[3.2-RC] Document Blazor loadBootResource (#18049)

Author: guardrex

aspnetcore/host-and-deploy/blazor/webassembly.md

@@ -5,13 +5,13 @@ description: Learn how to host and deploy a Blazor app using ASP.NET Core, Conte
 monikerRange: '>= aspnetcore-3.1'
 ms.author: riande
 ms.custom: mvc
-ms.date: 04/23/2020
+ms.date: 04/30/2020
 no-loc: [Blazor, SignalR]
 uid: host-and-deploy/blazor/webassembly
 ---
 # Host and deploy ASP.NET Core Blazor WebAssembly
 
-By [Luke Latham](https://github.com/guardrex), [Rainer Stropek](https://www.timecockpit.com), [Daniel Roth](https://github.com/danroth27), and [Ben Adams](https://twitter.com/ben_a_adams).
+By [Luke Latham](https://github.com/guardrex), [Rainer Stropek](https://www.timecockpit.com), [Daniel Roth](https://github.com/danroth27), [Ben Adams](https://twitter.com/ben_a_adams), and [Safia Abdalla](https://safia.rocks)
 
 [!INCLUDE[](~/includes/blazorwasm-preview-notice.md)]
 
@@ -325,3 +325,72 @@ The `--urls` argument sets the IP addresses or host addresses with ports and pro
 ## Configure the Linker
 
 Blazor performs Intermediate Language (IL) linking on each Release build to remove unnecessary IL from the output assemblies. For more information, see <xref:host-and-deploy/blazor/configure-linker>.
+
+## Custom boot resource loading
+
+A Blazor WebAssembly app can be initialized with the `loadBootResource` function to override the built-in boot resource loading mechanism. Use `loadBootResource` for the following scenarios:
+
+* Allow users to load static resources, such as timezone data or *dotnet.wasm* from a CDN.
+* Load compressed assemblies using an HTTP request and decompress them on the client for hosts that don't support fetching compressed contents from the server.
+* Alias resources to a different name by redirecting each `fetch` request to a new name.
+
+`loadBootResource` parameters appear in the following table.
+
+| Parameter    | Description |
+| ------------ | ----------- |
+| `type`       | The type of the resource. Permissable types: `assembly`, `pdb`, `dotnetjs`, `dotnetwasm`, `timezonedata` |
+| `name`       | The name of the resource. |
+| `defaultUri` | The relative or absolute URI of the resource. |
+| `integrity`  | The integrity string representing the expected content in the response. |
+
+`loadBootResource` returns any of the following to override the loading process:
+
+* URI string. In the following example (*wwwroot/index.html*), the following files are served from a CDN at `https://my-awesome-cdn.com/`:
+
+  * *dotnet.\*.js*
+  * *dotnet.wasm*
+  * Timezone data
+
+  ```html
+  ...
+
+  <script src="_framework/blazor.webassembly.js" autostart="false"></script>
+  <script>
+    Blazor.start({
+      loadBootResource: function (type, name, defaultUri, integrity) {
+        console.log(`Loading: '${type}', '${name}', '${defaultUri}', '${integrity}'`);
+        switch (type) {
+          case 'dotnetjs':
+          case 'dotnetwasm':
+          case 'timezonedata':
+            return `https://my-awesome-cdn.com/blazorwebassembly/3.2.0/${name}`;
+        }
+      }
+    });
+  </script>
+  ```
+
+* `Promise<Response>`. Pass the `integrity` parameter in a header to retain the default integrity-checking behavior.
+
+  The following example (*wwwroot/index.html*) adds a custom HTTP header to the outbound requests and passes the `integrity` parameter through to the `fetch` call:
+  
+  ```html
+  <script src="_framework/blazor.webassembly.js" autostart="false"></script>
+  <script>
+    Blazor.start({
+      loadBootResource: function (type, name, defaultUri, integrity) {
+        return fetch(defaultUri, { 
+          cache: 'no-cache',
+          integrity: integrity,
+          headers: { 'MyCustomHeader': 'My custom value' }
+        });
+      }
+    });
+  </script>
+  ```
+
+* `null`/`undefined`, which results in the default loading behavior.
+
+External sources must return the required CORS headers for browsers to allow the cross-origin resource loading. CDNs usually provide the required headers by default.
+
+You only need to specify types for custom behaviors. Types not specified to `loadBootResource` are loaded by the framework per their default loading behaviors.