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Using AddHandler

Xojo has a way to connect events to handlers outside of a Window or sublcass.

Issue: 12.1 (January/February 2014)
Author: Seth Verrinder
Author Bio: Seth is a consultant who uses Real Studio every day in his job at BKeeney Software, Inc.
Article Description: No description available.
Article Length (in bytes): 5,235
Starting Page Number: 70
Article Number: 12108
Related Link(s): None

Excerpt of article text...

Usually you handle events in Xojo by adding code to the window that contains the control. Xojo automatically calls your code when the event is raised. This only works for objects that are placed on a window, though. What if you want to handle an event raised by an object that's not placed on a window?

AddHandler and RemoveHandler give you a way to do just that.

Using a Timer

Let look at an example: say that you have a Calendar control that's a subclass of the Canvas. Your calendar has an AddEvent method that adds an event to your calendar and you want to automatically refresh the display when that happens. But someone might add a bunch of events and you don't want to repaint the canvas for each one. You could leave it up to the calling code to call Refresh when it's done. Or, you could add a method that takes a list of events. Another approach is to start a timer inside of AddEvent. When the timer fires, it calls Refresh.

One place where this is handy is for handling Timer events. Normally you can just place a timer on your window and add code to the Action event in the window. But, if you want to use a Timer that's not on a window, then you can't do that. There are a few options here:

• You can create a separate Timer subclass that knows about the class that you want to use it in. So, for example, if you have a Calendar class, then you might create CalendarUpdateTimer where you store a reference to the calendar that you want to update.

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