[Asp.net 5] DependencyInjection项目代码分析4-微软的实现（4）
这个系列已经写了6篇，链接地址如下：
如果想对本篇有个更好的了解，建议需要先看
“[Asp.net 5] DependencyInjection项目代码分析”
“[Asp.net 5] DependencyInjection项目代码分析4-微软的实现（1）”
“[Asp.net 5] DependencyInjection项目代码分析4-微软的实现（2）”。
"[Asp.net 5] DependencyInjection项目代码分析4-微软的实现（3）"
继续ServiceProvider类
在之前的讲解中我们提到过Service类调用CreateCallSite方法时会递归调用，但是我们没具体说明如何递归调的。实际上Service类，通过反射创建实例的时候，会实例化的参数对象，而实例话参数对象通过ServiceProvider类创建，而ServiceProvider类创建参数的实例，又需要通过Service类（如果是通过Type注册的）创建。下面我们把Service的CreateInstanceCallSite方法以及ServiceProvider相关的方法列出来。
public IServiceCallSite CreateCallSite(ServiceProvider provider, ISet<Type> callSiteChain) { ConstructorInfo[] constructors = _descriptor.ImplementationType.GetTypeInfo() .DeclaredConstructors .Where(IsInjectable) .ToArray(); // TODO: actual service-fulfillment constructor selection if (constructors.Length == 1) { ParameterInfo[] parameters = constructors[0].GetParameters(); IServiceCallSite[] parameterCallSites = new IServiceCallSite[parameters.Length]; for (var index = 0; index != parameters.Length; ++index) { parameterCallSites[index] = provider.GetServiceCallSite(parameters[index].ParameterType, callSiteChain); if (parameterCallSites[index] == null && parameters[index].HasDefaultValue) { parameterCallSites[index] = new ConstantCallSite(parameters[index].DefaultValue); } if (parameterCallSites[index] == null) { throw new InvalidOperationException(Resources.FormatCannotResolveService( parameters[index].ParameterType, _descriptor.ImplementationType)); } } return new ConstructorCallSite(constructors[0], parameterCallSites); } return new CreateInstanceCallSite(_descriptor); }
internal IServiceCallSite GetServiceCallSite(Type serviceType, ISet<Type> callSiteChain)
{
try
{
if (callSiteChain.Contains(serviceType))
{
throw new InvalidOperationException(Resources.FormatCircularDependencyException(serviceType));
}
callSiteChain.Add(serviceType);
ServiceEntry entry;
if (_table.TryGetEntry(serviceType, out entry))
{
return GetResolveCallSite(entry.Last, callSiteChain);
}
object emptyIEnumerableOrNull = GetEmptyIEnumerableOrNull(serviceType);
if (emptyIEnumerableOrNull != null)
{
return new EmptyIEnumerableCallSite(serviceType, emptyIEnumerableOrNull);
}
return null;
}
finally
{
callSiteChain.Remove(serviceType);
}
}
internal IServiceCallSite GetResolveCallSite(IService service, ISet<Type> callSiteChain)
{
IServiceCallSite serviceCallSite = service.CreateCallSite(this, callSiteChain);
if (service.Lifetime == ServiceLifetime.Transient)
{
return new TransientCallSite(serviceCallSite);
}
else if (service.Lifetime == ServiceLifetime.Scoped)
{
return new ScopedCallSite(service, serviceCallSite);
}
else
{
return new SingletonCallSite(service, serviceCallSite);
}
}
对于Service的CreateCallSite方法，之前我们已经介绍过，现在我们重点讲下ServiceProvider的GetServiceCallSite方法。从上面代码中我们发现参数中含有“ ISet<Type> callSiteChain”，这个参数是防止发生A的构造函数有B类型参数，B的构织函数中有A类型参数，当A，B都是通过类型注入的，那么系统会陷入死循环。而callSiteChain得作用就是防止这样的死循环发生，当创建A时，会在callSiteChain中查询历史中是否有A的创建过程，如果有则说明发生死循环了，直接抛出异常，结束；如果没有将A加入到callSiteChain中，继续创建其参数。GetResolveCallSite方法比较简单，对于ServiceProvider已经能够获取的IServiceCallSite实例，进行包装，已保证生成的实例能够适应不同的Scoped(该处应该使用设计模式中的代理模式，不过我设计模式不过关，请帮忙确认)。
对于TransientCallSite、ScopedCallSite、SingletonCallSite以及EmptyIEnumerableCallSite代码，如下所示：
private class EmptyIEnumerableCallSite : IServiceCallSite { private readonly object _serviceInstance; private readonly Type _serviceType; public EmptyIEnumerableCallSite(Type serviceType, object serviceInstance) { _serviceType = serviceType; _serviceInstance = serviceInstance; } public object Invoke(ServiceProvider provider) { return _serviceInstance; } public Expression Build(Expression provider) { return Expression.Constant(_serviceInstance, _serviceType); } } private class TransientCallSite : IServiceCallSite { private readonly IServiceCallSite _service; public TransientCallSite(IServiceCallSite service) { _service = service; } public object Invoke(ServiceProvider provider) { return provider.CaptureDisposable(_service.Invoke(provider)); } public Expression Build(Expression provider) { return Expression.Call( provider, CaptureDisposableMethodInfo, _service.Build(provider)); } } private class ScopedCallSite : IServiceCallSite { private readonly IService _key; private readonly IServiceCallSite _serviceCallSite; public ScopedCallSite(IService key, IServiceCallSite serviceCallSite) { _key = key; _serviceCallSite = serviceCallSite; } public virtual object Invoke(ServiceProvider provider) { object resolved; lock (provider._sync) { if (!provider._resolvedServices.TryGetValue(_key, out resolved)) { resolved = provider.CaptureDisposable(_serviceCallSite.Invoke(provider)); provider._resolvedServices.Add(_key, resolved); } } return resolved; } public virtual Expression Build(Expression providerExpression) { var keyExpression = Expression.Constant( _key, typeof(IService)); var resolvedExpression = Expression.Variable(typeof(object), "resolved"); var resolvedServicesExpression = Expression.Field( providerExpression, "_resolvedServices"); var tryGetValueExpression = Expression.Call( resolvedServicesExpression, TryGetValueMethodInfo, keyExpression, resolvedExpression); var captureDisposableExpression = Expression.Assign( resolvedExpression, Expression.Call( providerExpression, CaptureDisposableMethodInfo, _serviceCallSite.Build(providerExpression))); var addValueExpression = Expression.Call( resolvedServicesExpression, AddMethodInfo, keyExpression, resolvedExpression); var blockExpression = Expression.Block( typeof(object), new[] { resolvedExpression }, Expression.IfThen( Expression.Not(tryGetValueExpression), Expression.Block(captureDisposableExpression, addValueExpression)), resolvedExpression); return Lock(providerExpression, blockExpression); } private static Expression Lock(Expression providerExpression, Expression body) { // The C# compiler would copy the lock object to guard against mutation. // We don't, since we know the lock object is readonly. var syncField = Expression.Field(providerExpression, "_sync"); var lockWasTaken = Expression.Variable(typeof(bool), "lockWasTaken"); var monitorEnter = Expression.Call(MonitorEnterMethodInfo, syncField, lockWasTaken); var monitorExit = Expression.Call(MonitorExitMethodInfo, syncField); var tryBody = Expression.Block(monitorEnter, body); var finallyBody = Expression.IfThen(lockWasTaken, monitorExit); return Expression.Block( typeof(object), new[] { lockWasTaken }, Expression.TryFinally(tryBody, finallyBody)); } } private class SingletonCallSite : ScopedCallSite { public SingletonCallSite(IService key, IServiceCallSite serviceCallSite) : base(key, serviceCallSite) { } public override object Invoke(ServiceProvider provider) { return base.Invoke(provider._root); } public override Expression Build(Expression provider) { return base.Build(Expression.Field(provider, "_root")); } }
该工程所有类的关系图（包括内部类以及一些接口），如下所示：
补充说明
- IServiceCallSite中定义了Build方法，该方法使用了Expression，但是该篇文章没有对其进行具体的研究，并且Build方法是相对独立的。
- 对于OpenIEnumerableService泛型省略详解。
示例测试代码：
public static class TestServices { public static IServiceCollection DefaultServices() { var services = new ServiceCollection(); services.AddTransient<IFakeService, FakeService>(); services.AddTransient<IFakeMultipleService, FakeOneMultipleService>(); services.AddTransient<IFakeMultipleService, FakeTwoMultipleService>(); services.AddTransient<IFakeOuterService, FakeOuterService>(); services.AddInstance<IFakeServiceInstance>(new FakeService() { Message = "Instance" }); services.AddScoped<IFakeScopedService, FakeService>(); services.AddSingleton<IFakeSingletonService, FakeService>(); services.AddTransient<IDependOnNonexistentService, DependOnNonexistentService>(); services.AddTransient<IFakeOpenGenericService<string>, FakeService>(); services.AddTransient(typeof(IFakeOpenGenericService<>), typeof(FakeOpenGenericService<>)); services.AddTransient<IFactoryService>(provider => { var fakeService = provider.GetService<IFakeService>(); return new TransientFactoryService { FakeService = fakeService, Value = 42 }; }); services.AddScoped(provider => { var fakeService = provider.GetService<IFakeService>(); return new ScopedFactoryService { FakeService = fakeService, }; }); services.AddTransient<ServiceAcceptingFactoryService, ServiceAcceptingFactoryService>(); return services; } } public class ServiceProviderContainerTests : ScopingContainerTestBase { protected override IServiceProvider CreateContainer() { return TestServices.DefaultServices().BuildServiceProvider(); } [Fact] public void ScopedServiceCanBeResolved() { IServiceProvider container = CreateContainer(); var scopeFactory = container.GetService<IServiceScopeFactory>(); using (var scope = scopeFactory.CreateScope()) { var containerScopedService = container.GetService<IFakeScopedService>(); var scopedService1 = scope.ServiceProvider.GetService<IFakeScopedService>(); Thread.Sleep(200); var scopedService2 = scope.ServiceProvider.GetService<IFakeScopedService>(); Assert.NotEqual(containerScopedService, scopedService1); Assert.Equal(scopedService1, scopedService2); } } }
