[譯]ASP.NET Core 2.0 帶初始參數的中間件

問題

如何在ASP.NET Core 2.0向中間件傳入初始參數?app

答案

在一個空項目中,建立一個POCO(Plain Old CLR Object)來保存中間件所需的參數:async

public class GreetingOptions
{
    public string GreetAt { get; set; }
    public string GreetTo { get; set; }
}

添加一箇中間件:函數

public class GreetingMiddleware
{
    private readonly RequestDelegate _next;
    private readonly GreetingOptions _options;

    public GreetingMiddleware(RequestDelegate next, GreetingOptions options)
    {
        _next = next;
        _options = options;
    }

    public async Task Invoke(HttpContext context)
    {
        var message = $"Good {_options.GreetAt} {_options.GreetTo}";
        await context.Response.WriteAsync(message);
    }
}

答案1:實例類型

添加一個擴展方法來配置中間件:學習

public static IApplicationBuilder UseGreetingMiddleware(this IApplicationBuilder app, GreetingOptions options)
{
    return app.UseMiddleware<GreetingMiddleware>(options);
}

使用中間件:ui

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseGreetingMiddleware(new GreetingOptions {
        GreetAt = "Morning",
        GreetTo = "Tahir"
    });
}

答案2:函數類型

添加一個擴展方法來配置中間件:this

public static IApplicationBuilder UseGreetingMiddlewareAction(this IApplicationBuilder app, Action<GreetingOptions> optionsAction)
{
    var options = new GreetingOptions();
    optionsAction(options);

    return app.UseMiddleware<GreetingMiddleware>(options);
}

使用中間件:spa

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{
    app.UseGreetingMiddlewareAction(options =>
    {
        options.GreetAt = "Morning";
        options.GreetTo = "Tahir";
    });
}

上述兩種方法結果一致。code

運行,此時頁面顯示:orm

討論

以前咱們曾討論過,在單獨的類中定義中間件並使用擴展方法將其添加到請求管道中是最佳實踐。咱們也可能須要向中間件傳入參數,經過對ASP.NET Core源代碼以及其餘在線示例的學習,我總結出來上面兩種模式。中間件

上述的兩種解決方法都很是直觀。咱們將參數封裝到一個POCO類中,而後建立一個擴展方法來接受下面的參數:

1. POCO實例

2. 須要調用的函數(在函數內設置POCO)

注:POCO實例經過構造函數傳入中間件。UseMiddleware()方法接收可變參數params object[],並將這些參數傳入中間件構造函數。

配置服務

這些模式也能用於向服務容器中添加服務實例。爲了便於說明,咱們先添加一個服務:

public interface IMessageService
{
    string FormatMessage(string message);
}

public class MessageService : IMessageService
{
    private readonly GreetingOptions _options;

    public MessageService(GreetingOptions options)
    {
        _options = options;
    }

    public string FormatMessage(string message)
    {
        return $"Good {_options.GreetAt} {_options.GreetTo} - {message}";
    }
}

添加以下任一個擴展方法來配置服務:

public static IServiceCollection AddMessageService(this IServiceCollection services, GreetingOptions options)
{
    return services.AddScoped<IMessageService>(factory => new MessageService(options));
}

public static IServiceCollection AddMessageServiceAction(this IServiceCollection services, Action<GreetingOptions> optionsAction)
{
    var options = new GreetingOptions();
    optionsAction(options);

    return services.AddScoped<IMessageService>(factory => new MessageService(options));
}

在Configure()中使用此服務:

public void ConfigureServices(IServiceCollection services)
{
    services.AddMessageService(new GreetingOptions
    {
        GreetAt = "Morning",
        GreetTo = "Tahir"
    });

    services.AddMessageServiceAction(options =>
    {
        options.GreetAt = "Morning";
        options.GreetTo = "Tahir";
    });
}

由於ConfigureServices()先於Configure()執行,所以咱們能夠直接在Configure()注入此服務:

public void Configure(IApplicationBuilder app, IHostingEnvironment env, IMessageService msg)
{
    app.Run(async (context) =>
    {
        await context.Response.WriteAsync(msg.FormatMessage("by sanshi"));
    });
}

運行,此時頁面顯示:

 

 

源代碼下載

 

原文:https://tahirnaushad.com/2017/08/29/passing-parameters-to-middleware-in-asp-net-core-2-0/

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