TweetFollow Us on Twitter

Object-Oriented Programming with REALbasic

Volume Number: 20 (2004)
Issue Number: 4
Column Tag: Programming

REALBasic Best Practice

by Guyren G Howe

Object-Oriented Programming with REALbasic

Getting the most from a unique Object-Oriented paradigm

Welcome to the first issue of my regular REALbasic Best Practice column. Each month, I'll tackle the question of how one can get the most out of REALbasic. I'm going to mix things up: reviews, advice, adapting general programming techniques to REALbasic, but always with the theme of "best practice".

I'm going to start with something I touched on in last month's review: REALbasic's unique object-oriented programming feature, that I call the Event Model. This article assumes you are familiar with the basics of object-oriented programming.

A Little History

A little history will help us to understand where we are.

REALbasic's original designer, Andrew Barry (who sadly is no longer with REAL Software) came up with what I believe is an entirely novel, and powerful object-oriented programming model for the very first version of REALbasic, which he prosaically called Events. To distinguish this feature in general from particular events, I'll call the feature the Event Model.

Because it was too different from what folks were used to, and because it made porting code from other languages more difficult, Andrew bowed to pressure, and also provided support for basically the same OOP model that Java supports (single inheritance with interfaces).

Once Andrew left, the programmers and managers at REAL Software have, to my mind, quite reasonably focused on the many other ways in which they wanted to improve REALbasic, and they've done a bang-up job of that. But they have, in some ways unfortunately, lost focus on this cool language feature. It's still there, it still works just fine, but REAL doesn't correctly document it, they don't much talk about it, and they haven't extended it in a few obvious ways that would make it just about perfect.

Why You need events

You need the Events Model because it makes code development and maintenance easier, and because the result of code changes in a superclass under the Events Model is much more predictable than with traditional method overriding.

In fact, the Event Model brings such clarity, and maintainability benefits that, except for doing one particular thing, Events are superior in every way to traditional method overriding.

Events vs Method Overriding

I will now explain what the Event Model is, and provide an example to demonstrate the advantages I just mentioned.

What we're about here is the relationship between the code in a superclass, and the code in a subclass. In Object-Oriented Programming languages, one of the most interesting problems is how the language should support code re-use down through a class hierarchy. We want to be able to provide some code in a superclass, and then have code in a subclass somehow extend or modify that code -- and to perform further modification, and extension of behavior on down a class inheritance chain.

Every mainstream Object-Oriented language does this by allowing a subclass to override methods on its superclass, entirely replacing that code, and then allowing the subclass to invoke methods defined in the superclass, including quite often invoking the superclass's version of the overridden method. A common pattern will be a method on a superclass that provides code to complete some but not all of some task, with the expectation that a subclass will override the method, carry out the rest of the task, and at some point call the overridden method to do the common part.

Notice that in this scheme, the subclass is entirely in charge of the final carrying-out of the method. In particular, it is up to the subclass to determine when and how to invoke the code in the superclass as part of its design for the complete method. In an inheritance chain, control is usually passed in this way from the bottom up, with a subclass invoking the most immediate superclass's methods as it desires.

There are two serious problems with this model:

Good object-oriented design will push as much code as is reasonable toward the top of the class hierarchy, but that code has to be written without being able to rely on when and how it is being invoked by the subclasses. Even in code you're entirely writing yourself, you have to remember what conventions you intended to employ in this relationship, possibly when you come back to code six months after you wrote it; and

It is easy for a subclass to override a superclass in such a way that, although you want to make a change in behavior that is the superclass's responsibility, you can't do it without also having to change some or all of the subclasses' code as well.

The Events Model, on the other hand, turns this whole thing on its head, allowing the superclass to provide subclasses with explicit opportunities to act, and letting these opportunities pass down the inheritance chain rather than up it. This very neatly avoids both of the problems mentioned, as we will see.

An Example

Let's consider a simple example that clearly illustrates the advantage of the Events Model.

Assume we're writing an application with a non-standard user interface, so we use custom controls drawn using graphics primitives.

