README.md

BlazorObservers

BlazorObservers is a set of Blazor wrappers around the JavaScript Observer functionality.

Currently only the ResizeObserver is supported.

Introduction

Blazor not support resize events. This means that usually, a complicated JSInterop will be required, which adds a lot of boilerplate code to the project.

This library attempts to prevent this issue, by providing a manager, which creates and deletes JavaScript ResizeObservers. This allows efficient and clean code to execute dotNET methods on element(s) resize.

Usage

First add the ResizeObserverService to the dependency injection.

using BlazorObservers.ObserverLibrary.DI;

...

builder.Services.AddResizeObserverService();

And add the using statements to the imports.razor

@using BlazorObservers.ObserverLibrary.JsModels
@using BlazorObservers.ObserverLibrary.Services
@using BlazorObservers.ObserverLibrary.Tasks

Then you can inject the ResizeObserverService into your razor component.

Now register an element with the OnAfterRenderAsync method, and make sure the registration is removed on disposal.

@inject ResizeObserverService ResizeService
@implements IAsyncDisposable

<div @ref="targetElement" style="width: 100%; height: 100%; background-color: green;"></div>

private ElementReference targetElement { get; set; }
private ObserverTask? taskReference;

protected override async Task OnAfterRenderAsync(bool firstRender)
{
    if (firstRender)
        taskReference = await ResizeService.RegisterObserver(
            async (entries) => Console.WriteLine("Hello resizable world"), 
            targetElement);
}

public async ValueTask DisposeAsync()
{
    if (taskReference is not null)
        await ResizeService.DeregisterObserver(taskReference);
}

When changing the size of (one of) the observed element(s) in your callback method, make sure to halt callback execution, as otherwise an infinite loop can appear, and crash your application.

protected override async Task OnAfterRenderAsync(bool firstRender)
{
    if (firstRender)
    {
        taskReference = await ResizeService.RegisterObserver(ObserverResizeEvent, TabBar);
    }
}

private async Task ObserverResizeEvent(JsResizeObserverEntry[] entries)
{
    if (taskReference is null) return;
    taskReference.HaltTaskTriggering();
    await MethodThatChangesTheElementSize();
    taskReference.ResumeTaskTriggering();
}

Future

Completed:

• Make ResizeObserver callback parameters available
• Auto-package
• Add compatibility with element rerendering, so that the ResizeObserver stops observing the original element, and starts observing the newly rendered element.

Feature backlog:

• Add ExecuteFinal option to HaltTaskTriggering, so that the last callback with the accurate size change is executed.
• Add more observers, such as MutationObserver and IntersectionObserver
• Unit testing
• Performance analysis