TweetFollow Us on Twitter

MacApp Low Priority
Volume Number:11
Issue Number:2
Column Tag:Improving The Framework

Using Low Priority Events in MacApp

Fixing a minor bug gets your priorities straight

By Harry Haddon, Franklin & Marshall College

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.

Just like most standard Macintosh programs, MacApp has a main event loop, but as with many things, MacApp handles the gory details of the event loop for you while still giving you the flexibility to expand or improve upon it as needed. The focus of MacApp’s event loop is MacApp’s event list which usually contains commands but can also contain more generalized events. Commands and events posted to this list can have different priorities to change the order in which they are processed. The only problem is that MacApp 3.0 and 3.1 never actually process your low priority events.

This article gives a quick overview of the MacApp event list, explains why you might want to use an event with a low priority, and tells you how to fix MacApp-without modifying the MacApp source-so low priority events are properly processed.

Inside the Event List

The MacApp event list is of type TEventList and is a data member, named fEventList, of TApplication. TEventList contains objects of type TEvent and objects descended from TEvent including objects of type TCommand. Since TCommand is a descendent of TEvent, I will use the word “events” in this article to refer to both events and commands.

When you call PostAnEvent() or PostCommand(), the TEventHandler implementations of these two methods pass the event to the next event handler in the event handler chain until TApplication::PostAnEvent() gets the event and inserts it in fEventList sorted by priority. TApplication’s main event loop method retrieves events from fEventList and handles the events by calling their Process() method. The highest priority events, those with their fPriority field set to kPriorityHighest, are retrieved before the lower priority ones. The priorities defined by MacApp are:

// Low priority commands are considered last
const short kPriorityLowest = 127;
const short kPriorityLow = kPriorityLowest - 32;
// Normal priority: command default priority
const short kPriorityNormal = 64;
const short kPriorityHigh = kPriorityNormal - 32;
//High priority commands take precedence
const short kPriorityHighest = 0;

If you wish you can use priority values which are between these constants. The default priority for events is kPriorityNormal.

Events of equal priority in fEventList are not necessarily processed on a First-In, First-Out basis. MacApp uses a binary search when inserting events in fEventList and inserts the event at the first event it finds of equal priority. If you post an event and there are already two or more events of equal priority in the list, their order in the list is indeterminate and hence their order of processing is indeterminate. This is not normally a problem since the typical MacApp application does not have that many equal priority events in the list at one time, but it is something to consider if you’re posting multiple commands to the list at the same time and the order of processing is important.

One command you’ll always find in fEventList is the TEventRetrieverCommand that MacApp uses to fetch toolbox events from the toolbox’s Event Manager. The initialization method IApplication() creates this command with a priority of kPriorityLow and posts it to fEventList. The command stays in the list as long as the application is running, and its sole job is to check for toolbox events. Since TEventRetrieverCommand has a lower priority than normal, MacApp does not process it until after it processes the events in the list that have a normal priority. Thus MacApp won’t fetch any more events from the toolbox queue until after it has processed all of the normal priority events and commands in fEventList.

TEventRetrieverCommand::DoIt() checks for toolbox events by calling gApplication->PollToolboxEvent() which calls the toolbox trap WaitNextEvent(). If a toolbox event is available, it is encapsulated in a TToolboxEvent and processed by MacApp. If no toolbox event is available and TApplication.fAllowApplicationToSleep is true, MacApp figures out the various sleep parameters such as the sleep time and calls WaitNextEvent() to wait for the next toolbox event.

This all works great unless you try to post an event with a priority of kPriorityLow or lower. Then you will find that the TEventRetrieverCommand in fEventList acts as a road block for low priority events. Because it was posted first, it is processed before all events of the same priority (kPriorityLow). If no toolbox events are available from the Macintosh event queue, the TEventRetrieverCommand puts the application to sleep, preventing the processing of any low priority events remaining in fEventList. If a toolbox event is available, MacApp processes it, as it should, leaving no opportunity for the processing of low priority events.

Why Use Low Priority Events?

I ran into the bug with low priority events when I was developing a client application that fetches data from a server application. I used a descendent of TClientCommand, MacApp’s class for sending an Apple event and processing its reply, to fetch the data from the server. The server collects new data at the rate of 10 samples per second and the client needs to be updated at least several times a second so as soon as a reply is received, the client posts another TClientCommand to fetch the next chunk of data.

