TweetFollow Us on Twitter

Structures, Event Loop
Volume Number:2
Issue Number:6
Column Tag:The ABC's of C

Structures And The Event Loop

By Bob Gordon, Apropos Publications, Minneapolis, MN

Beginning this month, we will start following the chapters in the Using the Macintosh Toolbox with C book. This means we're will be learning about C and the Macintosh at the same time. While this is probably not the optimum way to learn a language, it means we can explore features of the language and the Mac and have a lot of time to play around and make mistakes. As you will see from this month's effort, I am not afraid to make lots of mistakes.

We are going to start right away with a program that demonstrates a bit about the Event Manager. Events are a critical component to the way Macintosh software functions. They are not visible like windows or pull-down menus, but they occupy a central location in most applications nonetheless.

From the point of view of the Event Manager, Macintosh program spend most of their time sitting around waiting for events. Events can come from the keyboard, the mouse, disk drives, the Window Manager, et cetera, as well as your application.

Structures

For the moment, we won't worry too much about how the Event Manager gives you an event. Instead, we'll use the event to examine an important C data construct called a structure. A structure is a collection of variables usually of different types organized under a single name (loosely paraphrased from K&R). An event from a software point of view is a structure:

 struct ER
 {short what;  /* kind of event */
  long  message; /* event info */
  long  when;  /* time of event */
  Point where; /* mouse location */
  short modifiers; /* other info */
 };

The variables collected together in the structure are members.

A structure declaration (as we have above) does not reserve any memory. We still need to define a variable:

 struct ER    event;

defines the variable, event, to be of type ER. Now, when the Event Manager gives us an event, we can examine its members to determine what to do. To access the members of a structure, use a period between the variable name and the member name:

 event.what

At this point, let us look more closely at the where member. It is obviously not one of the standard C types. It is in fact another structure:

 struct pt
 {short  v; /* vertical location */
  short  h; /* horizontal location */
 };

So structures may be nested in other structures (you can also have arrays of structures).

You noticed that I defined the structure "pt" not "Point." When we defined the event variable, above, we wrote struct ER event. C requires that you let it know each time you are dealing with a structure. Writing struct all over your programs can be a drag as well as making them less readable. C has a couple of ways to deal with this problem. One is the typedef, which is way of providing a new name for a type. Point then becomes:

 typdef struct pt   Point;

The other technique is to use the preprocessor define feature:

 #define EventRecord   struct ER

Both of these techniques will increase the clarity of our code. The typedef is preferred because it is actually a part of the compiler and can deal with certain situations that can confuse the lexical substitutions of the preprocessor. Some micro C compilers do not include typedefs, however.

Now that we have defined the EventRecord, we can get an event from the Event Manager and see what to do with it.

Switch Statement

The first thing to do is decide what kind of event it is (from the what member), and then take appropriate action. The clearest way to do this in C is with the switch or case statement (C calls it switch, but many C programmers call it a case statement). The switch is a multi-way branch:

 switch (integer expression)
 {
 case constant1 : code;
                                      break;
   case constant2 : more code;
                                      break;
 default                   : code;
                                      break;
 }

The break statements are needed to keep the code from falling through the cases. Without the break, if the expression matched constant2, it would continue execution with the default code. The break forces execution to continue after the closing }. The default is chosen if none of the other cases match. By the way, you can have several constants that match one section of code. Each is preceded by case and followed by a colon. In effect, they fall through to the first code to execute.

A Program that Demonstrates the Event Manager

/* Event Manager Demonstrator */
#include"stdio.h"
#include"MacCDefs.h"
#include"Events.h"

main()  /* This may look silly, but in      */
 { /* subsequent programs we will have */
 /* initialization routines up here. */
 mainloop();
 }
 
mainloop()
 {
 EventRecordevent; 
 while (True)
 {
 if (GetNextEvent(everyEvent,&event))
 switch (event.what)
 {
 case mouseDown:  printf("\nmouse down");
 break;
 case mouseUp:  printf("\nmouse up");
 break;
 case keyDown:   printf("\nkey down");
 break;
 case keyUp: printf("\nkey up");
 break;
 case autoKey:   printf("\nautokey");
 return;
  break;
 case updateEvt: break;
 case diskEvt:   printf("\ndisk event");
  break;
 case activateEvt: break;
 case networkEvt:  break;
 case driverEvt  : break;
 case nullEvent: break;
 }
 }
 }

GetNextEvent() is the Event Manager routine that returns the next event record each time its called. If there are no events to return, it returns the Null Event.

