Re: unit test for GUI code

From:
"andrewmcdonagh" <andrewmcdonagh@gmail.com>
Newsgroups:
comp.lang.c++
Date:
23 Feb 2007 03:59:14 -0800
Message-ID:
<1172231954.856057.167540@z35g2000cwz.googlegroups.com>
On Feb 23, 2:11 am, "Michael" <mchlg...@aol.com> wrote:

I was told that unit test is a powerful tool for progamming. If I am
writing a GUI code, is it possible to still using unit test?


I have an approach based on some stuff I read in Michael Feather's
book. It is not as developed as what Andrew posted (for example, I
don't have a clever name for it), but it works for me.

I've been doing MFC programming, so I'll give my example there,
although it's just an example. In MFC, there's a class called CDC,
which is the C (language) device context. It has a bunch of primitive
drawing methods like:

CPoint MoveTo(int x, int y);

Now I'm writing a class C that does a bunch of drawing, so it winds up
calling these. It has a method OnDraw that takes a CDC* and does the
drawing (in turn calling a bunch of other methods). This is the
method I want to test.

So basically, I created a class called DCInterface, which looks like
this:
class DCInterface {
public:
     virtual CPoint MoveTo(int x, int y) = 0;
     /* etc. */

};

Then, I created a class CDCWrapper that calls the actual one:

class CDCWrapper : public DCInterface {
private:
     CDC* pDC;
public:
     explicit CDCWrapper(CDC* p) : pDC(p) {}
     CPoint MoveTo(int x, int y) { return pDC->MoveTo(x, y); }
     /* etc. */

Then I refactor my code for class C to use a DCInterface instead of a
CDC. (The OnDraw method stays, since the MFC framework calls it, but
now it just creates a CDCWrapper and forwards on to the wrapped
version of OnDraw.)

So far, everything is just setup for unit testing.

Now, here's the cool part. I created a TestDC class that implements
all of the functionality by storing a string with messages like
"MoveTo(5, 6) called.\n";

So to do unit tests, I set everything up with my TestDC class, and the
output is a string (instead of drawing something on the screen). I
can then do all my assertions that the appropriate series of draw
operations were called, based on looking at the output string.

This obviously doesn't do everything, but it lets me unit test some
fairly complex graphics routines.

More generally, you can pull out a wrapper that has virtual copies of
the library interface. Create an "real" implementation class that
just delegates to the library, then create a separate "test"
implementation class that stores things to a string (or some other
structure that's easy to use in unit tests).

There's a fair amount of work to create the wrappers in the first
place - I took the approach of "only wrap functions you actually use,"
but that's a one-time cost and then you can reuse it for your unit
testing.

Michael


Its a great book for working with un tested code, and for seeing how
to create 'seams' between areas like GUI libraries and our code.

For those interested in unit testing, a good guide that M Features,
myself and plenty of others use is:

A unit test does NOT :
* Use any file IO
* Connect to a db
* Present anything on screen
* communicate across a network.

The rational for this, is many, but boils down to forcing us to create
a design that is decoupled from these areas, which nearly always
results in a good, clean & highly cohesive design. For example, how
many times have we in the past put logic directly into a dialog
class? Where as with this style of testing, we force ourselves to
separate out the UI aspect from the logic part, resulting in us using
the MVC or MVP or Humble Dialog design patterns, as they allow us to
unit test th elogic without going anywhere near the GUI.

The second major win is speed of test runs.

I can run 1200 unit tests in 42.7 seconds currently (and these are
Java unit tests!) because of following these guidelines. And because
they run soo fast the Team runs ALL tests ALL of the time.
Slow tests mean we run them fewer times, the less we run them, the
longer it is before we find out we broken something. The longer time
before finding out we broke something, the more costly it is to
fix.....

Andrew

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