Add data-protected browser storage library

src/ProtectedBrowserStorage/ProtectedBrowserStorage.sln

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src/ProtectedBrowserStorage/sample/ProtectedBrowserStorageSample/App.razor

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+<CascadingAuthenticationState>
+    <Router AppAssembly="typeof(Startup).Assembly">
+        <NotFoundContent>
+            <p>Sorry, there's nothing at this address.</p>
+        </NotFoundContent>
+    </Router>
+</CascadingAuthenticationState>

src/ProtectedBrowserStorage/sample/ProtectedBrowserStorageSample/Pages/LocalStorage.razor

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+@page "/local-storage"
+@using Microsoft.AspNetCore.ProtectedBrowserStorage
+@inject ProtectedLocalStorage ProtectedLocalStore
+
+<h1>Local storage</h1>
+
+<p>
+    This page demonstrates how data can be stored in the browser's <code>localStorage</code>
+    collection. This data is scoped to your browser. So, if you set a value on the
+    counter then reload, your data will be retained. If you open multiple browser tabs,
+    each one will overwrite the data for the others, as they are all sharing the same
+    storage slot. If you close and reopen the entire browser, your data will still be restored next time you come back here.
+</p>
+
+<p>
+    Tip: In many cases, it's safer to use <a href="">session storage</a> because with local
+    storage, if a user opens multiple tabs, you may have bugs or confusing user experience
+    because the tabs will overwrite each other's data. Which is the right choice depends on
+    whether tabs should be independent, and whether you need to store the data across browser
+    application restarts.
+</p>
+
+<fieldset>
+    @if (currentCount.HasValue)
+    {
+        <p>Current count: <strong>@currentCount</strong></p>
+        <button @onclick="@IncrementCount">Increment</button>
+    }
+</fieldset>
+
+@functions {
+    int? currentCount;
+
+    protected override async Task OnInitAsync()
+    {
+        currentCount = await ProtectedLocalStore.GetAsync<int>("localCount");
+    }
+
+    async Task IncrementCount()
+    {
+        currentCount++;
+        await ProtectedLocalStore.SetAsync("localCount", currentCount);
+    }
+}

src/ProtectedBrowserStorage/sample/ProtectedBrowserStorageSample/Pages/PrerenderingSupport.razor

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+@page "/prerendering-support"
+@using Microsoft.AspNetCore.ProtectedBrowserStorage
+@inject ProtectedLocalStorage ProtectedLocalStore
+@inject IComponentContext ComponentContext
+
+<h1>Prerendering support</h1>
+
+<p>
+    If you need to support prerendering while using browser-based storage, things are
+    a little more complicated. It's impossible to access browser-based storage during
+    prerendering, because there is no browser connection at that stage. So you need to avoid
+    calling <code>GetAsync</code> during prerendering, and defer the loading operation until
+    the client has connected back to the server.
+</p>
+
+<p>
+    This component demonstrates how to work with prerendering. To test this out, change
+    the value of <code>EnablePrerendering</code> to <code>true</code> inside <code>Startup.cs</code>,
+    then restart your application. You <strong>must</strong> then navigate directly to
+    this page's URL, otherwise you'll get an error, because the other pages don't support
+    prerendering.
+</p>
+
+<p>
+    Tip: In most cases, if you application is heavily built around use of browser-based storage,
+    it's better not to enable prerendering, since it adds complexity and doesn't really offer
+    any value, since you can't render a useful UI until the data has been loaded.
+</p>
+
+<fieldset>
+    @if (currentCount.HasValue)
+    {
+        <p>Current count: <strong>@currentCount</strong></p>
+        <button @onclick="@IncrementCount">Increment</button>
+    }
+    else
+    {
+        <p>Loading...</p>
+    }
+</fieldset>
+
+@functions {
+    int? currentCount;
+    bool isWaitingForConnection;
+
+    protected override async Task OnInitAsync()
+    {
+        if (ComponentContext.IsConnected)
+        {
+            // Looks like we're not prerendering, so we can immediately load
+            // the data from browser storage
+            await LoadStateAsync();
+        }
+        else
+        {
+            // We are prerendering, so have to defer the load operation until later
+            isWaitingForConnection = true;
+        }
+    }
+
+    protected override async Task OnAfterRenderAsync()
+    {
+        // By this stage we know the client has connected back to the server, and
+        // browser services are available. So if we didn't load the data earlier,
+        // we should do so now, then trigger a new render.
+        if (isWaitingForConnection)
+        {
+            isWaitingForConnection = false;
+            await LoadStateAsync();
+            StateHasChanged();
+        }
+    }
+
+    async Task LoadStateAsync()
+    {
+        currentCount = await ProtectedLocalStore.GetAsync<int>("prerenderedCount");
+    }
+
+    async Task SaveStateAsync()
+    {
+        await ProtectedLocalStore.SetAsync("prerenderedCount", currentCount);
+    }
+
+    Task IncrementCount()
+    {
+        currentCount++;
+        return SaveStateAsync();
+    }
+}

