Asp.NETCore让FromServices回来

起因

这两天,我忽然有点怀念 Asp.NET MVC 5 之前的时代,原因是我看到项目里面有这么一段代码(其实不止一段,几乎每个 Controller 都是)

    [Route("home")]
    [ApiController]
    public class HomeController : ControllerBase
    {
        private readonly IConfiguration configuration;
        private readonly IHostingEnvironment environment;
        private readonly CarService carService;
        private readonly PostServices postServices;
        private readonly TokenService tokenService;
        private readonly TopicService topicService;
        private readonly UserService userService;

        public HomeController(IConfiguration configuration,
                              IHostingEnvironment environment,
                              CarService carService,
                              PostServices postServices,
                              TokenService tokenService,
                              TopicService topicService,
                              UserService userService)
        {
            this.configuration = configuration;
            this.environment = environment;
            this.carService = carService;
            this.postServices = postServices;
            this.tokenService = tokenService;
            this.topicService = topicService;
            this.userService = userService;
        }

        [HttpGet("index")]
        public ActionResult<string> Index()
        {
            return "Hello world!";
        }
    }

在构造函数里面声明了一堆依赖注入的实例,外面还得声明相应的接收字段,使用代码克隆扫描,零零散散的充斥在各个 Controller 的构造函数中。在 Asp.NET MVC 5 之前,我们可以把上面的代码简化为下面的形式:

    [Route("home")]
    [ApiController]
    public class HomeController : ControllerBase
    {
        [FromServices] public IConfiguration Configuration { get; set; }
        [FromServices] public IHostingEnvironment Environment { get; set; }
        [FromServices] public CarService CarService { get; set; }
        [FromServices] public PostServices PostServices { get; set; }
        [FromServices] public TokenService TokenService { get; set; }
        [FromServices] public TopicService TopicService { get; set; }
        [FromServices] public UserService UserService { get; set; }

        public HomeController()
        {
        }

        [HttpGet("index")]
        public ActionResult<string> Index()
        {
            return "Hello world!";
        }
    }

但是,在 .NETCore 中,上面的这断代码是会报错的,原因就是特性:FromServicesAttribute 只能应用于 AttributeTargets.Parameter,导航到 FromServicesAttribute 查看源码

namespace Microsoft.AspNetCore.Mvc
{
    /// <summary>
    /// Specifies that an action parameter should be bound using the request services.
    /// </summary>
    /// <example>
    /// In this example an implementation of IProductModelRequestService is registered as a service.
    /// Then in the GetProduct action, the parameter is bound to an instance of IProductModelRequestService
    /// which is resolved from the request services.
    ///
    /// <code>
    /// [HttpGet]
    /// public ProductModel GetProduct([FromServices] IProductModelRequestService productModelRequest)
    /// {
    ///     return productModelRequest.Value;
    /// }
    /// </code>
    /// </example>
    [AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = true)]
    public class FromServicesAttribute : Attribute, IBindingSourceMetadata
    {
        /// <inheritdoc />
        public BindingSource BindingSource => BindingSource.Services;
    }
}

那么问题来了,AttributeUsage 是什么时候移除了 AttributeTargets.Property 呢?答案是:2015年11月17日,是一个叫做 Pranav K 的哥们革了 FromServiceAttribute 的命,下面是他的代码提交记录

Limit [FromServices] to apply only to parameters
https://github.com/aspnet/Mvc/commit/2a89caed05a1bc9f06d32e15d984cd21598ab6fb

这哥们的 Commit Message 很简洁:限制 FromServices 仅作用于 parameters 。高手过招,人狠话不多,刀刀致命!从此,广大 .NETCore 开发者告别了属性注入。经过我不懈努力的搜索后,发现其实在 Pranav K 提交代码两天后,他居然自己开了一个 Issue,你说气人不?

关于废除 FromServices 的讨论
https://github.com/aspnet/Mvc/issues/3578

在这个贴子里面,许多开发者表达了自己的不满,我还看到了有人像我一样,表达了自己想要一个简洁的构造函数的这样朴素的请求;但是,对于属性注入可能导致滥用的问题也产生了激烈的讨论,还有属性注入要求成员必须标记为 public 这些硬性要求,不得不说,这个帖子成功的引起了人们的注意,但是很明显,作者不打算修改 FromServices 支持属性注入。

自己动手,丰衣足食

没关系,官方没有自带的话,我们自己动手做一个也是一样的效果,在此之前,我们还应该关注另外一种从 service 中获取实例的方式,就是常见的通过 HttpContext 请求上下文获取服务实例的方式:

 var obj = HttpContext.RequestServices.GetService(typeof(Type));

上面的这种方式,其实是反模式的,官方也建议尽量避免使用,说完了废话,就自动动手撸一个属性注入特性类:PropertyFromServiceAttribute

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false, Inherited = true)]
public class PropertyFromServiceAttribute : Attribute, IBindingSourceMetadata
{
    public BindingSource BindingSource => BindingSource.Services;
}

没有多余的代码,就是标记为 AttributeTargets.Property 即可

应用到类成员
    [Route("home")]
    [ApiController]
    public class HomeController : ControllerBase
    {
        [PropertyFromService] public IConfiguration Configuration { get; set; }
        [PropertyFromService] public IHostingEnvironment Environment { get; set; }
        [PropertyFromService] public CarService CarService { get; set; }
        [PropertyFromService] public PostServices PostServices { get; set; }
        [PropertyFromService] public TokenService TokenService { get; set; }
        [PropertyFromService] public TopicService TopicService { get; set; }
        [PropertyFromService] public UserService UserService { get; set; }

        public HomeController()
        {

        }

        [HttpGet("index")]
        public ActionResult<string> Index()
        {
            return "Hello world!";
        }
    }

请大声的回答,上面的代码是不是非常的干净整洁!但是,像上面这样使用属性注入有一个小问题,在对象未初始化之前,该属性为 null,意味着在类的构造函数中,该成员变量不可用,不过不要紧,这点小问题完全可用通过在构造函数中注入解决;更重要的是,并非每个实例都需要在构造函数中使用,是吧。

示例代码

托管在 Github 上了 https://github.com/lianggx/Examples/tree/master/Ron.DI

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posted @ 2019-06-26 09:33  Ron.Liang  阅读(...)  评论(...编辑  收藏