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

Capture One 11.0.1.40 - RAW workflow sof...
Capture One is a professional RAW converter offering you ultimate image quality with accurate colors and incredible detail from more than 400 high-end cameras -- straight out of the box. It offers... Read more
GraphicConverter 10.5.4 - $39.95
GraphicConverter is an all-purpose image-editing program that can import 200 different graphic-based formats, edit the image, and export it to any of 80 available file formats. The high-end editing... Read more
Dash 4.1.3 - Instant search and offline...
Dash is an API documentation browser and code snippet manager. Dash helps you store snippets of code, as well as instantly search and browse documentation for almost any API you might use (for a full... Read more
Microsoft OneNote 16.9 - Free digital no...
OneNote is your very own digital notebook. With OneNote, you can capture that flash of genius, that moment of inspiration, or that list of errands that's too important to forget. Whether you're at... Read more
DEVONthink Pro 2.9.17 - Knowledge base,...
Save 10% with our exclusive coupon code: MACUPDATE10 DEVONthink Pro is your essential assistant for today's world, where almost everything is digital. From shopping receipts to important research... Read more
OmniGraffle 7.6 - Create diagrams, flow...
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
iFinance 4.3.7 - Comprehensively manage...
iFinance allows you to keep track of your income and spending -- from your lunchbreak coffee to your new car -- in the most convenient and fastest way. Clearly arranged transaction lists of all your... Read more
Opera 50.0.2762.58 - High-performance We...
Opera is a fast and secure browser trusted by millions of users. With the intuitive interface, Speed Dial and visual bookmarks for organizing favorite sites, news feature with fresh, relevant content... Read more
Microsoft Office 2016 16.9 - Popular pro...
Microsoft Office 2016 - Unmistakably Office, designed for Mac. The new versions of Word, Excel, PowerPoint, Outlook and OneNote provide the best of both worlds for Mac users - the familiar Office... Read more
SoftRAID 5.6.4 - High-quality RAID manag...
SoftRAID allows you to create and manage disk arrays to increase performance and reliability. SoftRAID allows the user to create and manage RAID 4 and 5 volumes, RAID 1+0, and RAID 1 (Mirror) and... Read more

Latest Forum Discussions

See All

Around the Empire: What have you missed...
Around this time every week we're going to have a look at the comings and goings on the other sites in Steel Media's pocket-gaming empire. We'll round up the very best content you might have missed, so you're always going to be up to date with the... | Read more »
The 7 best games that came out for iPhon...
Well, it's that time of the week. You know what I mean. You know exactly what I mean. It's the time of the week when we take a look at the best games that have landed on the App Store over the past seven days. And there are some real doozies here... | Read more »
Popular MMO Strategy game Lords Mobile i...
Delve into the crowded halls of the Play Store and you’ll find mobile fantasy strategy MMOs-a-plenty. One that’s kicking off the new year in style however is IGG’s Lords Mobile, which has beaten out the fierce competition to receive Google Play’s... | Read more »
Blocky Racing is a funky and fresh new k...
Blocky Racing has zoomed onto the App Store and Google Play this week, bringing with it plenty of classic kart racing shenanigans that will take you straight back to your childhood. If you’ve found yourself hooked on games like Mario Kart or Crash... | Read more »
Cytus II (Games)
Cytus II 1.0.1 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0.1 (iTunes) Description: "Cytus II" is a music rhythm game created by Rayark Games. It's our fourth rhythm game title, following the footsteps of three... | Read more »
JYDGE (Games)
JYDGE 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: Build your JYDGE. Enter Edenbyrg. Get out alive. JYDGE is a lawful but awful roguehate top-down shooter where you get to build your... | Read more »
Tako Bubble guide - Tips and Tricks to S...
Tako Bubble is a pretty simple and fun puzzler, but the game can get downright devious with its puzzle design. If you insist on not paying for the game and want to manage your lives appropriately, check out these tips so you can avoid getting... | Read more »
Everything about Hero Academy 2 - The co...
It's fair to say we've spent a good deal of time on Hero Academy 2. So much so, that we think we're probably in a really good place to give you some advice about how to get the most out of the game. And in this guide, that's exactly what you're... | Read more »
Everything about Hero Academy 2: Part 3...
In the third part of our Hero Academy 2 guide we're going to take a look at the different modes you can play in the game. We'll explain what you need to do in each of them, and tell you why it's important that you do. [Read more] | Read more »
Everything about Hero Academy 2: Part 2...
In this second part of our guide to Hero Academy 2, we're going to have a look at the different card types that you're going to be using in the game. We'll split them up into different sections too, to make sure you're getting the most information... | Read more »

