We have to augment our previous program in two aspects: statically, we have to provide another button, and dynamically, we have to react to this button being pressed by ending the program.
For the first part, we create the second button just like the first
part. When we place it, we have to specify where it is going to be
placed. We want it below the second button, and we want both buttons
to stretch out horizontally such that they are of the same length,
regardless of the size of the labels. This is done by adding
packing options to the pack command. Here,
Side says we want the first button at the top and the second
at the buttom, and Fill X specifies the stretching
behaviour mentioned above (Fig. 3 shows the resulting window):
Figure 3: A second example.
To change the dynamic behaviour, we first need the second button to create an event with the clicked function. However, we need to change the behaviour of the spawned event such that when this new clicked event occurs, the program is finished.
This combination of events as a case distinction -- when this event
occurs, do something, when the other event occurs, do something
different -- is achieved by the fourth important operation on
events, the choice combinator
Hence, we need to combine the previous dynamic behaviour and the new behaviour by +>. The new behaviour, finishing the program, is achieved by calling the destroy action on main. This closes the main window and lets the program terminate gracefully:
Note that the choice occurs inside the forever (why?). We could also have created two threads here, each listening to one button. While in this simple situation, this would have been easier, it is in general good practice to create only as many threads as needed, since one otherwise tends to run into memory leaks by unused threads lying around or even worse, nasty synchronisation problems.