Blazor async state management content (#12881)

aspnetcore/blazor/components.md

@@ -1,5 +1,5 @@
 ---
-title: Create and use Razor components
+title: Create and use ASP.NET Core Razor components
 author: guardrex
 description: Learn how to create and use Razor components, including how to bind to data, handle events, and manage component life cycles.
 monikerRange: '>= aspnetcore-3.0'
@@ -8,7 +8,7 @@ ms.custom: mvc
 ms.date: 06/16/2019
 uid: blazor/components
 ---
-# Create and use Razor components
+# Create and use ASP.NET Core Razor components
 
 By [Luke Latham](https://github.com/guardrex) and [Daniel Roth](https://github.com/danroth27)
 
@@ -547,6 +547,18 @@ protected override void OnAfterRender()
 }
 ```
 
+### Handle incomplete async actions at render
+
+Asynchronous actions performed in lifecycle events may not have completed before the component is rendered. Objects might be `null` or incompletely populated with data while the lifecycle method is executing. Provide rendering logic to confirm that objects are initialized. Render placeholder UI elements (for example, a loading message) while objects are `null`.
+
+In the Fetch Data component of the Blazor templates, `OnInitAsync` is overridden to asychronously receive forecast data (`forecasts`). When `forecasts` is `null`, a loading message is displayed to the user. After the `Task` returned by `OnInitAsync` completes, the component is rerendered with the updated state.
+
+*Pages/FetchData.razor*:
+
+[!code-cshtml[](components/samples_snapshot/3.x/FetchData.razor?highlight=9)]
+
+### Execute code before parameters are set
+
 `SetParameters` can be overridden to execute code before parameters are set:
 
 ```csharp
@@ -560,6 +572,8 @@ public override void SetParameters(ParameterCollection parameters)
 
 If `base.SetParameters` isn't invoked, the custom code can interpret the incoming parameters value in any way required. For example, the incoming parameters aren't required to be assigned to the properties on the class.
 
+### Suppress refreshing of the UI
+
 `ShouldRender` can be overridden to suppress refreshing of the UI. If the implementation returns `true`, the UI is refreshed. Even if `ShouldRender` is overridden, the component is always initially rendered.
 
 ```csharp

aspnetcore/blazor/components/samples_snapshot/3.x/FetchData.razor

@@ -0,0 +1,27 @@
+@page "/fetchdata"
+@using MyBlazorApp.Data
+@inject WeatherForecastService ForecastService
+
+<h1>Weather forecast</h1>
+
+<p>This component demonstrates fetching data from a service.</p>
+
+@if (forecasts == null)
+{
+    <p><em>Loading...</em></p>
+}
+else
+{
+    <table class="table">
+        <!-- forecast data in table element content -->
+    </table>
+}
+
+@code {
+    WeatherForecast[] forecasts;
+
+    protected override async Task OnInitAsync()
+    {
+        forecasts = await ForecastService.GetForecastAsync(DateTime.Now);
+    }
+}