通用FromEvent方法

Question

使用新的async/await模型,生成一个Task在事件触发时完成的模型非常简单; 你只需要遵循这种模式:

public class MyClass
{
    public event Action OnCompletion;
}

public static Task FromEvent(MyClass obj)
{
    TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();

    obj.OnCompletion += () =>
        {
            tcs.SetResult(null);
        };

    return tcs.Task;
}

这允许:

await FromEvent(new MyClass());

问题是你需要为FromEvent你想要的每个类中的每个事件创建一个新方法await.这可能会变得非常快,而且无论如何它主要只是样板代码.

理想情况下,我希望能够做到这样的事情:

await FromEvent(new MyClass().OnCompletion);

然后我可以FromEvent对任何实例上的任何事件重复使用相同的方法.我花了一些时间来尝试创建这样的方法,并且存在许多障碍.对于上面的代码,它将生成以下错误:

事件'Namespace.MyClass.OnCompletion'只能出现在+ =或 - =的左侧

据我所知,没有办法通过代码传递这样的事件.

所以,下一个最好的事情似乎是尝试将事件名称作为字符串传递:

await FromEvent(new MyClass(), "OnCompletion");

它并不理想; 如果该类型的事件不存在,你就不会得到intellisense并且会遇到运行时错误,但它仍然比大量的FromEvent方法更有用.

因此,使用反射和GetEvent(eventName)获取EventInfo对象很容易.下一个问题是该事件的委托在运行时是未知的(并且需要能够变化).这使得添加事件处理程序变得困难,因为我们需要在运行时动态创建方法,匹配给定签名(但忽略所有参数),访问TaskCompletionSource我们已有的并设置其结果.

幸运的是,我发现此链接包含有关如何[几乎]完全通过的说明Reflection.Emit.现在的问题是我们需要发出IL,我不知道如何访问tcs我拥有的实例.

以下是我为完成此任务所取得的进展:

public static Task FromEvent<T>(this T obj, string eventName)
{
    var tcs = new TaskCompletionSource<object>();
    var eventInfo = obj.GetType().GetEvent(eventName);

    Type eventDelegate = eventInfo.EventHandlerType;

    Type[] parameterTypes = GetDelegateParameterTypes(eventDelegate);
    DynamicMethod handler = new DynamicMethod("unnamed", null, parameterTypes);

    ILGenerator ilgen = handler.GetILGenerator();

    //TODO ilgen.Emit calls go here

    Delegate dEmitted = handler.CreateDelegate(eventDelegate);

    eventInfo.AddEventHandler(obj, dEmitted);

    return tcs.Task;
}

我可以发射什么IL,这将允许我设置结果TaskCompletionSource？或者,是否存在另一种创建方法的方法,该方法从任意类型返回任意事件的任务？

Answer 1

干得好:

internal class TaskCompletionSourceHolder
{
    private readonly TaskCompletionSource<object[]> m_tcs;

    internal object Target { get; set; }
    internal EventInfo EventInfo { get; set; }
    internal Delegate Delegate { get; set; }

    internal TaskCompletionSourceHolder(TaskCompletionSource<object[]> tsc)
    {
        m_tcs = tsc;
    }

    private void SetResult(params object[] args)
    {
        // this method will be called from emitted IL
        // so we can set result here, unsubscribe from the event
        // or do whatever we want.

        // object[] args will contain arguments
        // passed to the event handler
        m_tcs.SetResult(args);
        EventInfo.RemoveEventHandler(Target, Delegate);
    }
}

public static class ExtensionMethods
{
    private static Dictionary<Type, DynamicMethod> s_emittedHandlers =
        new Dictionary<Type, DynamicMethod>();

    private static void GetDelegateParameterAndReturnTypes(Type delegateType,
        out List<Type> parameterTypes, out Type returnType)
    {
        if (delegateType.BaseType != typeof(MulticastDelegate))
            throw new ArgumentException("delegateType is not a delegate");

        MethodInfo invoke = delegateType.GetMethod("Invoke");
        if (invoke == null)
            throw new ArgumentException("delegateType is not a delegate.");

        ParameterInfo[] parameters = invoke.GetParameters();
        parameterTypes = new List<Type>(parameters.Length);
        for (int i = 0; i < parameters.Length; i++)
            parameterTypes.Add(parameters[i].ParameterType);

        returnType = invoke.ReturnType;
    }

    public static Task<object[]> FromEvent<T>(this T obj, string eventName)
    {
        var tcs = new TaskCompletionSource<object[]>();
        var tcsh = new TaskCompletionSourceHolder(tcs);

        EventInfo eventInfo = obj.GetType().GetEvent(eventName);
        Type eventDelegateType = eventInfo.EventHandlerType;

        DynamicMethod handler;
        if (!s_emittedHandlers.TryGetValue(eventDelegateType, out handler))
        {
            Type returnType;
            List<Type> parameterTypes;
            GetDelegateParameterAndReturnTypes(eventDelegateType,
                out parameterTypes, out returnType);

            if (returnType != typeof(void))
                throw new NotSupportedException();

            Type tcshType = tcsh.GetType();
            MethodInfo setResultMethodInfo = tcshType.GetMethod(
                "SetResult", BindingFlags.NonPublic | BindingFlags.Instance);

            // I'm going to create an instance-like method
            // so, first argument must an instance itself
            // i.e. TaskCompletionSourceHolder *this*
            parameterTypes.Insert(0, tcshType);
            Type[] parameterTypesAr = parameterTypes.ToArray();

            handler = new DynamicMethod("unnamed",
                returnType, parameterTypesAr, tcshType);

            ILGenerator ilgen = handler.GetILGenerator();

