OOP: Better OOP part 2 2003-04-12 -- Dan Pressnell Newsgroups: comp.lang.clarion,topspeed.topic.oop March 5, 2006: Note that the code is now available at: http://www.icetips.com/downloadfile.php?FileID=59 To do object oriented programming in Clarion, first you have to understand what "objects" are in Clarion. Clarion objects come in two flavors. The most common is the CLASS. Less common, and less understood, is the INTERFACE, which is usually nothing more than a different way of using a class. Concentrate on classes, first. You have to understand classes before you understand how to write an interface. (You can almost always get by with classes and forget about interfaces.) A class can be thought of as a mini-program. Ideally, a class contains all the functionality and data that it needs to perform its tasks, in isolation from the rest of your program. This is a goal--not a rule--because in Clarion programs classes often need to access global data, most significantly the file declarations that are global to your program. Given that you can think of a class as a mini-program, think about how programs are constructed. A program has data, a map, and procedures. That exactly what a class has. Look at this class declaration: CustomerOrderClassType class, type Q &CustomerOrderQueueType CustomerID long GetData procedure() Construct procedure() Destruct procedure() end CustomerOrderQueueType queue, type CustomerID long CustomerName long OrderID long OrderDate long OrderAmount decimal(20,2) OrderBalance decimal(20,2) end That class has two data items, Q and CustomerID. That data belongs to the class. It doesn't belong to any other part of your program, in a strict sense, although if the rest of your program has access to the class, it has access to the class's data. The class also has three procedures. So the class has a data structure, and it has a collection of procedure declarations (like a MAP). Again, a mini-program. With the possible exception of references to application files, the class contains all the information it needs to function on its own, independently of any other part of your program. This does not mean that the class won't use other parts of your program, or even use other classes. It does mean that it is designed in a way that should not interfere with other things in your program, and it can be used from any part of your program without changes for the specific situation. Notice the Q that is part of the class declaration, and is a type, as defined later. To have a queue as class data, you have to declare a reference to a queue type. The queue that belongs to a specific instance of a class will be created typcally in the Construct method for the class, and disposed in the Destruct method. Like this: CustomerOrderClassType.Construct procedure() code self.q &= new CustomerOrderQueueType CustomerOrderClassType.Destruct procedure() code dispose(self.q) The Construct method will be executed automatically whenever you have a copy of the class come into focus (is created), and the Destruct method will execute automatically when the class goes out of focus (is destroyed). Creating queues (and classes) with NEW in the Construct method and disposing them in the Destruct method is a common technique in Clarion. The GetData method would retrieve information about orders for a customer. A possible use would be this: cc CustomerOrderClassType ! declare an instance of the class code cc.CustomerID = loc:CustomerID cc.GetData() At this point, the local instance of the class, "cc" would contain the data you have requested for the CustomerID that is held in the local variable loc:CustomerID. You have access to the data in the class by having access to the Q field in the class. For example, if you wanted to show the customer name in the first queue record, you could do this: get(cc.q, 1) message('Customer Name: ' & cc.q.CustomerName) Notice that here, in using the class, you have not concerned yourself with how the class does its work. You have simply primed it with a CustomerID, executed the method that retrieves the data, and used the data after it is retrieved. The complexities of retrieving the data are completely hidden from the part of your program that uses the class for it's purpose. (I leave the code for the GetData method to your imagination.) So how can this class be used? You can use it to display the customer's orders in a list box, print them on a report, put them into a spreadsheet, create an HTML file, or even export them to another data format. Assuming your GetData method works correctly, you will always get the same information correctly no matter how you are going to use it. This is what is meant by reusable code. Reusable code does not mean copy and paste. :) When writing reusable classes, there are some more mechanics to consider than what I've shown here. Those will be next. Dan Printed November 23, 2024, 2:15 am This article has been viewed/printed 35348 times. Google search has resulted in 27 hits on this article since January 25, 2004. |