wwwroot folder create a scripts folder.Run the application and observe how the element does not automatically gain focus, not even on first page load. In each of the pages replace the content with the same mark-up just below each directive.This is the problem we will solve using JavaScript Interop, and ElementReference. This means that putting autofocus attributes on inputs doesn’t work.

As Blazor apps do not really navigate (the HTML is simply rewritten and the browser URL changed), the browser will not scan for autofocus attributes when we navigate to a new URL and present the user with new content. The HTML specification has an autofocus attribute that can be applied to any focusable element when a page is loaded the browser will find the first element decorated with autofocus and give it focus. We’ll now use ElementReference, and JavaScript interop to solve a common problem. The addition of this specially formatted attribute is how Blazor uniquely identifies an element without having to hijack the element’s id parameter. The member that will be used when referencing the element decorated with we alter the Index.razor file of a new Blazor application to add an element reference to the h1 element and run the application, we’ll see something like the following generated HTML. '/'ĭefines an HTML element and uses to specify which member in our component we will use when referencing that element (MyElementReference). When we require a reference to an HTML element we should decorate that element (or Blazor component) with We identify which member in our component will hold a reference to the HTML element by creating a member with the type ElementReference and identify it on the element using the attribute. Blazor solves this problem with the element tag and the ElementReference struct. In statically generated HTML pages this is quite simple, but when pages are dynamically created by composing the output of many components it is difficult to ensure IDs are unique across all of the components. The standard JavaScript method for achieving this is to give our HTML element an id and have JavaScript locate it using document.getElementById('someId').

Sometimes we may wish to proceed anyway and let JavaScript interact with our generated HTML. When writing a Blazor application, manipulation of the Document Object Model (DOM) is discouraged as it may interfere with its incremental render tree, any changes to HTML should be managed in.