TweetFollow Us on Twitter

Modifying Objects At Runtime

Volume Number: 15 (1999)
Issue Number: 3
Column Tag: PowerPlant Workshop

Modifying Objects at Runtime in PowerPlant

By John C. Daub, Austin, Texas USA

A look at PowerPlant's Attachments mechanism

Welcome!

One of the things that makes the Mac so great and so much fun to use is the ability to customize your Mac just the way you like it. There are all sorts of cool functional utilities like Monaco Tuner/ProFont and FinderPop, and many cosmetic enhancers like the ubiquitous Kaleidoscope. Just how do these cool tools do the voodoo that they do so well? Through the magic of trap patching! Patching traps is a neat way to change the runtime behavior of an application and/or the OS itself without having to actually change the application or OS. Think of it like subclassing to implement new functionality for an object, but you didn't have to subclass to actually gain the new functionality - something else came in at runtime and modified the object's behavior.

If that explanation only served to further confuse you, I apologize. However, by the end of this article you will have a better understanding of the statement. You see, PowerPlant has a mechanism akin to patching called Attachments. Attachments are a way to modify the runtime behavior of PowerPlant objects, so in a sense it can be viewed as patching. By using PowerPlant's Attachments mechanism you can extend, limit, or otherwise change the behavior of a PowerPlant object at runtime, and all through a mechanism that's simple yet very powerful; in fact the theory of the Attachments mechanism will play a larger role throughout PowerPlant in future versions of the framework.

By the way, if you're curious about trap patching, check out a book like Dave Mark's Ultimate Mac Programming. Dave recruited Jorg Brown to write the chapter on trap patching, so you'll definitely be in good hands.

Overview

PowerPlant's Attachment mechanism is comprised of two parts: the Attachment's host and the Attachment itself.

Attachables

An object that can host Attachments is called an Attachable. The LAttachable class defines an Attachable object, and any class that inherits from LAttachable can have Attachments attached to it (you cannot attach Attachments to objects that are not Attachables). Within PowerPlant, LAttachable is used as a virtual base class for some key base objects: LCommander, LEventDispatcher, and LPane (see my introduction to PowerPlant article in the December 1998 MacTech or refer to the PowerPlant Book, Metrowerks' official documentation on PowerPlant, for an explanation of these classes)


Figure 1. LAttachable class hierarchy.

Although Figure 1 does not look like much, do not forget the great number of classes that inherit from LCommander and LEventDispatcher, and especially the multitude of classes that inherit from LPane (there was just no way to print such a large hierarchy within the constraints of this article). Since these key objects are Attachables, you can begin to see that many of PowerPlant's key behaviors can be modified through the use of Attachments.

The LAttachable class provides all the functionality a host needs to manage Attachments. One can AddAttachment() or RemoveAttachment() to add or remove Attachments from the Attachable's internal list of Attachments. If you're in a hurry, you can call RemoveAllAttachments() to remove all of an Attachable's Attachments. Accessors are provided to GetDefaultAttachable() and SetDefaultAttachable() (the default Attachable is the object to which an Attachment will be attached when it is created from a PPob DataStream). Finally LAttachable provides ExecuteAttachments() as the ever-important means to allow Attachments to carry out their task. Most of your interactions with LAttachable will be through AddAttachment() (and possibly RemoveAttachment()), as the rest is handled for you within PowerPlant itself. Of course the other part of the equation here are the Attachments themselves, so let's take a look at them.

Attachments

An Attachment is an object that can modify the runtime behavior of another object without having to modify that object itself. Through the use of Attachments, you can modify how an object: handles events, draws or prints, handles clicks and keypresses, adjusts the cursor, responds to commands and actions, and even how it finishes creating itself. Figure 2 presents the Attachment classes provided by PowerPlant.


Figure 2. LAttachment class hierarchy.

The points at which the Attachments are given the opportunity to modify the runtime behavior of its host object (i.e. the places within PowerPlant where LAttachable::ExecuteAttachments() is called) are well defined by the PowerPlant API. Since the intent of an Attachment is to modify the "normal" behavior of an object, this Attachment execution point occurs immediately before the host object executes its normal course of action. So just before that mouse click is given to the Pane object for processing, the Pane's Attachments are given first crack at the click. Listing 1 illustrates how this looks in code.

