Justin's Blog

March 12, 2012

Fixed Waiting Periods: What Are You Waiting For?

I've seen a lot of test code that looks like this:

/*Test deletion from a list*/
SWTBot bot = new SWTBot();

//Remove an entry
bot.buttonWithLabel("Delete").click();

//Wait for it to disappear
bot.wait(250);

//Check that the list is empty
String listId = "brokenList";
int listSize = bot.listWithId(listId).itemCount()
if(listSize != 0) {
  Assert.fail("An item was not deleted from " + listId);
}

Fixed waits like this are pretty standard, but they can cause problems later. If this is a resource-intensive proccess, your test could break in these cases:

Fixed waits can also make your code harder to read, because it isn't always easy to see what's being waited on.

So what do you do?

Conditions: Look Before You Leap

Consider this alternative:

//Test deletion from a list
SWTBot bot = new SWTBot();
bot.buttonWithLabel("Delete").click();

String listId = "brokenList";
SWTBotList list = bot.listWithId(listId);

// Wait for the list item to disappear
try {
  ICondition listIsEmpty = new ListHasRowCount(list, 0);
  bot.waitUntil(listIsEmpty, 1000);
}
catch (TimeoutException e) {
  Assert.fail("An item was not deleted from " + listId);
}

This is a faster, safer way to wait for events. Here's what the custom condition looks like:

/**
* An encapsulated condition: test() returns true if the 
* specified list contains the specified number of items 
* at the time of invocation.
*
* @author Justin Bangerter
*/
public class ListHasRowCount extends DefaultCondition {

  /** The list under testing */
  private SWTBotList list;

  /** The number of rows to test for */
  private int itemCt;

  /**
  * Create a condition: test() returns true if the list
  * contains itemCt items.
  *
  * @param list the list to check
  * @param itemCt how many items should be in the list
  */
  public ListHasItems(SWTBotList list, int itemCt) {
    this.list = list;
    this.itemCt = itemCt;
  }

  /*
  * @see org.eclipse.swtbot.swt.finder.waits.ICondition#test()
  */
  @Override
  public boolean test() throws Exception {
    return this.itemCt == list.itemCount();
  }

  /*
  * @see org.eclipse.swtbot.swt.finder.waits.ICondition#getFailureMessage()
  */
  @Override
  public String getFailureMessage() {
    String listMsg = "The list {0} contained {1} items.";
    String listId = list.getId();
    int itemCt = list.itemCount();
    return MessageFormat.format(listMsg, listId, itemCt);
  }
}

I didn't put any parameter checking in this example, but it can be nice to have if you need it.

Important: Good failure messages should simply declare the discovered state and say nothing about the expected state. This is because conditions can be passed into both bot.waitWhile(...) and bot.waitUntil(...).

Suppose we use this message instead:

"The list did not have " + this.itemCt + " items."

If you use the condition in bot.waitUntil(...);, and the test fails, the message will say "The list did not have 0 items." That makes sense.

However, if you use the condition in bot.waitWhile(...);, and the test fails the message will say the same thing. That doesn't make sense, because we want the list to have more than 0 items. The "failure" message will erroneously claim that the state was exactly what it should have been, which it was not.

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