            // declare local variable of type object[]
            LocalBuilder arr = ilgen.DeclareLocal(typeof(object[]));
            // push array's size onto the stack 
            ilgen.Emit(OpCodes.Ldc_I4, parameterTypesAr.Length - 1);
            // create an object array of the given size
            ilgen.Emit(OpCodes.Newarr, typeof(object));
            // and store it in the local variable
            ilgen.Emit(OpCodes.Stloc, arr);

            // iterate thru all arguments except the zero one (i.e. *this*)
            // and store them to the array
            for (int i = 1; i < parameterTypesAr.Length; i++)
            {
                // push the array onto the stack
                ilgen.Emit(OpCodes.Ldloc, arr);
                // push the argument's index onto the stack
                ilgen.Emit(OpCodes.Ldc_I4, i - 1);
                // push the argument onto the stack
                ilgen.Emit(OpCodes.Ldarg, i);

                // check if it is of a value type
                // and perform boxing if necessary
                if (parameterTypesAr[i].IsValueType)
                    ilgen.Emit(OpCodes.Box, parameterTypesAr[i]);

                // store the value to the argument's array
                ilgen.Emit(OpCodes.Stelem, typeof(object));
            }

            // load zero-argument (i.e. *this*) onto the stack
            ilgen.Emit(OpCodes.Ldarg_0);
            // load the array onto the stack
            ilgen.Emit(OpCodes.Ldloc, arr);
            // call this.SetResult(arr);
            ilgen.Emit(OpCodes.Call, setResultMethodInfo);
            // and return
            ilgen.Emit(OpCodes.Ret);

            s_emittedHandlers.Add(eventDelegateType, handler);
        }

        Delegate dEmitted = handler.CreateDelegate(eventDelegateType, tcsh);
        tcsh.Target = obj;
        tcsh.EventInfo = eventInfo;
        tcsh.Delegate = dEmitted;

        eventInfo.AddEventHandler(obj, dEmitted);
        return tcs.Task;
    }
}

此代码几乎适用于所有返回void的事件(无论参数列表如何).

如有必要,可以对其进行改进以支持任何返回值.

您可以在下面看到Dax和我的方法之间的区别:

static async void Run() {
    object[] result = await new MyClass().FromEvent("Fired");
    Console.WriteLine(string.Join(", ", result.Select(arg =>
        arg.ToString()).ToArray())); // 123, abcd
}

public class MyClass {
    public delegate void TwoThings(int x, string y);

    public MyClass() {
        new Thread(() => {
                Thread.Sleep(1000);
                Fired(123, "abcd");
            }).Start();
    }

    public event TwoThings Fired;
}

简单地说,我的代码支持真正的任何种类的委托类型.你不应该(也不需要)明确指定它TaskFromEvent<int, string>.

Answer 2

这将为您提供您所需要的而无需做任何ilgen,并且更简单.它适用于任何类型的事件代表; 您只需为事件委托中的每个参数数创建不同的处理程序.以下是0..2所需的处理程序,这应该是绝大多数用例.扩展到3及以上是从2参数方法的简单复制和粘贴.

这也比ilgen方法更强大,因为您可以在异步模式中使用事件创建的任何值.

// Empty events (Action style)
static Task TaskFromEvent(object target, string eventName) {
    var addMethod = target.GetType().GetEvent(eventName).GetAddMethod();
    var delegateType = addMethod.GetParameters()[0].ParameterType;
    var tcs = new TaskCompletionSource<object>();
    var resultSetter = (Action)(() => tcs.SetResult(null));
    var d = Delegate.CreateDelegate(delegateType, resultSetter, "Invoke");
    addMethod.Invoke(target, new object[] { d });
    return tcs.Task;
}

// One-value events (Action<T> style)
static Task<T> TaskFromEvent<T>(object target, string eventName) {
    var addMethod = target.GetType().GetEvent(eventName).GetAddMethod();
    var delegateType = addMethod.GetParameters()[0].ParameterType;
    var tcs = new TaskCompletionSource<T>();
    var resultSetter = (Action<T>)tcs.SetResult;
    var d = Delegate.CreateDelegate(delegateType, resultSetter, "Invoke");
    addMethod.Invoke(target, new object[] { d });
    return tcs.Task;
}

// Two-value events (Action<T1, T2> or EventHandler style)
static Task<Tuple<T1, T2>> TaskFromEvent<T1, T2>(object target, string eventName) {
    var addMethod = target.GetType().GetEvent(eventName).GetAddMethod();
    var delegateType = addMethod.GetParameters()[0].ParameterType;
    var tcs = new TaskCompletionSource<Tuple<T1, T2>>();
    var resultSetter = (Action<T1, T2>)((t1, t2) => tcs.SetResult(Tuple.Create(t1, t2)));
    var d = Delegate.CreateDelegate(delegateType, resultSetter, "Invoke");
    addMethod.Invoke(target, new object[] { d });
    return tcs.Task;
}

使用就像这样.如您所见,即使事件是在自定义委托中定义的,它仍然有效.并且您可以将公平值捕获为元组.

static async void Run() {
    var result = await TaskFromEvent<int, string>(new MyClass(), "Fired");
    Console.WriteLine(result); // (123, "abcd")
}

public class MyClass {
    public delegate void TwoThings(int x, string y);

    public MyClass() {
        new Thread(() => {
            Thread.Sleep(1000);
            Fired(123, "abcd");
        }).Start();
    }

    public event TwoThings Fired;
}

这里有一个帮助函数,如果以上三种方法对于您的首选项进行了太多的复制和粘贴,则允许您在每行中只编写一行TaskFromEvent函数.必须给予最大信用以简化我原来的.