We create an abstract Control superclass, whose task it is to keep track of where the control is on the screen. We give it a Draw method.

Next, we have an abstract Button subclass of Control, with code to draw a common background color and some other graphic details shared by all the buttons in the program.

We then create a whole variety of button subclasses for various tasks.

So we have an inheritance hierarchy that looks like this:


Figure 1: A simple inheritance hierarchy

The Method Overriding Way

When we use method overriding as our means of extension, as we go up the chain, each class overrides its superclass's Draw method and invokes that method as part of its own draw process.

This means that a control at the bottom of the chain will have a Draw method that looks something like this:

Pseudo-code for a Draw method at the bottom of the inheritance chain

Sub Draw
   Super.Draw
   ... code to draw specifics on top of background provided by super
End Sub

Button.Draw, in turn, has code like this:

Pseudo-code for Button.Draw
Sub Draw
   Super.Draw //Work out where I'm drawing
   ... code to draw background
End Sub

Control.Draw has code like this:

Pseudo-code for Control.Draw
Sub Draw
   ... code to work out where to draw
End Sub

This is all fine. We get nice abstraction and code re-use, all the usual OOP goodness.

But a problem arises when we make changes to the superclasses. Let's say we now want to make all our buttons have an Aqua-style highlight: we want to take whatever the subclass produces, and run a graphic filter over that to make it look like it's sitting in a drop of liquid.

We simply can't do it without wholesale code rewriting. With this inheritance scheme, the superclass has no way to obtain another chance to act after the subclass is done calling on it. The only solution is to make changes to every bottom-level subclass, even though we really don't need the subclasses to do their part of the drawing task any differently. A change in the shared logic can't be implemented in the shared code.

The Event Model

An Event declaration is essentially a declaration of the interface for a method that subclasses can implement (this "method" is called an Event Handler), that can only be called from the declaring superclass. Once declared, the superclass can invoke the event handler just like a regular method or function.

When an event handler is called, the most immediate subclass implementing a handler for it gets to act. If no subclass implements a handler, the call is ignored (if the call is to a function, a "null" -- 0, Nil, False, the empty string, and so on -- value is returned).

Note that once a handler for an event exists in the hierarchy for a particular class, further subclasses don't even see the event. Frequently, the subclass will do whatever it needs in response to the event invocation, and then invokes a new event it has declared, having exactly the same name and arguments, providing further subclasses with the opportunity to act.

Note that rather than the ill-defined mess that is method overriding, the language is providing a well-defined protocol for superclasses to delegate specific responsibilities down to their subclasses.

A good example of this scheme is the REALbasic EditField control, which declares a KeyDown event as a function with a string argument, returning a Boolean value. The event indicates to a subclass that the user has tried to type something into the EditField. If the subclass provides a handler for KeyDown and returns True in that handler, the keystroke is ignored. Otherwise, it is displayed in the EditField normally.

This scheme makes it easy to extend the EditField to create a control that, say, only lets the user enter a number.

The Button Example, Using the Event Model

Back to our example of custom buttons.

Under the Event model, rather than each subclass overriding its superclass's version of Draw, only the top-level Control class has a Draw method. In addition, Control declares a new DrawEvent event. Each subclass will provide a handler for the event, and will also declare a new version of the event, calling it at the appropriate time. So now we have:

Control.Draw has code like this:

Pseudo-code for Control.Draw
Sub Draw
   ... code to work out where to draw
   DrawEvent //Ask subclass to now draw itself there
End Sub

Button's DrawEvent handler looks like this:

Pseudo-code for Button.Draw
Sub DrawEvent
   ... code to draw background
   DrawEvent //Ask subclass to draw its content atop the background
End Sub

Finally, the class at the bottom of the chain just does its part of the task.

