This is tedious for Smalltalkers because the Smalltalk environment does not automatically manage external objects. This message executes 'ensureBlock' after 'doBlock' completes. And even if 'doBlock' does not complete because of an error, 'ensureBlock' is guaranteed to be executed. Let's continue with external files. The program creates a read stream on a file then writes the contents of a file to the transcript.
External streams can only write and read values such as bytes or words.
This is a very strict limitation in comparison with internal streams which can read and write any object. BOSS allows you to treat a file as a stream of objects. Program 21-12 reads three instances to display in a window one by one as you click the mouse. ScheduledControllers activeController view. BOSS manage persisten objects so that you can save objects from session to session.