My TClientCommand needed to be a lower priority than toolbox events so that the view that was changed by the TClientCommand would be updated via an update event before the next TClientCommand was processed. I also wanted the application to process toolbox events before it did the TClientCommand so that the application would be responsive to user actions such as mouse clicks. Experimentation with the TClientCommand’s priority set to kPriorityNormal on a slower Macintosh confirmed that being able to set its priority lower was a worthy goal.

You may have a similar situation where a low priority command would fit the bill. Remember that low priority commands really aren’t background or idle commands: they do not execute until after higher priority events have executed, but once they begin execution they can hog CPU cycles as much as any other event. If they take too much time to execute they can slow down the processing of user actions and create a less than enjoyable experience for your user. Design your commands accordingly.

Fixing the Low Priority Event Bug

I came up with a fairly simple fix that I have used with MacApp 3.0.1. This fix will probably also work with 3.1, since it appears that the relevant sections of code have not changed from 3.0 to 3.1. It is not a perfect fix in that events with the very lowest priority, kPriorityLowest, are still not processed, but this is not really a problem since you can use a priority of kPriorityLowest-1 for your lowest priority, and it will work fine.

The original TEventRetrieverCommand, which is installed by IApplication, is left in fEventList but its priority is changed to kPriorityLowest. This still allows the application to sleep-a Good Thing in the Macintosh world of cooperative multi-tasking-but it does not go to sleep until after all other commands are given a chance to execute. I changed the priority of TEventRetrieverCommand in IMyApplication() after calling IApplication():

TEventRetrieverCommand *originalEventRetriever;
originalEventRetriever = 
              (TEventRetrieverCommand *) fEventList->At(1);
originalEventRetriever->fPriority = kPriorityLowest;

if (qDebug && !originalEventRetriever->IsMemberClass(
             GetClassIDFromName("TEventRetrieverCommand")))
      ProgramBreak("First command in fEventList is not \
a TEventRetrieverCommand!");

This code doesn’t look for the TEventRetrieverCommand on the event list but just assumes that it's the first command on the list. The debug check will warn me if this isn’t true in future versions of MacApp. (Hopefully Apple will fix this in MacApp 3.5 and we won’t need this fix at all anymore.)

To keep processing toolbox events at kPriorityLow, I declared a new command that is a descendant of TEventRetrieverCommand. This command checks for toolbox events but never sleeps. It is posted at kPriorityLow to replace the original TEventRetrieverCommand that was demoted to kPriorityLowest.

class TNoSleepEventRetrieverCommand : 
 public TEventRetrieverCommand {
public:
  TNoSleepEventRetrieverCommand();
    // Empty constructor to satisfy compiler.  

  virtual pascal void INoSleepEventRetrieverCommand(
                      CommandNumber itsCommandNumber); 
    // Initialize the EventCommand procedurally.  
  virtual pascal Boolean IsReadyToExecute();                   
 // override 
    // Return true when event available

 virtual pascal void DoIt();
    // Retrieve and process an event without sleeping 
};

I put the declaration for TNoSleepEventRetrieverCommand in the header file that contains the declaration for TMyApplication.

I put the definitions for its methods in the .cp file that contains the methods of TMyApplication. The initialization method INoSleepEventRetrieverCommand() just calls IEventRetrieverCommand() and then sets the command's priority:

#pragma segment ASelCommand
pascal void TNoSleepEventRetrieverCommand::INoSleepEventRetrieverCommand(
 CommandNumber itsCommandNumber) 
{
  this->IEventRetrieverCommand(itsCommandNumber); 

    // Let more important stuff happen first
  fPriority = kPriorityLow;
}

Its IsReadyToExecute method returns true whenever a toolbox event is available:

#pragma segment ARes
pascal Boolean TNoSleepEventRetrieverCommand::IsReadyToExecute() 
{
 EventRecord theEvent;
 
 return EventAvail(gApplication->fMainEventMask, theEvent); 
}

When IsReadyToExecute() returns true, MacApp calls the command’s DoIt() method. The DoIt() for TNoSleepEventRetrieverCommand is just like DoIt() for TEventRetrieverCommand except it calls PollToolboxEvent() with the parameter allowApplicationToSleep set to false so the application doesn’t go to sleep on us:

#pragma segment ASelCommand
pascal void TNoSleepEventRetrieverCommand::DoIt() 
{
 gApplication->PollToolboxEvent(FALSE);
    // FALSE = never sleep
}