Price Scanner via MacPrices.net

Deals on clearance 15″ Apple MacBook Pros wit...
B&H Photo has clearance 2016 15″ MacBook Pros available for up to $800 off original MSRP. Shipping is free, and B&H charges NY & NJ sales tax only: – 15″ 2.7GHz Touch Bar MacBook Pro... Read more
Apple restocked Certified Refurbished 13″ Mac...
Apple has restocked a full line of Certified Refurbished 2017 13″ MacBook Airs starting at $849. An Apple one-year warranty is included with each MacBook, and shipping is free: – 13″ 1.8GHz/8GB/128GB... Read more
How to find the lowest prices on 2017 Apple M...
Apple has Certified Refurbished 13″ and 15″ 2017 MacBook Pros available for $200 to $420 off the cost of new models. Apple’s refurbished prices are the lowest available for each model from any... Read more
The lowest prices anywhere on Apple 12″ MacBo...
Apple has Certified Refurbished 2017 12″ Retina MacBooks available for $200-$240 off the cost of new models. Apple will include a standard one-year warranty with each MacBook, and shipping is free.... Read more
Apple now offering a full line of Certified R...
Apple is now offering Certified Refurbished 2017 10″ and 12″ iPad Pros for $100-$190 off MSRP, depending on the model. An Apple one-year warranty is included with each model, and shipping is free: –... Read more
27″ iMacs on sale for $100-$130 off MSRP, pay...
B&H Photo has 27″ iMacs on sale for $100-$130 off MSRP. Shipping is free, and B&H charges sales tax for NY & NJ residents only: – 27″ 3.8GHz iMac (MNED2LL/A): $2199 $100 off MSRP – 27″ 3.... Read more
2.8GHz Mac mini on sale for $899, $100 off MS...
B&H Photo has the 2.8GHz Mac mini (model number MGEQ2LL/A) on sale for $899 including free shipping plus NY & NJ sales tax only. Their price is $100 off MSRP. Read more
Apple offers Certified Refurbished iPad minis...
Apple has Certified Refurbished 128GB iPad minis available today for $339 including free shipping. Apple’s standard one-year warranty is included. Their price is $60 off MSRP. Read more
Amazon offers 13″ 256GB MacBook Air for $1049...
Amazon has the 13″ 1.8GHz/256B #Apple #MacBook Air on sale today for $150 off MSRP including free shipping: – 13″ 1.8GHz/256GB MacBook Air (MQD42LL/A): $1049.99, $150 off MSRP Read more
9.7-inch 2017 WiFi iPads on sale starting at...
B&H Photo has 9.7″ 2017 WiFi #Apple #iPads on sale for $30 off MSRP for a limited time. Shipping is free, and pay sales tax in NY & NJ only: – 32GB iPad WiFi: $299, $30 off – 128GB iPad WiFi... Read more

Jobs Board

*Apple* Data Center Site Selection and Strat...
# Apple Data Center Site Selection and Strategy Research Analyst Job Number: 83708609 Santa Clara Valley, California, United States Posted: 18-Jan-2018 Weekly Hours: Read more
Security Engineering Coordinator, *Apple* R...
# Security Engineering Coordinator, Apple Retail Job Number: 113237456 Santa Clara Valley, California, United States Posted: 18-Jan-2018 Weekly Hours: 40.00 **Job Read more
Firmware Engineer - *Apple* Accessories - A...
# Firmware Engineer - Apple Accessories Job Number: 113422485 Santa Clara Valley, California, United States Posted: 18-Jan-2018 Weekly Hours: 40.00 **Job Summary** Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Store Leader - Retail District Manag...
Job Description:Job SummaryAs more and more people discover Apple , they visit our retail stores seeking ways to incorporate our products into their lives. It's your Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.