Listing 1: Attachment execution point

LPane::Click()
void
LPane::Click(
   SMouseDownEvent&      inMouseDown)
{
   if (!inMouseDown.delaySelect) {
   
      PortToLocalPoint(inMouseDown.whereLocal);
      
      UpdateClickCount(inMouseDown);
      
      if (ExecuteAttachments(msg_Click, &inMouseDown)) {
         ClickSelf(inMouseDown);
      }
   }
}

When Click() is called, a little housekeeping is performed, then just before the actual handling of the click is to occur (ClickSelf()), the Pane's Attachments execute and could veto the execution of the Pane's normal click functionality. We'll look at how all of this works in the next section.

Like the LAttachable class, the LAttachment class is also quite simple. It contains constructors and a destructor, as one would expect of a C++ class. One item to note is LAttachment has an LStream constructor, which means that Attachment objects, like Pane objects, can be constructed from a PPob DataStream. You can edit Attachments in Constructor, they have class_ID's, and must be registered (see URegistrar) if you create your Attachments in this manner. If you've worked with Panes in PPob's, this procedure should be familiar to you. If you're not familiar with this procedure, it is covered in the PowerPlant Book.

There are three data members in LAttachment: mOwnerHost, mMessage, and mExecuteHost (and all three have associated Get/Set accessor methods). The first data member, mOwnerHost, is a pointer to the Attachment's host object for easy reference. The mMessage data member stores the particular message that the Attachment is to respond to. Finally, the mExecuteHost member is a Boolean value which specifies if the host should continue to execute its normal course of action or not. In the next section you'll see how these data members come into play.

The remaining two methods in LAttachment are Execute() and ExecuteSelf(). This is where all of the action happens, so to better explain them let's take a look at just how the Attachment mechanism actually works.

How Does It Work?

As you saw in the previous sections, the LAttachable and LAttachment classes are fairly simple, and the mechanisms by which they are implemented and execute are fairly simple as well. As mentioned previously, an Attachable iterates its list of Attachments at a well-defined point asking them to execute and passing them any relevant information the Attachment may need in order to properly execute in the given context. Figure 3 summarizes those points within PowerPlant.


Figure 3. Attachment execution points.

When in the course of an application's events it becomes necessary to execute an Attachment (the Where Called column in Figure 3), the host iterates its list of Attachments asking each if it cares to do something relevant to the given situation; this is done by calling the Attachment's Execute() method. Within Execute() the Attachment checks the message given to it by its host; if that message is the same as the Attachment's mMessage (or is the special msg_AnyMessage), then Attachment then calls its ExecuteSelf() to do its voodoo. Within ExecuteSelf() the Attachment of course does whatever it does (respond to the click, handle the event, modify the host's cosmetics, etc.), but also must specify if upon returning the host should continue to execute its normal action or not by calling SetExecuteHost(). If the Attachment wishes to completely override the host's behavior, the Attachment should SetExecuteHost(false). If the Attachment wishes to merely augment the behavior, SetExecuteHost(true). Note this only affects the execution of the host action; if the host has other Attachments that have not yet executed, they will still execute.

That's all there is to how the Attachment mechanism works! It is such a simple mechanism, but so much can be accomplished through it. If you still don't believe me, there is an example application on the CodeWarrior Reference CD (the Attacher demo, part of the PP Mini Demos) and I've written a sampler to accompany this article as well (which should be on the MacTech web/ftp site). The Attacher demo shows how the mechanics of Attachments work, and my demo can show you all of the things you can do with Attachments.

Working With Attachments

Now that you understand how Attachments work, I'm sure you want to start to actually work with Attachments. Just as the mechanism itself is fairly simple, working with Attachments isn't very difficult either. The two primary areas in which you will work with Attachments is how to add/use them within your own projects, and how to create your own Attachments.

