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

TotalFinder 1.12.2 - Adds tabs, hotkeys,...
TotalFinder is a universally acclaimed navigational companion for your Mac. Enhance your Mac's Finder with features so smart and convenient, you won't believe you ever lived without them. Features... Read more
Duet 2.3.0.3 - Use your iPad as an exter...
Duet is the first app that allows you to use your iDevice as an extra display for your Mac using the Lightning or 30-pin cable. Note: This app requires a $9.99 iOS companion app. Version 2.3.0.3:... Read more
FileMaker Pro Advanced 18.0.3 - Powerful...
FileMaker Pro Advanced is the tool you use to create a custom app. You also use FileMaker Pro Advanced to access your app on a computer. Start by importing data from a spreadsheet or using a built-in... Read more
OsiriX Lite 10.0.6 - 3D medical image pr...
OsiriX Lite is an image processing software dedicated to DICOM images (".dcm" / ".DCM" extension) produced by medical equipment (MRI, CT, PET, PET-CT, ...) and confocal microscopy (LSM and BioRAD-PIC... Read more
Ableton Live 10.1.5 - Record music using...
Ableton Live lets you create and record music on your Mac. Use digital instruments, pre-recorded sounds, and sampled loops to arrange, produce, and perform your music like never before. Ableton Live... Read more
Burn 2.7.8 - Easily burn data, audio, vi...
Burn... There are a lot of ways to approach burning discs. Burn keeps it simple, but still offers a lot of advanced options. Create data discs with advanced data settings like, file permissions, the... Read more
Malwarebytes 4.0.30.3073 - Adware remova...
Malwarebytes (was AdwareMedic) helps you get your Mac experience back. Malwarebytes scans for and removes code that degrades system performance or attacks your system. Making your Mac once again your... Read more
Acorn 6.5.3 - Bitmap image editor.
Acorn is a new image editor built with one goal in mind - simplicity. Fast, easy, and fluid, Acorn provides the options you'll need without any overhead. Acorn feels right, and won't drain your bank... Read more
Fantastical 2.5.13 - Create calendar eve...
Fantastical is the Mac calendar you'll actually enjoy using. Creating an event with Fantastical is quick, easy, and fun: Open Fantastical with a single click or keystroke Type in your event... Read more
A Better Finder Rename 11.05 - File, pho...
A Better Finder Rename is the most complete renaming solution available on the market today. That's why, since 1996, tens of thousands of hobbyists, professionals and businesses depend on A Better... Read more

Latest Forum Discussions

See All

Eternal Warfare is a new idle clicker fo...
Idle games are a popular genre on mobile, they might not be to everyone's taste but they're made with such regularity and receive a lot of downloads, so it's hard to argue it's not big business. Eternal Warfare is set to join the sea of idle games... | Read more »
New heroes and balance updates set to ar...
It feels like Hearthstone: Battlegrounds only launched yesterday, and already the auto batter addition to Blizzard's megahit card game is set to receive new heroes and balance updates. [Read more] | Read more »
Pre-register for Hello Kitty AR: Kawaii...
Hello Kitty — the cute cat that launched a multi-billion-pound franchise — has been brought to life… sort of. Sanrio has teamed up with the Bublar Group to create a new mobile game that uses AR tech to turn the real world into Hello Kitty’s... | Read more »
Gorgeous and tranquil puzzler Spring Fal...
One-man indie studio SPARSE//GameDev has now launched its tranquil puzzler, Spring Falls. It's described as "a peaceful puzzle game about water, erosion, and watching things grow". [Read more] | Read more »
Black Desert Mobile gets an official rel...
Pearl Abyss has just announced that its highly-anticipated MMO, Black Desert Mobile, will launch globally for iOS and Android on December 11th. [Read more] | Read more »
Another Eden receives new a episode, cha...
Another Eden, WFS' popular RPG, has received another update that brings new story content to the game alongside a few new heroes to discover. [Read more] | Read more »
Overdox guide - Tips and tricks for begi...
Overdox is a clever battle royale that changes things up by adding MOBA mechanics and melee combat to the mix. This new hybrid game can be quite a bit to take in at first, so we’ve put together a list of tips to help you get a leg up on the... | Read more »
Roterra Extreme - Great Escape is a pers...
Roterra Extreme – Great Escape has been described by developers Dig-It Games as a mini-sequel to their acclaimed title Roterra: Flip the Fairytale. It continues that game's tradition of messing with which way is up, tasking you with solving... | Read more »
Hearthstone: Battlegrounds open beta lau...
Remember earlier this year when auto battlers were the latest hotness? We had Auto Chess, DOTA Underlords, Chess Rush, and more all gunning for our attention. They all had their own reasons to play, but, at least from where I'm standing, most... | Read more »
The House of Da Vinci 2 gets a new gamep...
The House of Da Vinci launched all the way back in 2017. Now, developer Blue Brain Games is gearing up to deliver a second dose of The Room-inspired puzzling. Some fresh details have now emerged, alongside the game's first official trailer. [Read... | Read more »

