Friday, 10 August 2007

Adventures in test-driven development Part 3: Getting Rid Of the Ugly Stuff

As promised, today we are going to get rid of the Graphics dependency. We are also introducing Rhino Mocks into our project. We are going to see how the limitations of this framework are going to make our design better. It might sound as something strange at first, but we'll see how it works. Fortunately, we have written our first test using the all-mighty TypeMock, so we can be sure that nothing breaks.

In order to test our behavior and not print something in the process, we need to mock the Graphics object. However, it is impossible with Rhino, since the class is sealed, or NotInheritable. So, somehow we should eliminate this hardcoded dependency from our class.

Why is this good, apart from testability? First, the Graphics class is a heavy dependency. The sole fact that it is IDisposable tells us that it depends on some unmanaged resources. In addition, it brings a dependency on Win32, which is bad, since we want to develop a universal component. Sure if we want to use our component on Mono or whatever CLR implementation exists, we could find a Graphics analog, but there's no guarantee that these implementations, that are not part of the standards, are implemented the same way.

Also, it carries too much with it that we just don't need. We don't want our main object perform all the low-level work, we want to serve it as a main controller, leaving all individual details to other objects.

Therefore, we are going to introduce a Canvas class, which is going to serve as our main printing device. Immediately, we have two advantages. First, we can provide multiple inheritors of the Canvas class, so, in theory, we can print to PDF, HTML, Word, anything. Second, we are ruling the interface of our Canvas class. Whenever we look at it, we immediately know its responsibilities within our project. We don't have to adapt to the Graphics interface (this is done in the implementation). Since we don't need mysterious stuff that Graphics has, like GetHdc() or FromHwndInternal(), we don't include it as part of our class, so those who use our component won't be tempted to play with it.

Enough said, let's code! Before we even start with our second test, we should do a little redesign.We should move in small steps, running our test after each step. This way we immediately know if we make a wrong step. Remember that RhinoMocks requires that we mock an interface or an inheritable class. The first step is to construct a thin wrapper around the Graphics class, giving out just the methods we need. Let's call it GraphicsCanvas, and let's put it into WinForms namespace. This is the only namespace that is going to reference the System.Drawing assembly, and eventually we are going to extract it into a separate project. Although it seems like adding complexity, actually it makes our design cleaner, since the project itself becomes clean of "the earthly stuff" in some way. For the same reason, I'm going to introduce my analog of the System.Drawing.Point structure.

The first step is, unfortunately, relatively big. We have to write some basic stuff into our new class so that we have something to test. Let's have it:

Namespace WinForms

Public Class GraphicsCanvas

Private _graphics As Drawing.Graphics

Private Sub New()

End Sub

Public Sub New(ByVal graphics As System.Drawing.Graphics)

Me._graphics = graphics

End Sub

Sub DrawString(ByVal text As String, ByVal position As Printing.Point)

Me._graphics.DrawString(text, New Drawing.Font("Verdana", 8.0, Drawing.FontStyle.Regular), Drawing.Brushes.Black, 0, 0)

End Sub

End Class

End Namespace

What do we have here? First, we hold a private variable for our Graphics object. Next, we have a private parameterless constructor, which makes it impossible to construct our class without parameters. Indeed, it cannot exist without the Graphics object. Next, we have a public constructor. I should say here that while the class itself and the constructor can reference the Graphics class, all other public methods shouldn't, since we are going to convert them into an interface later. So, the last is our first printing method. The least we should provide for printing is the text and its location. In fact, it's about 90% of our needs, or close to that.

Now, let's figure out our testing strategy. The way Rhino Mocks, as well as most other mock frameworks, work is quite different from TypeMock. TypeMock places a sort of hook on a class before the concrete object is created, so that when it is created, some method calls can be intercepted. So, it's fine that the object itself is created somewhere in our code. Just like the Graphics object, which is created somewhere deep in the framework. On the other hand, a Rhino Mocks mock should be created explicitly in the test. That forces us to redesign our code, so that we could somehow feed the mock object into our production code. This is done via constructor arguments, property setters, or method parameters. This procedure is generally called "dependency injection".