Adding Attachments

There are two ways to add an Attachment to an Attachable: on the fly or through a PPob. To add an Attachment on the fly you first create the Attachment via its "parameterized" constructor, then add it to a host by calling the host's AddAttachment() method. Listing 2 illustrates adding an LClipboard Attachment to your Application object (CApplication represents your Application object subclass, and remember that LApplication inherits from LCommander, which inherits from LAttachable).

Listing 2: Adding an Attachment on the fly

CApplication::Initialize()
void
CApplication::Initialize()
{
      // Create the LClipboard Attachment
   LClipboard*   theClipAttachment = new LClipboard;

      // Attach it to our Application object
   AddAttachment(theClipAttachment);
}

Although some Attachments, like LClipboard must be created and added on the fly as above, other Attachments can be created from a PPob DataStream (many Attachments support both creation mechanisms). Just like you can edit the Panes in your PPob's within Constructor for that wonderful WYSIWYG experience, you can add and edit some Attachments in Constructor as well.


Figure 4. Editing Attachments in Constructor.

As mentioned previously, to create your Attachment via the PPob DataStream, the Attachment must have an LStream constructor and a class_ID, just like Panes that you create from the PPob DataStream. As well, you must register your Attachment with URegistrar so the reanimation process will work correctly (again, for more information on PowerPlant's reanimation and registration mechanisms, consult the PowerPlant Book). If you create your Attachments in your PPob's, the Attachment is attached to whatever object it is hierarchically beneath. In Figure 4, the LWindowThemeAttachment will be attached to the LWindow; the LBeepAttachment is attached to the LStaticText. This is where the default attachable comes into play. Take a look at LAttachment's LStream constructor and the routines in UReanimator to see exactly how the default attachable is used.

In addition to editing Attachments in PPob's, you can also edit the CTYP information for the Attachment's themselves. By creating your own CTYP's for your Constructor objects, they'll appear in the Catalog window and can facilitate working in Constructor. Figure 5 shows the Constructor CTYP editor. Of course you can find more information on creating CTYP's and using Constructor in the Constructor Manual, Metrowerks documentation for Constructor.


Figure 5. Editing Attachment CTYP's in Constructor.

Creating your own Attachments

With what I've shown you so far, I hope you've been able to see just how simple it is to work with Attachments. But if there is anything difficult about Attachments it might be writing your own Attachment classes. The process itself isn't difficult - it's the first step of finding the behavior to implement that might throw you off a bit. Before you begin to write the Attachment you must design it well. You must decide what behavior you wish to implement, and then how it will fit into the Attachment mechanism: what message(s) to respond to; if the host should execute or not, and then under what circumstances to execute or not; if the Attachment can be created from a PPob DataStream; etc.

As soon as you have your design and goal(s) for the Attachment set, the implementation process is fairly short and simple. You need to have at least one constructor that allows the Attachment to be created on the fly. If the Attachment can be created from a PPob DataStream you must provide an LStream constructor and class_ID. Then implement your ExecuteSelf() to do the Attachment's magic, and you're done! In the implementation of ExecuteSelf(), be aware to SetExecuteHost() as needed. And if you wish to add that finishing touch, create a CTYP file for your Attachment so it can be easily edited within Constructor.

Take a look at the Attachments that PowerPlant provides, such as those in UAttachments.cp. There isn't much to their implementation, but as you use Attachments more and more you'll find yourself appreciating that simplicity. Conversely, if you look at my HCmdButtonAttachment (which is part of the demo application that accompanies this article), you'll see that Attachments aren't always lightweight, but the power they can bring makes them a great tool to have in your toolchest.

Why Use Attachments?

If you've made it this far, I hope that you've been able to gain an understanding of what Attachments are and just how they work within PowerPlant. You've seen the LAttachable and LAttachment classes, how the Attachments mechanism works, and how to build your own Attachments. But there is one question that remains: why would you use Attachments?