src/ProtectedBrowserStorage/sample/ProtectedBrowserStorageSample/Pages/SessionStorage.razor

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+@page "/"
+@using Microsoft.AspNetCore.ProtectedBrowserStorage
+@inject ProtectedSessionStorage ProtectedSessionStore
+
+<h1>Session storage</h1>
+
+<p>
+    This page demonstrates how data can be stored in the browser's <code>sessionStorage</code>
+    collection. This data is scoped to your current browser tab. So, if you set a value on the
+    counter then reload, your data will be retained. If you open multiple browser tabs, each
+    one will persist its own independent value for the count across reloads, even though the
+    code uses the same storage key for them all.
+</p>
+
+<fieldset>
+    @if (currentCount.HasValue)
+    {
+        <p>Current count: <strong>@currentCount</strong></p>
+        <button @onclick="@IncrementCount">Increment</button>
+    }
+</fieldset>
+
+@functions {
+    int? currentCount;
+
+    protected override async Task OnInitAsync()
+    {
+        currentCount = await ProtectedSessionStore.GetAsync<int>("sessionCount");
+    }
+
+    async Task IncrementCount()
+    {
+        currentCount++;
+        await ProtectedSessionStore.SetAsync("sessionCount", currentCount);
+    }
+}

src/ProtectedBrowserStorage/sample/ProtectedBrowserStorageSample/Pages/_Host.cshtml

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+@page "/"
+@namespace ProtectedBrowserStorageSample.Pages
+@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
+
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>ProtectedBrowserStorageSample</title>
+    <base href="~/" />
+</head>
+<body>
+    <app>
+        @if (Startup.EnablePrerendering)
+        {
+            @(await Html.RenderComponentAsync<App>())
+        }
+        else
+        {
+            <text>Loading...</text>
+        }
+    </app>
+
+    <script src="_framework/blazor.server.js"></script>
+    <script src="_content/Microsoft.AspNetCore.ProtectedBrowserStorage/protectedBrowserStorage.js"></script>
+</body>
+</html>

src/ProtectedBrowserStorage/sample/ProtectedBrowserStorageSample/Pages/_Imports.razor

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+@layout MainLayout

src/ProtectedBrowserStorage/sample/ProtectedBrowserStorageSample/Program.cs

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+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+
+namespace ProtectedBrowserStorageSample
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            CreateHostBuilder(args).Build().Run();
+        }
+
+        public static IHostBuilder CreateHostBuilder(string[] args) =>
+            Host.CreateDefaultBuilder(args)
+                .ConfigureWebHostDefaults(webBuilder =>
+                {
+                    webBuilder.UseStartup<Startup>();
+                });
+    }
+}

src/ProtectedBrowserStorage/sample/ProtectedBrowserStorageSample/ProtectedBrowserStorageSample.csproj

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+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <PropertyGroup>
+    <TargetFramework>netcoreapp3.0</TargetFramework>
+    <LangVersion>7.3</LangVersion>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Microsoft.AspNetCore.ProtectedBrowserStorage.csproj" />
+  </ItemGroup>
+
+</Project>

src/ProtectedBrowserStorage/sample/ProtectedBrowserStorageSample/Shared/MainLayout.razor

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+@inherits LayoutComponentBase
+
+<NavLink href="">Session storage</NavLink> |
+<NavLink href="local-storage">Local storage</NavLink> |
+<NavLink href="prerendering-support">Prerendering support</NavLink>
+
+<hr />
+
+@Body