In our case, each page corresponds to a separate Graphics object, so it makes sense that we provide a separate GraphicsCanvas object for each page. So, the first refactoring is to extract the call to DrawString() to a separate method:

Protected Overrides Sub OnPrintPage(ByVal e As System.Drawing.Printing.PrintPageEventArgs)


End Sub

Sub PrintCurrentPage(ByVal graphics As System.Drawing.Graphics)

For Each section In Me.Sections.Values

For Each element In section.Elements.Values

graphics.DrawString(CType(element, Elements.LabelElement).Text, New Drawing.Font("Verdana", 8.0, Drawing.FontStyle.Regular), Drawing.Brushes.Black, 0, 0)



End Sub

(Remember to run our first test each time we do some refactoring)

In our second test, we are not going to call the Report.Print() method, since there's no way to inject the Canvas dependency. Instead, we'll call the PrintCurrentPage, but only after we figure out how to call it. So, this method should take our wrapper as an argument. The next version looks like this:

Protected Overrides Sub OnPrintPage(ByVal e As System.Drawing.Printing.PrintPageEventArgs)

Dim canvas = New WinForms.GraphicsCanvas(e.Graphics)


End Sub

Sub PrintCurrentPage(ByVal canvas As WinForms.GraphicsCanvas)

For Each section In Me.Sections.Values

For Each element In section.Elements.Values

Dim Text = CType(element, Elements.LabelElement).Text

canvas.DrawString(Text, New Fresh.Printing.Point)



End Sub

But now that we try to write our test, we have a problem. Since our GraphicsCanvas object can't be created without a Graphics object, so can't the corresponding mock. We imposed this limitation on ourselves when we added a private parameterless constructor to our class. We could remove it now, but it would be a bad idea. Instead, let's get our design driven by this limitation. We'll see that it's for the good.

What are the requirements for the type of the canvas argument? First, it should allow us to pass our GraphicsCanvas object, so it should be a base class. Second, it should be mockable. In addition, it should provide our base functionality (the DrawString method) without implementing it (so the method should be abstract, or MustOverride). So, it's either an abstract class or an interface. Typically, such things are done with interfaces. Perhaps historically mock frameworks could work with interfaces but not with abstract classes. For me, an interface is some common functionality among unrelated classes (like IDisposable), whereas an abstract class is a conceptual common ground, like Shape for all geometrical shapes.

So, let's shoot our Canvas class:

Namespace Printing

Public MustInherit Class Canvas

MustOverride Sub DrawString(ByVal text As String, ByVal position As Point)

End Class

End Namespace

Next, we just modify or GraphicsCanvas class so that it inherits from Canvas. And let the PrintCurrentPage accept a Canvas object as its argument:
Sub PrintCurrentPage(ByVal canvas As Printing.Canvas)
So, did it make our design better? First, we have greater control over printing. We have extracted our PrintCurrentPage method and now can call it directly. This is a big step towards being independent from the PrintDocument class by the way. Next, we can print to anything we like, provided we can implement a custom Canvas class. Our main Report object has been relieved from the printing burden, and can concentrate on more important tasks (such as handling the report structure and routing commands to other objects). This is called the Separation of Responsibilities (SoC) principle.

Finally, let's see our test:

<Test()> Sub TestWithRhino()

Dim mocks As New MockRepository()

Dim canvasMock = mocks.CreateMock(Of Printing.Canvas)()

Dim TestReport = New Report

Dim TestSection As New Core.Section

TestReport.Sections.Add("", TestSection)

Dim TestElement As New Elements.LabelElement

TestSection.Elements.Add("", TestElement)

TestElement.Text = "test"

Using mocks.Record

canvasMock.DrawString("", New Printing.Point()) 'we can provide any arguments here

LastCall.Constraints(Rhino.Mocks.Constraints.Text.Like("test"), New Rhino.Mocks.Constraints.Anything)

End Using

Using mocks.Playback


End Using

End Sub

First goes some preparation. Next we have a Using mocks.Record statement -- it's a first part of the Record-Replay pattern, where we first write down what method calls we expect, what arguments should be there, and which results should the methods return. In our case, the first line states that we should call the DrawString method. The arguments are irrelevant here, but without them the code won't be compiled. The second line tells us that the first argument should be equal to our test string, and the second can be anything.

Last, we have a Playback block where we put our tested method.

Our test actually verifies that the appropriate call is made to the Canvas object. It doesn't verify that the string is actually printed. This is where Rhino Mocks can't help us. However, we can easily write a separate test (using TypeMock) for it. Since we have two separate objects, we can test them independently. We can do all kinds of tests verifying that DrawString is invoked correctly, and only one test verifying that it actually prints something. When we have other Canvas objects, we'll have to make one test for each object, instead of testing all possible report layouts and data with each canvas. That's what the term unit testing is about.

Another thing that's not tested here is that calling the Print() method actually calls the PrintCurrentPage method. So far, we have our first test to verify it, but this is an indication that this piece has to be refactored as well. I guess I'll be making a separate class ReportPrinter that inherits from PrintDocument and manages the interaction with the actual printing and previewing (it can even be used at design-time in a form), and our Report class will be completely ignorant of these implementation details. However, I'll wait till I implement paging and let my tests drive my design.

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