TweetFollow Us on Twitter

January 94 - Using TGridView

Using TGridView

Bob Hablutzel

This is the second article in what will hopefully be a monthly series of technical question and answers on object oriented programming. The intent of this column is to go beyond simple answers. I intend to take a question, answer it, and then use that question as a springboard to a more general discussion on programming. I hope you find the results interesting.

The first articles will be culled from frequently asked questions on MacApp3Tech$, but I need your questions! I can be reached electronically at AppleLink: B.HABLUTZEL; by telephone at 708.328.0130; by fax at 708.328.2417 or by post at Bob Hablutzel, Hablutzel Consulting, 606 Asbury, Evanston, IL, USA 60202.

QI would like to display a list of strings. MacApp seems to have a class for doing just this (TTextListView), but I cannot figure out how to attach the TList to it. How is the list associated with the view?

AThe immediate answer is that the list in TList and the list in TTextListView are not the same list. Unfortunately, it appears from the names of the classes that they were designed to go together. While they do work nicely together, it is not as transparent as the names would imply.

To understand the problem, we need to take a step back and look at the overall view hierarchy. In general, the view classes are fully functional; you can instantiate them, and they will perform the actions you expect. The classes in the category include TView, TButton, TRadio, etc.

A few of the view classes, however, are really abstract superclasses, and are not meant to be instantiated. TGridView, TTextGridView, and TTextListView all fall into this category. They are there for you to customize, not for you to use as–is. Another important fact to realize is that the classes, unlike the toolbox List Manager, do not maintain the data for you; this is something you need to maintain in the subclass you create.

TGridView, the parent of these views, describes a basic cell-oriented view. It knows a great deal about displaying cell oriented data: how to select cells, how to scroll cells, managing different column widths and row heights. It explicitly does not, however, know how to draw any particular cell.

This makes some sense, from a framework perspective. There is really no way to predict what kind of cell you will be imaging, and therefore it makes sense to make no assumptions. This leaves the doors open to icon cells, text cells, numeric cells, or even a mixed bag of all of these.

What TGridView does define, however, is a routine that is called to draw a particular cell. This routine, TGridView::DrawCell(), takes two arguments. The first argument defines which cell is being imaged. The cell is passed as a point–like structure defining the column and row numbers of the cell. The second argument is the rectangle to draw into.

If you decide to implement a view based on TGridView, you need to override DrawCell. (You may not need to override any other method). In the DrawCell override, you would determine the data you are going to image (based on the column and row numbers passed in), and draw the data. Again, this data can be anything you choose; you are in complete control of the drawing.

The designers of MacApp realized, however, that text-only grids are very common and important grid types. In order to make it easier to support these grid types, they created the TTextGridView and TTextListView classes. In these classes, a text-only override of DrawCell has already been provided for you, and you therefore do not have to worry about the imaging of the data. You do, however, need to provide a means of obtaining the data.

TTextGridView defines a view that understands multiple–column grids of text. It defines a routine, GetText(), that takes two arguments. The first argument is the cell to return text for, and the second is the text being returned. By default, this routine returns an empty string, so if you include an unspecialized TTextGridView in your programs, you will have the right number of cells, but nothing in them. You need to provide an override to GetText to provide the text for the individual cells.

TTextListView defines a view that is subclassed from TTextGridView, and is specialized to handle single–column grids of text. It overrides the default definition of GetText, and in turns calls a more specialized TTextListView defined routine, GetItemText. This routine takes two arguments, much like TTextGridView::GetText, but the first argument is simply the row number, not the cell. This allows you to return the string based on row number only.1

If you subclass either of TTextGridView or TTextListView, you need to at least override GetText or GetItemText, respectively. This give the view some text to draw. Of course, you need to get the text from somewhere. You basically have three choices: calculate the data on the fly; store the data in the view; or store the data extermally to the view, most likely in a document. The override of GetText/GetItemText needs to know how to obtain the text given the cell/row number.

We have talked about displaying the data contained in a grid view, but there is another problem to be solved: defining the number of rows and columns. For some applications, this value can be pre-determined, and coded into your view creation (or set with your favorite view editor). However, for most applications the size of the grid needs to be determined at run time. TGridView provides a number of useful routines for handling this problem, such as InsRowFirst to insert rows, InsColFirst to insert columns, DelRowFirst to delete rows, and DelColFirst to delete columns.