Remember that Attachments modify the runtime state and/or behavior of their host object. By being able to add to, remove from, or otherwise modify how an object behaves, you can do some nifty stuff and have a lot of code to reuse to boot. For example, a common feature in Mac applications today is the ability to simulate a mouse click on a button in a dialog with a keystroke; for instance, cmd-D could simulate a click on the "Don't Save" button in a "Save Changes" dialog. If you wanted to implement this behavior in a button class, you would subclass your main button class and implement the functionality to support the keystroke-as-click behavior. You did this for your PushButton class, but now you want the same functionality in your BevelButton class, so you have to reimplement the same code all over again. Then you want to have it in your CheckBox class, then your RadioButton class, and then you realize you want it in so many of your classes that it would be better to place it into a common parent class, but then this would break PowerPlant's existing inheritance chains so you would have to reimplement every single button widget you used... and you give up because this is getting ridiculous.

Consider the Attachment. You are implementing a behavior: handle a keystroke as if it was a click on a button (or any control for that matter). You create an Attachment that can intercept msg_KeyPress's, and whose ExecuteSelf(), upon receiving the correct keystroke calls SimulateHotSpotClick(), and you've now clicked the button without ever putting your hands on your mouse. Furthermore you've factored this behavior into stand-alone and reusable code. You can easily add this Attachment to your PushButtons, your BevelButtons, your RadioButtons, CheckBoxes, and any other Control you might desire. All the same code, no need to repeat nor reimplement the code, and no need to subclass anything - you can use stock objects. This is essentially what all Attachments do, and slightly more specific to this example, what the HCmdButtonAttachment Attachment does.

Attachments are also good for implementing new behaviors for legacy code. PowerPlant's Debugging Classes are centered around a menu that is added to your application's menubar and provides functionality to help debug and stress-test your application (see the Factory Floor column in the November, 1998 issue of MacTech for an overview of the Debugging Classes). Because the menu is implemented as an Attachment (LDebugMenuAttachment) it makes it quite simple to add and remove this functionality from an existing application. Furthermore, it enables you to have the Debug menu only in the debug builds of your application and not in your release builds. The modularity and "drop-in" capabilities of Attachments is another strength.

Conclusion

I hope you've been able to see what Attachments have to offer you as you continue you work with PowerPlant. If you have not used Attachments, do try the two demo applications and read over the various Attachment classes' source code; give them a try in your own applications. Most of the PowerPlant code that I've written recently has been Attachment based (e.g. LCMAttachment, LDebugMenuAttachment, HURLAttachment), and I find it to be one of the niftier and more flexible mechanisms that PowerPlant has to offer. After you're more familiar with Attachments, I'm sure you'll also find a place for them in your toolbox. What you can do with Attachments is pretty limitless, and after you write your first Attachment class I'm sure you'll agree with me.

Until next time... Happy programming!

Bibliography

  • Mark, Dave. Ultimate Mac Programming. Programmers Press, A Division of IDG Books Worldwide, Inc., Foster City, California. 1994.
  • Metrowerks Corporation. Constructor Manual. 1998.
  • Metrowerks Corporation. PowerPlant Book. 1998.
  • Metrowerks Corporation. PowerPlant Reference. 1998.
  • Monaco Tuner and ProFont: ftp://members.aol.com/squeegee/.
  • FinderPop: http://www.finderpop.com/.
  • Kaleidoscope: http://www.kaleidoscope.net/.

John C. Daub is one of Metrowerks Corporation's PowerPlant engineers. John wonders if you really care to know quirky biographical information about him, and if so, why? If you really want to know strange things about John or wish to ask him any questions (about PowerPlant or otherwise), you can reach John via email at hsoi@metrowerks.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

