Switch fallthrough in Dart

问题

I started learning Dart today, and I've come across something that my google skills are having trouble finding.

How do I have a fall-through in a non-empty case?

My use case is this: I'm writing a sprintf implementation (since dart doesn't have this too), which would work except for this fall-through thing. When parsing the variable type you can, for example, have "%x" versus "%X" where the upper case type tells the formatter that the output is supposed to be uppercase.

The semi-pseudocode looks like:

bool is_upper = false;
switch (getType()) {
    case 'X':
      is_upper = true;
    case 'x':
      return formatHex(is_upper);
}

The other ways I can think of doing this, would one of the following

switch (getType()) {
  case 'X': case 'x':
    return formatHex('X' == getType());
}

var type = getType();
if (type in ['x', 'X']) {
   return formatHex('X' == getType());
}

Now, the second choice almost looks good, but then you have to remember that there are eleven cases, which would mean having eleven if (type in []), which is more typing that I'd like.

So, does dart have some // //$FALL-THROUGH$ that I don't know about?

Thanks.

回答1:

The Dart specification gives a way for a switch case to continue to another switch case using "continue":

switch (x) {
  case 42: print("hello");
           continue world;
  case 37: print("goodbye");
           break;
  world:  // This is a label on the switch case.
  case 87: print("world");
}

It works in the VM, but sadly the dart2js switch implementation doesn't yet support that feature.

回答2:

You can't have a non-empty case body in Dart that falls through, this will raise an error.

What I tend to do with anything other than very simple switch statements is to refactor all the common code out into functions, so that you don't have this multi-level control flow in the switch itself.

In other words, something like:

switch (getType()) {
    case 'X':
        return formatHex(true);
    case 'x':
        return formatHex(false);
}

There's no reason why you need to have fallthrough. It comes in handy when the actions in a case section can be carried out in toto at the end of another case section, but this method can do that without fallthrough and without making your switch statement complex.

It can also handle more complex cases where there are common actions that aren't included in toto at the end. For example, you may want to do something at the start or in the middle of the case section. Calling common functions handles that more than well enough:

switch (getType()) {
    case 'X':
        doSomethingOnlyForUpperCase();
        doSomethingCommon();
        doSomethingElseOnlyForUpperCase();
        return formatHex(true);
    case 'x':
        doSomethingCommon();
        return formatHex(false);
}

I actually also do this for languages (such as C) that support this sort of non-empty fall-through since I believe it aids in readability and maintainability.

回答3:

From the dart language tour, your example of (2) should be correct.

var command = 'CLOSED';
switch (command) {
  case 'CLOSED':     // Empty case falls through.
  case 'NOW_CLOSED':
    // Runs for both CLOSED and NOW_CLOSED.
    executeClose();
    break;
}

It would be an error if you tried to do something as follows

var command = 'OPEN';
switch (command) {

  case 'OPEN':
    executeOpen();
    // ERROR: Missing break causes an exception to be thrown!!

  case 'CLOSED':
    executeClose();
    break;
}

回答4:

Also, a simplification of Lasse's answer is to use a simple continue; instead of continue <label>; as in:

bool is_upper = false;
switch (getType()) {
    case 'X':
      is_upper = true;
      continue;
    case 'x':
      return formatHex(is_upper);
}

来源：https://stackoverflow.com/questions/12550164/switch-fallthrough-in-dart

标签

switch-statement

dart

fall-through