All the constants (nullEvent, mouseUp, et cetera) and the EventRecord structure are defined in Events.h. I recommend you take a look at the copy that comes with your compiler.

The program exits on an autokey event. To stop the program, simply hold a key down.

The first parameter to GetNextEvent() is the event mask. With it, you can select the events to which you wish to respond. It's a bit mapped mask; you can combine events by adding the constants defined in events.h together. EveryEvent is all 1 bits (-1 decimal).

You will probably not see the key up event. It is generated when you release a key. It took me awhile to find out why I wasn't seeing it, but I finally took a look at Inside Macintosh.

There is a second event mask that controls which events get entered into the event queue. Since GetNextEvent() gets events from the queue, if an event is masked out, GetNextEvent() will never be able to return it. This event mask is initialized to:

 everyEvent - keyUpMask

so it will not even post key up events. There is a function, SetEventMask(), that will set this, but it is apparently not included in Mac C.

Some Things to do

This program is very brief, but you can easily add a few lines to get a better feel of how the Event Manager works and how to use some C functions. Try examining the where field on each event. Remember this is a Point structure. The message and modifier fields provide essential information for some events, especially key events. Print these out as well.

There are a number of other functions in the Event Manager. They allow reading the mouse, keyboard, and time without waiting for an event. There is also a function to read the event queue that leaves the event in the queue.

We will not do any more with the Event Manager at this time, but we will use it in probably every program we write.

Finally, if anyone is reading along, I would appreciate hearing from you. Let me know if this is useful or if you have an idea for a short program we can do in the column.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Remotix 3.1.4 - Access all your computer...
Remotix is a fast and powerful application to easily access multiple Macs (and PCs) from your own Mac. Features Complete Apple Screen Sharing support - including Mac OS X login, clipboard... Read more
DesktopLyrics 2.6.6 - Displays current i...
DesktopLyrics is an application that displays the lyrics of the song currently playing in "iTunes" right on your desktop. The lyrics for the song have to be set in iTunes; DesktopLyrics does nothing... Read more
VOX 2.5.1 - Music player that supports m...
VOX is a beautiful music player that supports many filetypes. The beauty is in its simplicity, yet behind the minimal exterior lies a powerful music player with a ton of features and support for all... Read more
NetNewsWire 4.0.0 - RSS and Atom news re...
NetNewsWire is the best way to keep up with the sites and authors you read most regularly. Let NetNewsWire pull down the latest articles, and read them in a distraction-free and Mac-like way. Native... Read more
MacUpdate Desktop 6.0.6 - Search and ins...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
ForkLift 2.6.5 - Powerful file manager:...
ForkLift is a powerful file manager and ferociously fast FTP client clothed in a clean and versatile UI that offers the combination of absolute simplicity and raw power expected from a well-executed... Read more
Drive Genius 4.1.0 - Powerful system uti...
Drive Genius 4 gives you faster performance from your Mac while also protecting it. The award-winning and improved DrivePulse feature alerts you to hard drive issues before they become major problems... Read more
OnyX 2.9.7 - Maintenance and optimizatio...
OnyX is a multifunctional utility for OS X. It allows you to verify the startup disk and the structure of its System files, to run miscellaneous tasks of system maintenance, to configure the hidden... Read more
DEVONthink Pro 2.8.5 - Knowledge base, i...
DEVONthink Pro is your essential assistant for today's world, where almost everything is digital. From shopping receipts to important research papers, your life often fills your hard drive in the... Read more
Backblaze 4.0.1.878 - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac.With unlimited storage available for $5 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more