BusyContacts 1.6.4 - Fast, efficient con...
BusyContacts is a contact manager for OS X that makes creating, finding, and managing contacts faster and more efficient. It brings to contact management the same power, flexibility, and sharing... Read more
Steam 4.0 - Multiplayer and communicatio...
Steam is a digital distribution, digital rights management, multiplayer and communications platform developed by Valve Corporation. It is used to distribute a large number of games and related media... Read more
OmniGraffle Pro 7.19.3 - Create diagrams...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
OmniGraffle 7.19.3 - Create diagrams, fl...
OmniGraffle helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use Graffle to... Read more
Hopper Disassembler 5.3.3- - Binary disa...
Hopper Disassembler is a binary disassembler, decompiler, and debugger for 32- and 64-bit executables. It will let you disassemble any binary you want, and provide you all the information about its... Read more
calibre 5.35.0 - Complete e-book library...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital librarian... Read more
Sound Studio 4.10.0 - Robust audio recor...
Sound Studio lets you easily record and professionally edit audio on your Mac. Easily rip vinyls and digitize cassette tapes, or record lectures and voice memos. Prepare for live shows with live... Read more
Sparkle Pro 4.0 - Visual website creator...
Sparkle Pro will change your mind if you thought building websites wasn't for you. Sparkle is the intuitive site builder that lets you create sites for your online portfolio, team or band pages, or... Read more
Dropbox 140.4.1951 - Cloud backup and sy...
Dropbox for Mac is a file hosting service that provides cloud storage, file synchronization, personal cloud, and client software. It is a modern workspace that allows you to get to all of your files... Read more
FotoMagico 6.0.5 - Powerful slideshow cr...
FotoMagico lets you create professional slideshows from your photos and music with just a few, simple mouse clicks. It sports a very clean and intuitive yet powerful user interface. High image... Read more

Latest Forum Discussions

See All

‘Horizon Chase’ China Spirit DLC Release...
Following the release of the excellent reveal of the Horizon Chase Senna Forever expansion, the game will be getting a new DLC on mobile platforms today. Today, the Horizon Chase China Spirit DLC pack will release on iOS and Android bringing in 9... | Read more »
‘PUZZLED’ from SNK and Hamster Is Out No...
Following ZED BLADE ACA NeoGeo earlier this month, SNK has brought over another game in the ACA NeoGeo series to both iOS and Android in the form of PUZZLED. SNK and Hamster originally brought the series to mobile with Samurai Shodown IV, Alpha... | Read more »
A House Full of Covid – The TouchArcade...
It’s been a rough week as both of our young children tested positive for Covid, and since recording this early on Friday my wife has tested positive now too. Thankfully the kids seemed to recover fairly quickly and are mostly back to normal, and I... | Read more »
TouchArcade Game of the Week: ‘Krispee S...
Krispee Street is a new hidden object game from Frosty Pop that is based on their popular and almost painfully sweet webcomic Krispee. This is one of the latest titles to be added to the Netflix Games catalog, which means you’ll need to log into... | Read more »
SwitchArcade Round-Up: ‘Escape Lala’, ‘B...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for January 21st, 2022. In today’s article, we’ve got a lot of new releases. A lot. There were eight on the schedule when I went to bed last night. There were twenty-four when I woke up... | Read more »
Beta Testers Needed for Huge Version 2.0...
Ya’ll remember Dungeon Raid, right? The phenomenal matching RPG hybrid that launched on mobile more than a decade ago, but was more or less abandoned by its developer only to die a slow death on the App Store before the 32-bit Appocalypse finally... | Read more »
‘Ark Legends’ Gives Players a Chance to...
It’s Airpods and Amazon gift cards galore as Melting Games opens pre-registration for Ark Legends. The upcoming mobile RPG is giving away tons of in-game goodies such as gold, energy, iron core, hero summon chest and rare iron core to players who... | Read more »
‘Nickelodeon Extreme Tennis’ Out Now on...
Nickelodeon Extreme Tennis () from Old Skull Games and Nickelodeon is this week’s new Apple Arcade release. Nickelodeon Extreme Tennis features characters from old and new Nickelodeon shows including SpongeBob, TMNT, and many more. The tennis game... | Read more »
SwitchArcade Round-Up: ‘RPGolf Legends’,...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for January 20th, 2022. In today’s article, we’ve got a massive amount of new releases to check out. We’ve got summaries of all of them, from heaven to hell. We also have the lists of... | Read more »
‘Zed Blade ACA NEOGEO’ Review – Well, It...
SNK’s NEOGEO platform played host to a great many classics, both famous and under-the-radar. The Metal Slug games. The King of Fighters series. Magician Lord. Shock Troopers. Sengoku 3. NEO Turf Masters. Fatal Fury. Samurai Shodown. Twinkle Star... | Read more »

