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.

 
AAPL
$100.95
Apple Inc.
-0.01
MSFT
$47.02
Microsoft Corpora
-0.50
GOOG
$587.45
Google Inc.
-8.63

MacTech Search:
Community Search:

Software Updates via MacUpdate

Monosnap 2.2.2 - Versatile screenshot ut...
Monosnap allows you to save screenshots easily, conveniently, and quickly, sharing them with friends and colleagues at once. It's the ideal choice for anyone who is looking for a smart and fast... Read more
Tunnelblick 3.4beta36 - GUI for OpenVPN...
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
SoftRAID 5.0.4 - High-quality RAID manag...
SoftRAID allows you to create and manage disk arrays to increase performance and reliability. SoftRAID's intuitive interface and powerful feature set makes this utility a must have for any Mac OS X... Read more
Audio Hijack Pro 2.11.3 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Airfoil 4.8.9 - Send audio from any app...
Airfoil allows you to send any audio to AirPort Express units, Apple TVs, and even other Macs and PCs, all in sync! It's your audio - everywhere. With Airfoil you can take audio from any... Read more
WhatRoute 1.13.0 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the... Read more
Chromium 37.0.2062.122 - Fast and stable...
Chromium is an open-source browser project that aims to build a safer, faster, and more stable way for all Internet users to experience the web. FreeSMUG-Free OpenSource Mac User Group build is... Read more
Attachment Tamer 3.1.14b9 - Take control...
Attachment Tamer gives you control over attachment handling in Apple Mail. It fixes the most annoying Apple Mail flaws, ensures compatibility with other email software, and allows you to set up how... Read more
Duplicate Annihilator 5.0 - Find and del...
Duplicate Annihilator takes on the time-consuming task of comparing the images in your iPhoto library using effective algorithms to make sure that no duplicate escapes. Duplicate Annihilator detects... Read more
jAlbum Pro 12.2 - Organize your digital...
jAlbum Pro has all the features you love in jAlbum, but comes with a commercial license. With jAlbum, you can create gorgeous custom photo galleries for the Web without writing a line of code!... Read more

Latest Forum Discussions

See All

Kitty Powers’ Matchmaker – Tips, Tricks,...
Hey There, Kittens: | Read more »
Major New Update for CSR Racing Adds Fer...
Major New Update for CSR Racing Adds Ferrari and Multiplaye​r Posted by Jessica Fisher on September 22nd, 2014 [ permalink ] | Read more »
Fruit Ninja Will be Reborn With a Massiv...
Fruit Ninja Will be Reborn With a Massive Update and Origins Animation Series Posted by Jessica Fisher on September 22nd, 2014 [ permalink ] Halfbrick Studios is rebuilding | Read more »
CloudMagic Updated for iOS 8 – Adds Inte...
CloudMagic Updated for iOS 8 – Adds Interactive Notifications, Share Extension, and More Posted by Jessica Fisher on September 22nd, 2014 [ | Read more »
Starbase Annex (Games)
Starbase Annex 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: "it’s really very clever... a little bit of Hearthstone and a dash of Eclipse" - PocketTactics.com From the creator of Starbase... | Read more »
Star Nomad Review
Star Nomad Review By Jordan Minor on September 22nd, 2014 Our Rating: :: BUMPY RIDEiPad Only App - Designed for the iPad Star Nomad will only appeal to the most hardcore space explorers.   | Read more »
Partyrs Review
Partyrs Review By Jordan Minor on September 22nd, 2014 Our Rating: :: LIFE OF THE PARTYUniversal App - Designed for iPhone and iPad Partyrs makes party planning as fun as partying itself.   | Read more »
View Source – HTML, JavaScript and CSS...
View Source – HTML, JavaScript and CSS 1.0 Device: iOS Universal Category: Utilities Price: $.99, Version: 1.0 (iTunes) Description: View Source is an app plus an iOS 8 Safari extension that makes it easy to do one key web developer... | Read more »
Avenged Sevenfold’s Hail To The King: De...
Avenged Sevenfold’s Hail To The King: Deathbat is Coming to iOS on October 16th Posted by Jessica Fisher on September 19th, 2014 [ permalink ] Just in time for Halloween, on October 16 Avenged Sevenfold will be launching | Read more »
Talisman Has Gone Universal – Can Now be...
Talisman Has Gone Universal – Can Now be Played on the iPhone Posted by Jessica Fisher on September 19th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Apple restocks some refurbished 2014 MacBook...
The Apple Store has restocked some Apple Certified Refurbished 2014 MacBook Airs, with prices starting at $769. An Apple one-year warranty is included with each MacBook, and shipping is free. These... Read more
13-inch 128GB MacBook Air on sale for $949, s...
B&H Photo has the new 2014 13″ 1.4GHz/128GB MacBook Air on sale for $949.99 including free shipping plus NY tax only. Their price is $50 off MSRP. B&H will also include free copies of... Read more
Apple offering free $25 iTunes Gift Card with...
The Apple Store is offering a free $25 iTunes Gift Card with the purchase of a $99 Apple TV for a limited time. Shipping is free. Read more
Apple refurbished iPod touch available for up...
The Apple Store has Apple Certified Refurbished 5th generation iPod touches available starting at $149. Apple’s one-year warranty is included with each model, and shipping is free. Most colors are... Read more
iFixIt Tears Down iPhone 6; Awards Respectabl...
iFixit notes that even the smaller 4.7″ iPhone 6 is a giant among iPhones; so big that Apple couldn’t fit it into the familiar iPhone form factor. In a welcome reversal of a recent trend to more or... Read more
Phone 6 Guide – Tips Book For Both iPhone 6...
iOS Guides has announced its latest eBook: iPhone 6 Guide. Brought to you by the expert team at iOS Guides, and written by best-selling technology author Tom Rudderham, iPhone 6 Guide is packed with... Read more
How to Upgrade iPhone iPad to iOS 8 without D...
PhoneClean, a iPhone cleaner utility offered by iMobie Inc., reveals a solution for upgrading iPhone and iPad to iOS 8 without deleting photos, apps, the new U2 album or anything. Thanks to more than... Read more
Inpaint 6 – Photo Retouching Tool Gets Faster...
TeoreX has announced Inpaint 6, a simple retouching tool for end users that helps remove scratches, watermarks, and timestamps as well as more complex objects like strangers, unwanted elements and... Read more
Worldwide PC Monitor Market Sees Growth in To...
Worldwide PC monitor shipments totaled 32.5 million units in the second quarter of 2014 (2Q14), a year-over-year decline of -2.9%, according to the International Data Corporation (IDC) Worldwide... Read more
Updated Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more

Jobs Board

Position Opening at *Apple* - Apple (United...
…customers purchase our products, you're the one who helps them get more out of their new Apple technology. Your day in the Apple Store is filled with a range of Read more
*Apple* Retail - Multiple Positions (US) - A...
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
Position Opening at *Apple* - Apple (United...
**Job Summary** At the Apple Store, you connect business professionals and entrepreneurs with the tools they need in order to put Apple solutions to work in their Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** The Apple Store is a retail environment like no other - uniquely focused on delivering amazing customer experiences. As an Expert, you introduce people Read more
Position Opening at *Apple* - Apple (United...
**Job Summary** As businesses discover the power of Apple computers and mobile devices, it's your job - as a Solutions Engineer - to show them how to introduce these Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.