Take, for example, the method TGridView::InsRowFirst. This routine creates a new row of cells for data to be displayed in at the beginning of the grid. Since grid views do not know about data, it has no way of affecting the data that corresponds to these cells. It is your responsibility to manipulate the data to correspond to the new cell state. (More properly, it is your responsibility to call this routines to manipulate the grid to correspond to changes in the data's state.) The changing of the data, and the changing of the display of the data, are two separate actions that need to take place simultaneously. (This is also true for the routines that TTextListView defines, such as InsItemFirst.)

It is important to stress that these routines manipulate cells, not data. By adding or deleting cells, you provide room for data to be displayed; you do not affect data in any way. It is worth noting at this point that, if your grid view changes size and lives in a scroller, you probably want to change the size determiner of the view to sizeVariable. If you don't, the view will not really change size when you add or delete rows, confusing the scroller.

There are a number of tricks you can use to make your life easier when managing the number of cells. For example, you could make the grid view object dependent upon the document it is associated with. Then, when the document data changes, the grid view can catch the change in an override of DoUpdate, and adjust the number of cells accordingly. Grid views provide a simple means of display cell-oriented data. You need to remember that it is your responsibility to define a means for them to obtain the data, and, in the case of direct TGridView subclasses, to display the data. You also need to tell the grid how large it is, so that it can display the right number of cells for your data. Forgetting either of these two actions will result in no data being displayed in the view.

1 This might seem like a trivial reason for an override, and by itself it would be. However, TTextListView also provides some functionality, such as type-selection, that make sense for single column grids but not for multiple–column grids.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

ExpanDrive 5.4.6 - $49.99
ExpanDrive builds cloud storage in every application, acts just like a USB drive plugged into your Mac. With ExpanDrive, you can securely access any remote file server directly from the Finder or... Read more
Apple Pages 6.1.1 - Apple's word pr...
Apple Pages is a powerful word processor that gives you everything you need to create documents that look beautiful. And read beautifully. It lets you work seamlessly between Mac and iOS devices, and... Read more
Apple Numbers 4.1.1 - Apple's sprea...
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
Apple Keynote 7.1.1 - Apple's prese...
Easily create gorgeous presentations with the all-new Keynote, featuring powerful yet easy-to-use tools and dazzling effects that will make you a very hard act to follow. The Theme Chooser lets you... Read more
Espresso 5 - Powerful HTML, XML, CSS, an...
Espresso is back. For people who make delightful, innovative, and fast websites -- in an app to match. Espresso helps you write, code, design, build, and publish with flair and efficiency.... Read more
Vitamin-R 2.47 - Personal productivity t...
Vitamin-R creates the optimal conditions for your brain to work at its best by structuring your work into short bursts of distraction-free, highly focused activity alternating with opportunities for... Read more
Dash 4.0.3 - 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
Posterino 3.3.6 - Create posters, collag...
Posterino offers enhanced customization and flexibility including a variety of new, stylish templates featuring grids of identical or odd-sized image boxes. You can customize the size and shape of... Read more
Together 3.8.3 - Store and organize all...
Together helps you organize your Mac, giving you the ability to store, edit and preview your files in a single clean, uncluttered interface. Features Smart storage. With simple drag-and-drop... Read more
iClock Pro 3.4.9 - Customize your menuba...
iClock Pro is a menu bar replacement clock for Apple's default clock. iClock Pro is an update, total rewrite and improvement to the popular iClock. Have the day, date and time in different fonts and... Read more

Latest Forum Discussions

See All

Minimalist puzzle game, Cuts, free on iO...
If you're looking for a gorgeous puzzle experience on iOS devices, developer Gamebra.in's aesthetically interesting puzzler, Cuts, is discounted to free on the iOS App Store right now. [Read more] | Read more »
Anime tactical RPG, War of Crown, comes...
If you're looking for another tactical RPG fix to go alongside your Fire Emblem Heroes campaigns check out Gamevil's newest, anime-inspired tactics RPG, War of Crown, which comes out tomorrow. [Read more] | Read more »
Fantasy MMORPG MU Origin adds new modes,...
MU Origin, Webzen’s highly popular fantasy MMORPG is getting ready to shake things up for the second time this year, as a new update makes its way to the Google Play and App Store from today. Introducing new systems, modes, and events, the land of... | Read more »
Blizzard is looking to hire a mobile dev...
A new thread on the popular video game rumor forum, NeoGAF, uncovered an interesting job listing over at Blizzard Entertainment. It appears the studio behindStarCraft, World of WarCraft, Hearthstone,andOverwatch is looking to bring on a new hire... | Read more »
Legend of Zelda meets Cooking Mama in ne...
Dungeon Chef is what happens when you mix the RPG elements (and style) of a Legend of Zelda game, with cooking elements. Although, now that The Legend of Zelda: Breath of the Wild also has cookingelements, so maybe the gameplay is not so novel.... | Read more »
ChordFlow (Music)
ChordFlow 1.0.0 Device: iOS Universal Category: Music Price: $6.99, Version: 1.0.0 (iTunes) Description: ChordFlow is a chord sequencer with a unique 4-track polyphonic arpeggiator, extensive chord library, MIDI out and Ableton Link... | Read more »
The Walking Dead: A New Frontier is out...
The newest season of Telltale Games'The Walking Dead is well underway. After the release of the third episode, "Above the Law" about a month ago, episode four, "Thicker Than Water" is hot and ready for more zombies and gut-wrenching emotional... | Read more »
Best games we played this week
Another week, another new wave of mobile games do dive into. We've dug through the list of apps that came out this week to tell you which apps are worth your sweet time. And while there weren't too many games this week, there were some big ones.... | Read more »
Vignettes (Games)
Vignettes 1.0.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.1 (iTunes) Description: Vignettes is a casual but unique exploration game without text or characters, where objects shapeshift as you spin them around... | Read more »
Get Me Outta Here is an 80s retro shoote...
Are you ready to fight some aliens? Because Crescent Moon Games has released the retro shooter Get Me Outta Here on iOS devices today. [Read more] | Read more »

