将回调动作传递给方法

Question

It is an asynchronous method that goes and queries the database and sometime returns. Currently It is something like this:

this.Model.GetStudentKeys(keysList);
this.Foo1();
this.Foo2();

I am not proficient with the syntax but what I need to have is to change the code above in a way that those two Foo methods get called after the Async query is done. So I kind of know I need something like this:

this.Model.GetStudentKeys(keysList, callBack);

Action<object> callback {
    this.Foo1();
    this.Foo2();
}

But like I said I am not familiar with the syntax. Can you please help me with it?

Answer 1

You can use callbacks, but they are being superseded by C#'s async/await syntax . Before I explain said syntax however, I'll show you how you can achieve the same with Action s:

/*Action is a reference type, so its assignment is no different. Its value is a lambda
  expression (http://msdn.microsoft.com/en-us/library/bb397687.aspx),
  which you can think of as an anonymous method.*/
private Action<object> callback = o =>
                             {
                                 object result = o;

                                 this.Foo1();
                                 this.Foo2();
                             };

/*It's good practice in C# to append 'Async' to the name of an asynchronous method
  in case a synchronous version is implemented later.*/
this.Model.GetStudentKeysAsync(keysList, callback);

With this approach, your GetStudentsAsync method must invoke the callback, like so:

public void GetStudentsAsync(List<string> keys, Action<object> callback)
{
    var returnValue = null;
    //do some stuff...
    callback(returnValue);
} 

You can also pass the lambda expression directly as a parameter:

this.Model.GetStudentKeysAsync(keysList, o =>
                             {
                                 this.Foo1();
                                 this.Foo2();
                             });

With .NET 4.5 (and .NET 4.0 using the Microsoft Async package ) however, you can use the async and await keywords provided by C# 5:

//notice the 'async' keyword and generic 'Task(T)' return type
public async Task<IEnumerable<StudentModel>> GetStudentsAsync(IEnumerable<int> keyList)
{
    /*to invoke async methods, precede its invocation with
      the 'await' keyword. This allows program flow to return
      the line on which the GetStudentsAsync method was called.
      The flow will return to GetStudentsAsync once the database
      operation is complete.*/

    IEnumerable<StudentModel> students = await FakeDatabaseService.ExecuteSomethingAsync();

    return students;
}

Simply invoke the above method as follows:

IEnumerable<StudentModel> students = await this.Model.GetStudentsAsync(keyList);

Answer 2

Assuming the GetStudentKeys method returns a Task object (due to it being async), you could use the (new in C# 5.0) construct async and await ,

public async void MethodName()
{
    await this.Model.GetStudentKeysAsync(keysList);    // <-- Async operation
    this.Foo1();
    this.Foo2();
}

Once the async operation completes, the code will then continue after the await keyword.

Answer 3

You can define your function like that

    void GetStudentKeys(keysList, Func<bool> callback)
    {
        Thread th = new Thread(delegate(){
            //do some work in separate thread and inside that thread call
            if(callback != null)
                callback();
        });
        th.Start();
    }

and change the function call to

    this.Model.GetStudentKeys(keysList, () => {
        this.Foo1();
        this.Foo2();
        return true;
    });

Something like that.