Mastering Perl/Tk

Nonblocking Wait Activities

Perl/Tk provides three commands that wait for particular events to occur. Although the wait is nonblocking (Tk events continue to be processed), program flow is logically suspended at the wait point only until the appropriate event occurs. The commands are:

$widget->waitVariable( varRef )

Waits until the variable referenced by varRef changes (i.e., it is written or undef).

$widget->waitVisibility

Waits until $widget’s visibility state changes. The most common use for this command is to wait for a window to appear on the display. (Event type = Visibility.)

$widget->waitWindow

Waits until $widget is destroyed. (Event type = Destroy.)

waitVariable can be employed in a number of ingenious situations. In Chapter 23, we use it as a means of effecting interprocess communications. But perhaps the most common is waiting for a user response to, say, a Dialog widget. A Dialog posts a message and one or more Buttons, then waits for the user to make a selection by clicking a Button. The specified Button label text is then stored in the variable that waitVariable is watching, and logical program flow continues.

Tk::waitVariableX

Although waitVariable is nonblocking in the sense that Tk events continue to be processed, the program flow at the wait point is blocked until the variable changes. If the variable never changes, then that thread of execution can never continue. So, we can imagine a waitVariable with a timeout such that, after a certain amount of time, ...