It would be nice to have a watcher task as preLaunchTask in launch.json. At the moment this is not possible because the launch task will not start before the preLaunchTask ends.
A long running preLaunchTask should have a regex similar to problemMatcher to wait for a specific text in the output window before executing the launch task.
In my scenario I'm running webpack dev server as a watcher task. I want this task to run if F5 is hit and the preLaunchTask is not running. To delay the debugger launch it would be useful to test for a specific string in the output window webpack: bundle is now VALID.