Pseudo-code for a Draw method at the bottom of the inheritance chain
Sub DrawEvent
   ... code to draw specifics on top of background provided by super
End Sub

Now what happens when we want to add the new graphical tweak? We only have to change the code in one place, Button's DrawEvent handler, which now looks like this:

Pseudo-code for an extended Button.Draw

Sub DrawEvent
   ... code to draw background
   DrawEvent //Ask subclass to draw its content atop the background
   ... code to draw water on top
End Sub

Once you get the Event Model, you miss it in every other object-oriented language.

There are many advantages to the Event Model. For example, if you're working with a team of programmers, or you're shipping a framework for others to use, with method overriding, you need all sorts of tortuous documentation about how subclasses should interact with their superclasses. With the Event model, the language itself enforces the relationship. So not only does the Event Model make maintenance easier, but it better explicates the logic of the program.

The Exception that Proves the Rule

...except that there is one feature for which we still need overriding: functions for which the subclass should return a more specific type than the superclass does. The canonical example of this is a generic data structure. We can write, say, a generic data store base class that stores Objects. But this is needlessly difficult to use as it is, because we have to cast the objects we get from it to a more specific type in order to use them, every single time we use this class.

What we want is to extend the generic data structure to one storing objects of more specific types -- a list of controls, for example, where the casting to the correct return type is done for us. Unfortunately, the Events Model provides no way to do this, because the function that yields the objects must be declared to return values of type Object (or perhaps Variant).

The method overriding paradigm provides what we need here. We create a Protected function toward the top of the inheritance chain, then declare and invoke suitable events so that subclasses can carry out the function. Then we create a 'public face' on that function in the bottom-level class in the form of a function that returns the type we want, and have it just call up to the protected internal function, at the top of the class hierarchy, that provides the generic functionality.

So the call sequence goes like this:


Figure 2: Using Method Overriding to Create a More Restricted Type

We invoke the public Get() function on the bottom-level ControlStore class.

ControlStore invokes the protected GenericStore.GetInternal() function on the base GenericStore class.

GenericStore invokes its GetEvent.

The most immediate subclass declaring a handler is GenericList, which fetches the result, returning it to GenericStore.

GenericStore returns the result to ControlStore.Get().

ControlStore.Get() casts the result to a Control and returns it to the original caller.

Interestingly, notice that in a sense, we aren't actually extending the GenericList class's behavior: we're actually restricting it. All of this calling up, and back down the inheritance chain is a little weird when you first do it. Quite often, in the bottom class, you will be creating the public function, calling the protected version in the base class, then turning around and handling the event again the same bottom-level class. When I first started doing this, it felt a bit baroque, to be going up, and then down the inheritance chain like this, and having the public function actually fulfill its role in the event handler. But I'm quite comfortable with it now because I don't see overriding as an extension mechanism at all any more.

It might help to see that in fact, you could do this with just Events by creating a wrapper class that does the type casting. I just prefer to do it all within the same class hierarchy, because it's cleaner and clearer to do this rather than create the generic store, the wrapper class, and suitable Interface.

It would be nice if REALbasic had templates, or if some other mechanism could be found that let us stick entirely to the Events Model without any of that, but this constrained use of overriding is just fine for now.

Improvements Wanted

I'd like to see REAL Software emphasize the Events Model in their documentation. I'd also like to see them look into making some improvements to the Events Model to make it more powerful.

One obvious improvement would be to provide a compiler directive that could execute different code depending on whether an Event has a handler available or not. A common style of programming you'll want to do with Events is to provide default behavior at some point in the code for a class, but allow a subclass to extend or replace that behavior (as the EditField does with the KeyDown Event).

The only way to do that right now with Events is to call a function-style Event Handler, and do your default behavior if you get back the "null" response. But in some circumstances, you need to treat the null value as a result, rather than an indication that you didn't provide an event handler, and you wind up doing all sorts of gymnastics to work around the problem.

Conclusion

I hope I've convinced you of the virtue of the Events Model. Once you "get it", in my experience, you'll wonder how you ever lived without it.


Guyren G Howe works in artificial intelligence research, after years of work as a technical writer and developer. He is married with one child, is an Australian, and lives in Austin, Texas. Guyren has been working with REALbasic for several years. Most notably, he wrote the REALbasic Curriculum Project, an extensive computer science curriculum, for REAL Software (available from the REALbasic website).