Price Scanner via MacPrices.net

B&H offers $100 discounts on 4-Core and 6...
B&H Photo has new 4-Core and 6-Core Mac minis in stock and on sale today for $100 off Apple’s MSRP. Prices start at $699. Overnight shipping is free to many addresses in the US: – 3.6GHz Quad-... Read more
Save $200 today on a 2019 13″ MacBook Air wit...
Apple has a full line of Certified Refurbished 2019 13″ MacBook Airs available starting at only $929 and up to $200 off the cost of new Airs. Each MacBook features a new outer case, comes with a... Read more
New Verizon Pre-Black Friday 2019 deal: Buy o...
Buy one new Apple iPhone 11 model or 2018 iPhone XS model at Verizon and get a second one for free. One new line of service required. Offer is valid from November 21, 2019 to November 27, 2019. Here... Read more
AirPods with Wireless Charging Case on sale t...
Abt Electronics has 2019 AirPods with the Wireless Charging Case on sale today for $163 shipped. Their price is $36 off Apple’s MSRP, and it’s currently the cheapest price for these AirPods from any... Read more
Apple continues to offer 2017 13″ Dual-Core n...
Apple has Certified Refurbished 2017 13″ 2.3GHz Dual-Core non-Touch Bar MacBook Pros still available starting at $1019. An standard Apple one-year warranty is included with each model, outer cases... Read more
Save up to $120 on the new 16″ MacBook Pro at...
Apple’s resellers are starting to receive stock of new 16″ MacBook Pros, and the first set of sales & deals are now available: (1) Amazon 16″ MacBook Pros start on sale for $100-$116 off Apple’s... Read more
Apple Watch Series 3 models on sale at Amazon...
Amazon has Apple Watch Series 3 GPS models on sale for $30 off MSRP, starting at only $169. There prices are the lowest we’ve ever seen for these models from any Apple reseller. Choose Amazon as the... Read more
The ‘Mac Potpourri’ Mailbag: Edition #1- Info...
COMMENTARY: 11.20.19- Welcome to the inaugural edition of the “Mac Potpourri” Mailbag where we take a look at correspondence received from readers of this column from all over the world who write in... Read more
13″ 2.4GHz MacBook Pros available for up to $...
Apple has a full line of Certified Refurbished 2019 13″ 2.4GHz 4-Core Touch Bar MacBook Pros available starting at $1529 and up to $300 off MSRP. Apple’s one-year warranty is included, shipping is... Read more
New at T-Mobile: Switch to T-Mobile, and get...
T-Mobile is offering a free 64GB iPhone 8 for new customers who switch to T-Mobile and open a new line of service. Eligible trade-in required, and discount applied over a 24 month period. The fine... Read more

Jobs Board

Best Buy *Apple* Computing Master - Best Bu...
**747303BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Sales **Store NUmber or Department:** 001413-Cypress-Store **Job Description:** **What Read more
*Apple* Mobility Pro - Best Buy (United Stat...
**743221BR** **Job Title:** Apple Mobility Pro **Job Category:** Store Associates **Store NUmber or Department:** 000230-Greenwood-Store **Job Description:** At Best Read more
*Apple* Mobility Pro - Best Buy (United Stat...
**747338BR** **Job Title:** Apple Mobility Pro **Job Category:** Store Associates **Store NUmber or Department:** 000254-Superstition Springs-Store **Job Read more
Best Buy *Apple* Computing Master - Best Bu...
**745516BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Store Associates **Store NUmber or Department:** 001101-Manhattan-Store **Job Read more
Best Buy *Apple* Computing Master - Best Bu...
**746655BR** **Job Title:** Best Buy Apple Computing Master **Job Category:** Sales **Store NUmber or Department:** 002518-Atlantic Center-Store **Job Description:** Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.