我们定义了如下这个DynamicActionProvider类型实现了IActionDescriptorProvider接口。针对提供的源代码向ActionDescriptor列表的转换体现在AddControllers方法中：它利用ICompiler对象编译源代码，并在生成的程序集中解析出有效的Controller类型，然后利用ApplicationModelFactory创建出代表应用模型的ApplicationModel对象，后者作为参数调用ControllerActionDescriptorBuilder的静态方法Build创建出描述所有Action方法的ActionDescriptor对象。
public class DynamicActionProvider : IActionDescriptorProvider { private readonly List<ControllerActionDescriptor> _actions; private readonly Func<string, IEnumerable<ControllerActionDescriptor>> _creator; public DynamicActionProvider(IServiceProvider serviceProvider, ICompiler compiler) { _actions = new List<ControllerActionDescriptor>(); _creator = CreateActionDescrptors; IEnumerable<ControllerActionDescriptor> CreateActionDescrptors(string sourceCode) { var assembly = compiler.Compile(sourceCode, Assembly.Load(new AssemblyName("System.Runtime")), typeof(object).Assembly, typeof(ControllerBase).Assembly, typeof(Controller).Assembly); var controllerTypes = assembly.GetTypes().Where(it => IsController(it)); var applicationModel = CreateApplicationModel(controllerTypes); assembly = Assembly.Load(new AssemblyName("Microsoft.AspNetCore.Mvc.Core")); var typeName = "Microsoft.AspNetCore.Mvc.ApplicationModels.ControllerActionDescriptorBuilder"; var controllerBuilderType = assembly.GetTypes().Single(it => it.FullName == typeName); var buildMethod = controllerBuilderType.GetMethod("Build", BindingFlags.Static | BindingFlags.Public); return (IEnumerable<ControllerActionDescriptor>)buildMethod.Invoke(null, new object[] { applicationModel }); } ApplicationModel CreateApplicationModel(IEnumerable<Type> controllerTypes) { var assembly = Assembly.Load(new AssemblyName("Microsoft.AspNetCore.Mvc.Core")); var typeName = "Microsoft.AspNetCore.Mvc.ApplicationModels.ApplicationModelFactory"; var factoryType = assembly.GetTypes().Single(it => it.FullName == typeName); var factory = serviceProvider.GetService(factoryType); var method = factoryType.GetMethod("CreateApplicationModel"); var typeInfos = controllerTypes.Select(it => it.GetTypeInfo()); return (ApplicationModel)method.Invoke(factory, new object[] { typeInfos }); } bool IsController(Type typeInfo) { if (!typeInfo.IsClass) return false; if (typeInfo.IsAbstract) return false; if (!typeInfo.IsPublic) return false; if (typeInfo.ContainsGenericParameters) return false; if (typeInfo.IsDefined(typeof(NonControllerAttribute))) return false; if (!typeInfo.Name.EndsWith("Controller", StringComparison.OrdinalIgnoreCase) && !typeInfo.IsDefined(typeof(ControllerAttribute))) return false; return true; } } public int Order => -100; public void OnProvidersExecuted(ActionDescriptorProviderContext context) { } public void OnProvidersExecuting(ActionDescriptorProviderContext context) { foreach (var action in _actions) { context.Results.Add(action); } } public void AddControllers(string sourceCode) => _actions.AddRange(_creator(sourceCode)); }
四、让应用感知到变化
DynamicActionProvider 解决了将提供的源代码向对应ActionDescriptor列表的转换，但是MVC默认情况下对提供的ActionDescriptor对象进行了缓存。如果框架能够使用新的ActionDescriptor对象，需要告诉它当前应用提供的ActionDescriptor列表发生了改变，而这可以利用自定义的IActionDescriptorChangeProvider来实现。为此我们定义了如下这个DynamicChangeTokenProvider类型，该类型实现了IActionDescriptorChangeProvider接口，并利用GetChangeToken方法返回IChangeToken对象通知MVC框架当前的ActionDescriptor已经发生改变。从实现实现代码可以看出，当我们调用NotifyChanges方法的时候，状态改变通知会被发出去。
public class DynamicChangeTokenProvider : IActionDescriptorChangeProvider { private CancellationTokenSource _source; private CancellationChangeToken _token; public DynamicChangeTokenProvider() { _source = new CancellationTokenSource(); _token = new CancellationChangeToken(_source.Token); } public IChangeToken GetChangeToken() => _token; public void NotifyChanges() { var old = Interlocked.Exchange(ref _source, new CancellationTokenSource()); _token = new CancellationChangeToken(_source.Token); old.Cancel(); } }
五、应用构建
到目前为止，核心的两个类型DynamicActionProvider和DynamicChangeTokenProvider已经定义好了，接下来我们按照如下的方式将它们注册到MVC应用的依赖注入框架中。