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In previous sections we used procedures or pairs of object and message as actions. Each time an action is invoked, a new thread is created. While this is fine as it comes to efficiency (threads in Oz are light weight), it may cause trouble in that the order in which actions are invoked might be lost: the threads are created in the right order but there is no guarantee that they will run in that order.
The class Tk.listener fixes this. An instance of a subclass of Tk.listener has a thread of its own in which it serves action messages in order of invocation. For example, in
L ={New class $ from Tk.listener
meth b1 {Browse b1} end
meth b2 {Browse b2} end
end tkInit}
B1={New Tk.button tkInit(parent:W text:'One' action: L#b1)}
B2={New Tk.button tkInit(parent:W text:'Two' action: L#b2)}
{Tk.send pack(B1 B2 side:left)} the methods b1 and b2 are always executed in the same order in which the corresponding buttons are pressed.
When the tkInit method of the class Tk.listener is executed, a new thread together with a message stream is created. Whenever an action is invoked, where the object O of an object message pair O#M is an instance of Tk.listener, no new thread is created but M is appended at the end of the message stream. The thread then serves the message M as soon as all previous messages on the stream have been served completely. It serves M by executing the object application {O M}.
An additional message M to be served can be given to a listener by the method tkServe. For example, by
{L tkServe(b1)} the message b1 is served by L.
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