Auroch Digital is Bringing Back Games Wo...
| Read more »
Carbo - Handwriting in the Digital Age...
Carbo - Handwriting in the Digital Age 1.0 Device: iOS Universal Category: Productivity Price: $3.99, Version: 1.0 (iTunes) Description: | Read more »
The Apple Watch isn't Great as a Fi...
| Read more »
Show the World What You See With Stre.am...
Live broadcasting is getting popular on mobile devices, which is why you can now get Stre.am, by Infinite Takes. [Read more] | Read more »
PhotoTime's 2.1 Update Adds Apple W...
The latest PhotoTime update is adding even more functionality to the handy photo organizing app. Yep, including Apple Watch support. [Read more] | Read more »
Oh My Glob! Adventure Time Puzzle Quest...
Finn and Jake are taking over D3 Go!'s popular puzzle game series in the upcoming Adventure Time Puzzle Quest. [Read more] | Read more »
Earthcore: Shattered Elements - Tips, Tr...
At first glance, Earthcore: Shattered Elements seems like a rather simple card-battling game. Once you’re introduced to skills that will change quite a bit. Even more so once you start to acquire hero cards. But it’s not so complicated that we... | Read more »
Dungeon999F (Games)
Dungeon999F 1.33 Device: iOS Universal Category: Games Price: $.99, Version: 1.33 (iTunes) Description: "The game you must play at least once in your life!" "The game with potential of million downloads globally!" ...is what the... | Read more »
Mixels Rush - Use Mixes, Maxes and Murps...
Mixels Rush - Use Mixes, Maxes and Murps to Outrun the Nixels 1.0.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.0 (iTunes) Description: Use awesome new Mixels and make crazy combinations to beat the annoying... | Read more »
Battles of the Ancient World II (Games)
Battles of the Ancient World II 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

Sale! 16GB iPad mini 3 for $349, save $50
B&H Photo has the 16GB iPad mini 3 WiFi on sale for $349 including free shipping plus NY sales tax only. Their price is $50 off MSRP, and it’s the lowest price available for this model. Read more
Price drop on 2014 15-inch Retina MacBook Pro...
B&H Photo has dropped prices on 2014 15″ Retina MacBook Pros by $200. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799.99 save $200 - 15″ 2.5GHz... Read more
Will iOS 9 Finally Bring Productivity Friendl...
Ah, the irony. From its original announcement in 2010, Apple has doggedly insisted that the iPad remain “simple,” thus arbitrarily limiting its considerable potential as a content creation and... Read more
13-inch 2.5GHz MacBook Pro (refurbished) avai...
The Apple Store has Apple Certified Refurbished 13″ 2.5GHz MacBook Pros available for $829, or $270 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.... Read more
Mac Pros on sale for up to $260 off MSRP
B&H Photo has Mac Pros on sale for up to $260 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.7GHz 4-core Mac Pro: $2799, $200 off MSRP - 3.5GHz 6-core Mac Pro: $3719.99... Read more
Save up to $600 with Apple refurbished Mac Pr...
The Apple Store has Apple Certified Refurbished Mac Pros available for up to $600 off the cost of new models. An Apple one-year warranty is included with each Mac Pro, and shipping is free. The... Read more
Apple drops price on refurbished 15-inch 2.2G...
The Apple Store has dropped their price on the Apple Certified Refurbished 2014 15″ 2.2GHz Retina MacBook Pro to $1609, $390 off original MSRP. Apple’s one-year warranty is included, and shipping is... Read more
Apple restocks refurbished Mac minis for up t...
The Apple Store has restocked Apple Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz... Read more
Cover For iPhone Helps You Create Beautiful P...
Taking photos, editing and sharing them is one of the things most of people do on their iPhones for fun, especially if they’re users of social media. Unfortunately, many iOS photo-editing solutions... Read more
Password Keyboard 1.0 For iOS Handles Logins...
Pfaeffikon, Switzerland based Power APP has introduced Password Keyboard 1.0.0, their new password utility developed for iPhone, iPad and iPod touch devices. Password Keyboard is designed to simplify... Read more

Jobs Board

Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
Visual Merchandise Manager %u2013 Accessories...
…to be part of an incredible team. Imagine what you could do here. At Apple , great ideas have a way of becoming reality very quickly. Our Accessories assortment makes Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Solutions Consultant - Retail Sales...
**Job Summary** The ASC is an Apple employee who serves as the Apple business manager and influencer in a hyper-business critical Reseller's store which delivers Read more
Technical Project Manager - *Apple* Pay - A...
**Job Summary** Apple Pay is seeking an experienced technical PM…manage the rollout of features to merchants for the Apple Pay platform in the US Within this role Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.