The TNoSleepEventRetrieverCommand is created and posted in TMyApplication after the priority of the original TEventRetrieverCommand is changed:

TNoSleepEventRetrieverCommand *aEventCommand = 
                         new TNoSleepEventRetrieverCommand;
aEventCommand->INoSleepEventRetrieverCommand(cNoCommand); 
this->PostAnEvent(aEventCommand);

That’s it. With these fixes in place you can post a command with a priority of kPriorityLow or lower, and MacApp will process it as it should.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Macs Fan Control 1.5.14 - Monitor and co...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more
VueScan 9.7.96 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
FileMaker Pro 19.6.1 - Quickly build cus...
FileMaker Pro is the tool you use to create a custom app. You also use FileMaker Pro to access your app on a computer. Start by importing data from a spreadsheet or using a built-in Starter app to... Read more
Duet 3.1.0.0 - 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 iOS companion app. Release notes were... Read more
Firefox 107.0.1 - Fast, safe Web browser...
Firefox offers a fast, safe Web browsing experience. Browse quickly, securely, and effortlessly. With its industry-leading features, Firefox is the choice of Web development professionals and casual... Read more
War Thunder 2.21.1.91 - Multiplayer war...
In War Thunder, aircraft, attack helicopters, ground forces and naval ships collaborate in realistic competitive battles. You can choose from over 1,500 vehicles and an extensive variety of combat... Read more
Numbers 12.2.1 - Apple's spreadshee...
With Apple Numbers, sophisticated spreadsheets are just the start. The whole sheet is your canvas. Just add dramatic interactive charts, tables, and images that paint a revealing picture of your data... Read more
DEVONthink Pro 3.8.7 - Knowledge base, i...
DEVONthink is DEVONtechnologies' document and information management solution. It supports a large variety of file formats and stores them in a database enhanced by artificial intelligence (AI). Many... Read more
Drive Genius 6.2.3 - $79.00
Drive Genius features a comprehensive Malware Scan. Automate your malware protection. Protect your investment from any threat. The Malware Scan is part of the automated DrivePulse utility. DrivePulse... Read more
VLC Media Player 3.0.18 - Popular multim...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more

Latest Forum Discussions

See All

‘Genshin Impact’ Version 3.3 Pre-Install...
Following the reveal of the release date and more for Genshin Impact (Free) version 3.3 ‘All Senses Clear, All Existence Void’, HoYoverse showcased the Genius Invokation TCG that arrives this week in the update. | Read more »
TouchArcade Game of the Week: ‘Sling Min...
The world of PC games has always blown my mind because there’s just SO MUCH stuff out there that it’s not uncommon at all for there to be a game that’s well-liked and well-reviewed, and seemingly quite popular with a solid fanbase, and have it be... | Read more »
SwitchArcade Round-Up: Reviews Featuring...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for December 2nd, 2022. So, today turned out a little quieter than the usual Friday. It was so quiet, in fact, that I decided to pen a few reviews. The Knight Witch, Railbound, and Donut... | Read more »
Blue Archive reveals its latest event st...
Nexon has announced the new update for Blue Archive, under the name of An Unconcealed Heart. Featuring a battle between two academies, the story will follow a group struggling to gain recognition, and will bring three new students to recruit. [... | Read more »
Dead Cells+ Is Out Now on Apple Arcade a...
Following the major update for Dead Cells on iOS and Android a few days ago, Playdigious has brought Dead Cells+ () to Apple Arcade. As an App Store Great, Dead Cells+ includes all prior paid DLC and content updates. It also has exclusive mobile... | Read more »
SwitchArcade Round-Up: ‘Romancing SaGa’,...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for December 1st, 2022. Wow, December. We’re already at the last month of the year? Phew. I have a lot of work to finish in the next few weeks. As for today, we’ve got a little news, a... | Read more »
‘Railbound’ Update Now Available Adding...
One of our favorite puzzlers released this year is Railbound from Afterburn Games, which hit in early September and earned our Game of the Week recognition for being an absolutely ace logic puzzler. The goal is to place rail pieces down in order to... | Read more »
The Seven Deadly Sins: Grand Cross celeb...
Netmarble Corporation has pulled out all the stops to celebrate the 3 and a half year anniversary of The Seven Deadly Sins: Grand Cross. The Grand Cross 3.5th Year Anniversary the Ultimate One, a rather wordy title, brings with it a brand new... | Read more »
‘Skullgirls Mobile’ Major Update 5.2 Out...
Developer Hidden Variable pushed out a major update for Skullgirls Mobile (Free) a few hours ago. The version 5.2 update brings in Black Dahlia (before the console and PC game), Retakes, XP Treats, free gifts, and more. Since launch, Skullgirls... | Read more »
Out Now: ‘Disgaea 4’, ‘Romancing SaGa: M...
Each and every day new mobile games are hitting the App Store, and so each week we put together a big old list of all the best new releases of the past seven days. Back in the day the App Store would showcase the same games for a week, and then... | Read more »

