Extracting pattern and simplifying testing

Question

I observe the same pattern in a number of my classes but can't extract/abstract it due the tight coupling inside each particular implementation.

Having the class accepting an optional number of dependencies onto its constructor:

public Repository(IBuilder, IFormatter, ILoader, ISelector)

With the single method passing its arguments to the first dependency. Passing result to the second. And so on. Returning result of the last dependency:

public XElement Do(int i)
{
    string q = _builder.Build(i);
    string f = _formatter.Format(q);
    XDocument d = _loader.Load(f);
    XElement r = _selector.Select(d);
    return r;
}

(names of the classes, methods and variables are random, just to illustrate the pattern)

Can it be refactored to decouple the flow from the types of dependencies, arguments and return values?

It also will simplify the (unit) testing because currently I make sure that a dependency N passes its result to a dependency N+1:

[TestMethod]
public void Do_Should_Pass_Result_Of_Build_To_Format()

Answer 1

This is basically a pipe and filter pattern - where you construct some number of filters into a chain and just pass the outputs to the next filter. If you control all instances, then the simple thing is to abstract out to an interface:

 interface IFilter {
     object Filter(object o);
 }

Note the weak typing of using object - if these classes are used somewhere else, implementing IFilter as an explicit interface would probably be preferable. You could do generics, but that makes the chaining harder.

 interface IBuilder : IFilter {
     string Build(int i);
 }

 class Builder : IBuilder {
     public string Build(int i) {
         // Do stuff
     }

     object IBuilder.Filter(object o) {
         return (object)this.Build((int)o);
     }
 }

Now, each filter simply takes an input and transforms to an output. We need a pipe to tie to it all together:

 class Pipeline {
     private IFilter[] Filters { get; set; }

     public Pipeline(params IFilter[] filters) {
        this.Filters = filters;
     }

     public object Execute(object input) {
        foreach (var f in this.Filters) {
            input = f.Filter(input);
        }
        return input;
     }
 }

And, in use:

 class Repository {
     private Pipeline { get; set; }

     public Repository(IBuilder b, IFormatter f, ILoader l, ISelector s) {
         this.Pipeline = new Pipeline(b, f, l, s);
     }

     public XElement Do(int i) {
         return (XElement)this.Pipeline.Execute(i);
     }
 }

Your unit tests for Repository now only need to be concerned with the results of Do - which verifies the Pipeline was constructed properly (an important bit to test, since it's weakly typed).

You can dress it up with generics, extension methods, builder patterns, etc. - but that's the basic pattern.

Answer 2

One option would be to create a generic method that accepts delegates to the methods:

var combined = Combine(_builder.Build, _formatter.Format, _loader.Load, _selector.Select);
return combined(i);

The implementation would look like this:

public static Func<T1, T5> Combine<T1, T2, T3, T4, T5>(
    Func<T1, T2> f1, Func<T2, T3> f2, Func<T3, T4> f3, Func<T4, T5> f4)
{
    return input =>
    {
        var r1 = f1(input);
        var r2 = f2(r1);
        var r3 = f3(r2);
        var r4 = f4(r3);
        return r4;
    };
}

The implementation would need to have overloads for different number of delegates.

Also, I'm not completely sure whether this actually improves the situation. Your original code is very straightforward, this code is harder to understand, because it's not immediatelly clear what does Combine() do.

Answer 3

An alternative to @svick's answer is to use an extension method which only takes in two functions, and then chain them together, as follows:

public static class ExtensionHelper
{
    public static Func<T1,T3> Chain<T1,T2,T3> (this Func<T1,T2> f1, Func<T2,T3> f2)
    {
       return item => f2(f1(item));
    }
}

Then, you can chain any number of functions as follows:

static void Main(string[] args)
{
   Func<int,int> f1 = AddOne;

   var chained3 = f1.Chain(Double).Chain(i => i - 1); // overall: i => ((i + 1) * 2) - 1
   var val = chained3(1); // returns 3
}

public static int AddOne(int i)
{
   return i + 1;
}

public static int Double(int i)
{
   return i * 2;
}

The downside is that you have to declare the initial function as a Func type to start the chain. You cannot use the method group or a lambda directly.