Dart Switch

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There are several places in my Dart code that could benefit from a good switch statement. The question is, does Dart have a good switch statement?

In my simple canvas player animation, I respond to event key codes with a series of if statements:

attachMover(me, context) {
  // Move on key down
  document.
    on.
    keyDown.
    add((event) {
      String direction;

      // Listen for arrow keys
      if (event.keyCode == 37) direction = 'left';
      if (event.keyCode == 38) direction = 'up';
      if (event.keyCode == 39) direction = 'right';
      if (event.keyCode == 40) direction = 'down';

      if (direction != null) {
        event.preventDefault();
        me.move(direction);
        draw(me, context);
      }
    });

Assigning direction in each of those if statements feels un-DRY. I would prefer assigning the direction variable once—to the result of a switch statement:

      String direction = switch(event.keyCode) {
        case 37: 'left';
        case 38: 'up';
        case 39: 'right';
        case 40: 'down';
      };

Only that does not work in Dart.

When I run that statement, Dart informs me that:

Internal error: 'file:///home/cstrom/repos/dart-book/book/includes/animation/main.dart': Error: line 65 pos 26: unexpected token 'switch'
      String direction = switch(event.keyCode) {
                         ^

So it seems that I am stuck setting the direction inside the case statements just as I did with the if statements:

      String direction;
      switch(event.keyCode) {
        case 37: direction = 'left';
        case 38: direction = 'up';
        case 39: direction = 'right';
        case 40: direction = 'down';
      };

I suppose that is a slight improvement over the equivalent if statements—at least with switch, I no longer need to check event.keyCode on each line.

So yes, an improvement except that it does not work. Working is kind of a big deal.

The crash message:

Exception: 'file:///home/cstrom/repos/dart-book/book/includes/animation/main.dart': Switch case fall-through at line 69.
Stack Trace:  0. Function: 'FallThroughError._throwNew@127eafe4' url: 'bootstrap' line:595 col:3
 1. Function: '::function' url: 'file:///home/cstrom/repos/dart-book/book/includes/animation/main.dart' line:69 col:9
 2. Function: '_EventListenerListImpl@33cc944a.function' url: 'dart:html' line:8653 col:35

Fallthrough? You mean that I have to break each statement in the switch()?:

      String direction;
      switch(event.keyCode) {
        case 37: direction = 'left'; break;
        case 38: direction = 'up'; break;
        case 39: direction = 'right'; break;
        case 40: direction = 'down'; break;
      };

Indeed, that is exactly what I have to do because, not only does that compile again, but also my player animation is again working:

And then:

But in the end, I think I prefer if-statement:

      if (event.keyCode == 37) direction = 'left';
      if (event.keyCode == 38) direction = 'up';
      if (event.keyCode == 39) direction = 'right';
      if (event.keyCode == 40) direction = 'down';

The equivalent case-statement-break buries the assignment inside too much code:

      switch(event.keyCode) {
        case 37: direction = 'left'; break;
        case 38: direction = 'up'; break;
        case 39: direction = 'right'; break;
        case 40: direction = 'down'; break;
      };

I really do not care for that trailing break. It obscures the intent of the code (the direction assignment), decreasing long-term maintainability. I suppose that I could move it to a separate line, but my switch() statement already has two more lines of code than the equivalent if-statement structure.

The only time that I can think to use the switch statement in Dart code is when I have significant, multi-line calculation to perform for each case. Even then it might be better to break those mulii-line statements out into separate functions.

Which begs the question: is there a use-case for Dart's switch()? Unless I can think of one, I may give it a miss in Dart for Hipsters.


Day #334