Price Scanner via MacPrices.net

Verizon’s 2022 iPad promo: $100-$310 off any...
Verizon has cellular-capable iPads on sale for $100-$310 off MSRP when purchased with an Unlimited service plan. Sale price is applied to your account monthly over a 24 or 30 month period, depending... Read more
Sunday Sale: Apple AirPods are on sale for up...
Amazon has Apple AirPods on sale for $10-$100 off MSRP today, depending on the model. All are in stock today with free delivery: – AirPods Max headphones (Blue): $449 $100 off MSRP – AirPods Max... Read more
These Apple resellers are offering 13″ M1 Mac...
Apple resellers are offering discounts on 13″ MacBook Pros with M1 Apple Silicon processors ranging up to $150 off MSRP. Here’s where to get one today: (1): Apple’s 13″ MacBook Pros with M1 Apple... Read more
Amazon lowers prices on select 13″ M1 MacBook...
Amazon has select Apple 13″ M1 MacBook Airs on sale for $150 off MSRP this weekend, starting at only $849. Their prices are the lowest available for new MacBook Airs today. Stock may come and go, so... Read more
Apple has 13″ M1 MacBook Airs back in stock s...
Apple has restocked a full line of 13″ M1 MacBook Airs, Certified Refurbished, starting at only $849 and up to $190 off original MSRP. These are the cheapest M1-powered MacBooks for sale today at... Read more
In stock and on sale! 16″ 10-Core M1 Pro MacB...
Amazon has new 16″ 10-Core/512GB M1 Pro MacBook Pros in stock today and on sale for $50 off MSRP including free shipping. Their prices are the lowest available for new M1 Pro 16″ MacBook Pro from any... Read more
Deal Alert!: 14″ M1 Pro with 10-Core CPU in s...
Amazon has the new 14″ M1 Pro MacBook Pro with a 10-Core CPU and 16-Core GPU in stock today and on sale for $2299.99 including free shipping. Their price is $200 off Apple’s standard MSRP, and it’s... Read more
Apple has 24-inch M1 iMacs (8-Core CPU/8-Core...
Apple has restocked a wide array of 24-inch M1 iMacs with 8-Core CPUs and 8-Core GPUs in their Certified Refurbished store. Models are available starting at only $1269 and range up to $260 off... Read more
Select 24″ M1 iMacs are on sale for $100 off...
Sales of Apple’s new 24″ M1 iMacs have been rare since its introduction, perhaps due to global supply issues. However, B&H is offering a $100 discount on select 24″ iMacs, and they’re in stock... Read more
M1 Mac minis are back in stock today at Apple...
Apple has M1-powered Mac minis available in their Certified Refurbished section starting at only $589 and up to $140 off MSRP. Each mini comes with Apple’s one-year warranty, and shipping is free: –... Read more

Jobs Board

Registered Nurse (RN) Employee Health PSJH -...
…is calling for a Registered Nurse (RN) Employee Health PSJH to our location in Apple Valley, CA.** We are seeking a Registered Nurse (RN) Employee Health PSJH to be Read more
Systems Administrator - Pearson (United State...
…and troubleshoot Windows operating systems (workstation and server), laptop computers, Apple iPads, Chromebooks and printers** + **Administer and troubleshoot all Read more
IT Assistant Level 1- IT Desktop Support Anal...
…providing tier-1 or better IT help desk support in a large Windows and Apple environment * Experience using IT Service Desk Management Software * Knowledge of IT Read more
Human Resources Business Partner PSJH - Provi...
…**is calling a** **Human Resources Business Partner, PSJH** **to our location in Apple Valley, CA.** **Applicants that meet qualifications will receive a text with Read more
Manager Community Health Investment Programs...
…is calling a Manager Community Health Investment Programs PSJH to our location in Apple Valley, CA.** **Qualified candidates will be invited to do a self-paced video Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.