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Ember 1.8.3 - Versatile digital scrapboo...
Ember (formerly LittleSnapper) is your digital scrapbook of things that inspire you: websites, photos, apps or other things. Just drag in images that you want to keep, organize them into relevant... Read more
Apple iTunes 12.1 - Manage your music, m...
Apple iTunes lets you organize and play digital music and video on your computer. It can automatically download new music, app, and book purchases across all your devices and computers. And it's a... Read more
LibreOffice 4.4.3 - Free, open-source of...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
FoldersSynchronizer 4.2.1 - Synchronize...
FoldersSynchronizer is a popular and useful utility that synchronizes and backs-up files, folders, disks and boot disks. On each session you can apply special options like Timers, Multiple Folders,... Read more
Simon 4.0.2 - Monitor changes and crashe...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
Cocktail 8.1.2 - General maintenance and...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
Cyberduck 4.6.4 - FTP and SFTP browser....
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more
Herald 5.0.2 - Notification plugin for M...
Note: Versions 2.1.3 (for OS X 10.7), 3.0.6 (for OS X 10.8), and 4.0.8 (for OS X 10.9) are no longer supported by the developer. Herald is a notification plugin for Mail.app, Apple's Mac OS X email... Read more
DEVONthink Pro 2.8.3 - Knowledge base, i...
Save 10% with our exclusive coupon code: MACUPDATE10 DEVONthink Pro is your essential assistant for today's world, where almost everything is digital. From shopping receipts to important research... Read more
Boom 2 1.0.1 - System-wide pro audio app...
Boom 2 is a system-wide volume booster and equalizer app that is designed especially for OS X 10.10 Yosemite. It comes with a smart interface, self-calibrates itself according to your Mac, offers... Read more

Playworld Superheroes Review
Playworld Superheroes Review By Tre Lawrence on January 30th, 2015 Our Rating: :: HERO CRAFTINGUniversal App - Designed for iPhone and iPad It’s all about the imagination, fighting bad creatures — and looking good while doing so.   | Read more »
Join the SpongeBob Bubble Party in this...
Join the SpongeBob Bubble Party in this New Match 3 Bubble Poppin’ Frenzy Posted by Jessica Fisher on January 30th, 2015 [ permalink ] | Read more »
Handpick Review
Handpick Review By Jennifer Allen on January 30th, 2015 Our Rating: :: TANTALIZING SUGGESTIONSiPhone App - Designed for the iPhone, compatible with the iPad Handpick will make you hungry, as well as inspire you to cook something... | Read more »
Storm the Halls of Echo Base in First St...
Storm the Halls of Echo Base in First Star Wars: Galactic Defense Event Posted by Jessica Fisher on January 30th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Contradiction Review
Contradiction Review By Tre Lawrence on January 30th, 2015 Our Rating: :: SPOT THE LIEiPad Only App - Designed for the iPad Contradiction is a live action point and click adventure that’s pretty engaging.   Developer: Tim Follin... | Read more »
Unlock Sunshine Girl in Ironkill with th...
Unlock Sunshine Girl in Ironkill with this special 148Apps code Posted by Rob Rich on January 29th, 2015 [ permalink ] Robo-fighter Ironkill has been out on iOS a | Read more »
Crossroad Zombies Review
Crossroad Zombies Review By Jordan Minor on January 29th, 2015 Our Rating: :: CROSSWALKING DEADiPad Only App - Designed for the iPad Crossroad Zombies is a rough draft of a cool genre mash-up.   | Read more »
Blood Brothers 2 – Tips, Cheats, and Str...
War is hell: Is it the kind of hell you want to check out? Read our Blood Brothers 2 review to find out! Blood Brothers 2, DeNA’s follow-up to the original Blood Brothers, is an intriguing card collecting / role-playing / strategy hybrid. There’s... | Read more »
Blood Brothers 2 Review
Blood Brothers 2 Review By Nadia Oxford on January 29th, 2015 Our Rating: :: AN AGGRAVATING RELATIVEUniversal App - Designed for iPhone and iPad Blood Brothers 2 is built on a simple, solid foundation, but its free-to-play system... | Read more »
I AM BREAD, the Toast of the Town, is Ro...
Have you ever dreamt of being deliciously gluten-y? Do you feel passionate about Rye and Wheat? The guys at Bossa Studios do and that is why they are bringing I AM BREAD to iOS soon. The loafy app will feature all the new content that is being... | Read more »

