Time for action – getting values from the UI

If the test tries to access a property from the returned widget, there will be an exception thrown. For example, ctabs.get(0).getText() will result in an Invalid thread access SWT error.

To perform testing on widgets, the code has to run in the UI thread. Either the Display.getDefault().syncExec() or the equivalent Synchronizer class can be used, but SWTBot has a UIThreadRunnable that can launch code and a general interface called StringResult, which is like a Runnable that can return a String value through syncExec.

  1. At the end of the testTimeZone method of the UITest class, create a new StringResult and pass it to UIThreadRunnable.syncExec().
  2. In the run method, get the first CTabItem and return its text value.
  3. After the Runnable method has run, assert that the value is Africa.
  4. The code looks like:
    String tabText = UIThreadRunnable.syncExec(new StringResult() {
  @Override
  public String run() {
    return ctabs.get(0).getText();
  }
});
assertEquals("Africa", tabText);
  5. Run the tests and ensure they all pass.

What just happened?

To interact with widgets, code must be run on the UI thread. To run code on the UI thread in SWT, it needs to be wrapped into a Runnable, which needs to be posted to the display (or Synchronizer) and executed there.

Using a syncExec method call means that the result is guaranteed to be available for testing. If an asyncExec method call is used, then the result may not be available by the time the following assert operation runs.

To pass a value back from the UI thread to a non-UI thread, the result has to be stored in a variable. This has to be either a field on the class or a value in a final array. The StringResult of the SWTBot package effectively wraps this up in the UIThreadRunnable, in which an ArrayList is created to hold the single element.

Java 8 makes this significantly easier in that it allows lambda methods to be executed. For example, the execution can be replaced with the much shorter version as a lambda:

String tabText = UIThreadRunnable.syncExec(() ->
 ctabs.get(0).getText());
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