Blazor Lifecycle Methods

The Blazor application provides different synchronous as well as asynchronous lifecycle methods.

• OnInitialized
• OnInitializedAsync
• OnParametersSet
• OnParametersSetAsync
• OnAfterRender
• OnAfterRenderAsync
• ShouldRender

OnInit & OnInitAsync

The synchronous and asynchronous version of the application methods which gets executed when the component gets Initialized.

• The OnInitialized is called first, then OnInitializedAsync.
• It is executed when the component is completely loaded.
• You can use this method to load data from services because each control in the UI is loaded after this method.
• It is executed when the component is ready and when it has received the values from the parent in the render tree.
• Any asynchronous operations, which require the component to re-render once they complete, should be placed in the OnInitializedAsync method.

@page "/"

<h1>OnInit & OnInitAsync Demo</h1>

@foreach (var item in EventType)
{
    @item <hr />
}

@functions{
    List<string> EventType = new List<string>();

    protected override void OnInitialized()
    {
        EventType.Add("OnInitialized is called");
    }
    protected override async Task OnInitializedAsync()
    { 
        EventType.Add("OnInitializedAsync is called");
        await Task.Delay(1000);          
    }
}

OnParametersSet & OnParametersSetAsync

The synchronous and asynchronous way of setting the parameter when the component receives the parameter from its parent component.

• The OnParametersSet and OnParametersSetAsync methods are called when a component is first initialised.
• After initialisation, OnParametersSet and OnParametersSetAsync are called each time new or updated parameters are receieved from the parent in the render tree.

@page "/"

<h1>OnParametersSet & OnParametersSetAsync Demo</h1>

@foreach (var item in EventType)
{
   @item <hr />
}

@functions{
   List<string> EventType = new List<string>();

   protected override void OnParametersSet()
   {
       EventType.Add("OnParameterSet is called");
   }
   protected override async Task OnParametersSetAsync()
   {
       EventType.Add("OnParametersSetAsync is called");
       await Task.Delay(1000);         
   }
}

OnAfterRender & OnAfterRenderAsync

The synchronous and asynchronous version of the application methods to perform the additional steps like initializing the other components.

• The OnAfterRender and OnAfterRenderAsync methods are called after each render of the component.
• At the point they are called you can expect that all element and component references are populated.
• It means that if you need to perform an action, such as attaching an event listener, it requires the elements of the component to be rendered in the DOM.
• Another great use for these lifecycle methods is for JavaScript library initialization, which requires DOM elements to be in place to work.

@page "/"

<h1>OnInit & OnInitAsync Demo</h1>

@foreach (var item in EventType)
{
   @item <hr />
}

@functions{
   List<string> EventType = new List<string>();

   protected override void OnAfterRender()
   {
       EventType.Add("OnAfterRender is called");
   }

   protected override  async Task OnAfterRenderAsync()
   {
       EventType.Add("OnAfterRenderAsync is called");
       await Task.Delay(1000);           
   }
}

ShouldRender

This method returns a boolean value, if returns true, it means refresh the UI, otherwise changes are not sent to UI. The ShouldRender method always does the initial rendering despite its return value.

@page "/"

 <h1>ShouldRender Method Demo</h1>
@foreach (var item in EventType){  @item <hr />}
 @functions{
    List<string> EventType = new List<string>();
 
    protected override bool ShouldRender()
    {
        EventType.Add("ShouldRender is called");
        return true;
    }
}

StateHasChanged

This method notifies the component that its state has changed.

• It is called after any lifecycle method has been called.
• It can also be invoked manually to trigger a UI re-render.
• This method looks at the value returned from ShouldRender to decide if a UI re-render should happen.
• However, this only happens after the component has been rendered for the first time.

@page "/refresh-ui-manually"
@using System.Threading;

<h1>@Count</h1>

<button onclick=@StartCountdown>Start Countdown</button>

@functions {
    private int Count { get; set; } = 10;

    void StartCountdown()
    {
        var timer = new Timer(new TimerCallback(_ =>
        {
            if (Count <= 0) return;
            Count--;

            // Note that the following line is necessary because otherwise
            // Blazor would not recognize the state change and not refresh the UI
            this.StateHasChanged();
        }), null, 1000, 1000);
    }
}