Price Scanner via MacPrices.net

Apple Becomes World’s Largest Smartphone Vend...
According to the latest research data from Strategy Analytics, as global smartphone shipments grew 31 percent annually to reach a record 380 million units in the fourth quarter of 2014. Apple became... Read more
Cut the Cord: OtterBox Resurgence Power Case...
Dead batteries and broken phones are two of the biggest issues for smartphone users today. Otterbox addresses both with the new Resurgence Power Case for Apple iPhone 6, promising to make those panic... Read more
13-inch Retina MacBook Pros on sale for up to...
B&H Photo has 13″ Retina MacBook Pros on sale for $200 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.6GHz/128GB Retina MacBook Pro: $1199.99 save $100 - 13″ 2.6GHz/... Read more
15-inch 2.5GHz Retina MacBook Pro on sale for...
 B&H Photo has the 15″ 2.5GHz Retina MacBook Pro on sale for $2319.99 including free shipping plus NY sales tax only. Their price is $180 off MSRP, and it’s the lowest price available for this... Read more
Back in stock: Refurbished iPod nanos for $99...
The Apple Store has Apple Certified Refurbished 16GB iPod nanos available for $99 including free shipping and Apple’s standard one-year warranty. That’s $50 off the cost of new nanos. Most colors are... Read more
Apple lowers price on refurbished 256GB MacBo...
The Apple Store has lowered prices on Apple Certified Refurbished 2014 MacBook Airs with 256GB SSDs, now available for up to $200 off the cost of new models. An Apple one-year warranty is included... Read more
New Good Management Suite Simplifies Enterpri...
Good Technology has announced the availability of the Good Management Suite, a comprehensive cross-platform solution for organizations getting started with mobile business initiatives. Built on the... Read more
15-inch 2.0GHz Retina MacBook Pro (refurbishe...
The Apple Store has Apple Certified Refurbished previous-generation 15″ 2.0GHz Retina MacBook Pros available for $1489 including free shipping plus Apple’s standard one-year warranty. Their price is... Read more
15-inch 2.5GHz Retina MacBook Pro on sale for...
Amazon has the 15″ 2.5GHz Retina MacBook Pro on sale for $2319.99 including free shipping. Their price is $180 off MSRP, and it’s the lowest price available for this model from any reseller. Read more
iPad Turns Five – The ”Book Mystique
Five years ago this week, the late Steve Jobs took the stage at an Apple special press event to unveil the first Apple iPad — a slab-shaped, hand-holdable device that was to shake the personal... Read more

Jobs Board

At-Home Chat Specialist- *Apple* Online Stor...
**Job Summary** At Apple , we believe in hard work, a fun environment, and the kind of creativity and innovation that only comes about when talented people from diverse Read more
SW QA Engineer - *Apple* TV - Apple (United...
**Job Summary** The Apple TV team is looking for experienced Quality Assurance Engineers with a passion for delivering first in class home entertainment solutions. **Key Read more
*Apple* Retail - Multiple Positions(US) - Ap...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you re also the Read more
*Apple* Systems Engineer - Pre Sales, Educat...
…is responsible for proactively providing technical expertise to drive sales of Apple solutions into assigned accounts. The SE architects, validates, and assists in Read more
*Apple* Solutions Consultant - Retail Sales...
As an ambassador of the Apple brand, the ASC is accountable for driving sales performance by: Connecting with customers. Discovering customers' needs and values. Showing Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.