Price Scanner via MacPrices.net

Holiday Sale: Apple AirPods Pro for only $199...
Amazon has new 2022 AirPods Pro in stock and on sale for $199.99 shipped as part of their Holiday sale. Their price is $50 off Apple’s MSRP, equaling their Black Friday price, and it’s the lowest... Read more
New Holiday Sale: Apple retailers are offerin...
Several Apple retailers lowered prices on 10.9″ iPad Airs overnight to lows of $100 off MSRP starting at $499. Their prices are the lowest available for iPad Airs anywhere this Holiday season right... Read more
New Holiday sale at Amazon: Take $50 off Appl...
Amazon has Apple’s new 10th-generation iPads in stock and on sale, for the first time, for $50 off MSRP starting at only $399. Their discount applies to all models and all colors. With the discount,... Read more
Holiday Sale: Get an 8.3″ Apple iPad mini for...
Sams Club has 10.9″ 64GB iPad minis on Holiday sale for $80-$100 off MSRP through December 7, 2022. With their discount, prices start at $399 — the cheapest price for a new iPad mini from any of the... Read more
Sams Club Holiday December Event sale: Apple...
Apple AirPods Max headphones are on sale at Sams Club for $110 off MSRP ($439) as part of their December Event sale, ending on December 7, 2022, valid for all colors. Sale price for online orders... Read more
Apple’s 10.2″ 64GB 9th-generation iPads are o...
Sams Club has 9th-generation 64GB iPads on Holiday sale for $60 off MSRP through December 7, 2022. With their discount, prices start at $259 — the cheapest price for a new iPad from any of the Apple... Read more
11″ 128GB WiFi M2 iPad Pro on sale for $749,...
B&H Photo has the new 11″ 128GB WiFi M2-powered iPad Pro (in Space Gray or Silver) on Holiday sale for $749 including free 1-2 day shipping to most US addresses. Their price is $50 off MSRP and... Read more
Find the best Holiday sale price on an iPad u...
We’ve updated our iPad Price Trackers with the latest information on the new 10th-generation iPads, M2-powered iPad Pros, M1 iPad Airs, iPad minis, and 9th generation iPads from Apple’s authorized... Read more
Apple retailers are offering $100-$150 Holida...
Apple retailers have posted their most-recent Holiday sale prices on 13″ MacBook Airs. Take up to $150 off MSRP on M2-powered Airs with these sales with prices starting at only $1099. Free shipping... Read more
Holiday Sale: Apple’s 14″ MacBook Pros with M...
B&H Photo is offering $200-$300 discounts on Apple’s 14″ MacBook Pros with M1 Pro CPUs as part of their Holiday 2022 sale, with prices starting at $1799. Free 1-2 day shipping is available to... Read more

Jobs Board

Support Technician II - *Apple* Support - O...
…problems and acting as a liaison between customers and resolving groups. As an Apple Technical Specialist, you will be supporting many of our popular Apple Read more
*Apple* Electronic Repair Technician - PlanI...
…a highly motivated individual to join our Production Department as an Apple Electronic Repair Technician. The computer repair technician will diagnose, assemble, Read more
Lead Developer - *Apple* tvOS - Rumble (Uni...
…earnings, and positive sentiment About the role: We are looking for a Lead Apple tvOS Developer to join our application engineering team to expand our video centric Read more
Tier 1 Endpoint Engineer - *Apple* - Red Ri...
…Desk on site, at our Client's location, with a focus on support to Apple products. This position will handle technical support requests directly from customers and Read more
Product Manager II - *Apple* - DISH (United...
…you will be doing We seek an ambitious, data-driven thinker to assist the Apple Product Development team as our new Retail Wireless division continues to grow and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.