Price Scanner via MacPrices.net

HyperX Ships Pulsefire FPS Gaming Mouse, Winn...
Your reporter is a longtime fan of gaming mice for general purpose coomnputing use, finding them typically superior in comfort and performance. HyperX, a division of Kingston Technology Company, Inc... Read more
Penske Truck Leasing Unveils “Penske Fleet” M...
Penske Truck Leasing has introduced a free mobile app called “Penske Fleet” to benefit its full-service truck leasing and contract maintenance customers. The mobile app enables Penske’s customers to... Read more
DEVONthink To Go 2.1.3 For iOS Adds New Declu...
DEVONtechnologies has updated DEVONthink To Go, its iOS companion to DEVONthink for Mac. Enhancements and bug fixes in Version 2.1.3 include DEVONtechnologies’ new decluttering service for clipping... Read more
Use Apple’s Education discount to save up to...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free: -... Read more
Apple refurbished Apple Watches available sta...
Apple is now offering Certified Refurbished Series 1 and Series 2 Apple Watches for 14-16% off MSRP, starting at $229. An Apple one-year warranty is included with each watch. Shipping is free: Series... Read more
15-inch Touch Bar MacBook Pros, Apple refurbi...
Apple is offering Certified Refurbished 2016 15″ Touch Bar MacBook Pros for $360 to $420 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2.6GHz... Read more
13-inch MacBook Airs on sale for up to $150 o...
Overstock.com has 13″ MacBook Airs on sale for up to $150 off MSRP including free shipping: - 13″ 1.6GHz/128GB MacBook Air (sku MMGF2LL/A): $869.99 $130 off MSRP - 13″ 1.6GHz/256GB MacBook Air (sku... Read more
15-inch Touch Bar MacBook Pros on sale for $1...
B&H Photo has the new 2016 15″ Apple Touch Bar MacBook Pros in stock today and on sale for up to $200 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
15-inch 2.7GHz Touch Bar MacBook Pros on sale...
Amazon has 2016 15″ 2.7GHz Apple Touch Bar MacBook Pros in stock today and on sale for $150-$200 off MSRP. Shipping is free: - 15″ 2.7GHz Touch Bar MacBook Pro Space Gray (sku MLH42LL/A): $2599 $200... Read more
Apple now offering Certified Refurbished 13-i...
Apple is now offering Certified Refurbished 2016 13″ Touch Bar MacBook Pros for $270-$300 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.9GHz... Read more

Jobs Board

*Apple* Mac Computer Technician - GeekHampto...
…complex computer issues over the phone and in person? GeekHampton, Long Island's Apple Premium Service Provider, is looking for you! Come work with our crew 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* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 56881986 Middletown, NY, New York, United States Posted: Apr. 17, 2017 Weekly Hours: 40.00 **Job Summary** As an Apple Read more
*Apple* Mobile Master - Best Buy (United Sta...
**489879BR** **Job Title:** Apple Mobile Master **Location Number:** 000402-Utica-Store **Job Description:** **What does a Best Buy Apple Mobile Master do?** At Read more
Music Marketing Lead, iTunes & *Apple*...
# Music Marketing Lead, iTunes & Apple Music Job Number: 56868140 Culver City, California, United States Posted: Apr. 17, 2017 Weekly Hours: 40.00 **Job Summary** Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.