In 1.5.1 the select??? methods opened a modal dialog box in the main thread which means that the sketch execution stopped on the select??? method call until the dialog box was closed.

In 2.0 the select??? methods create a secondary thread and uses this to lauch the dialog box. It means that the main thread continues even while the dialog box is open. When the dialog box is closed it will call the callback method specified in the original select??? method call. It does mean that in the main thread you cannot assume that the file has been saved/loaded/processed you have to flag this yourself. The easiest way is to have a boolean variable set to false before calling the select??? method and then set this to true in the callback method.

Unfortunately it means that it is not always straightforward to convert a sketch using these methods from a 1.5.1 sketch to a 2.0 sketch.