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.

 
AAPL
$112.00
Apple Inc.
-0.65
MSFT
$47.70
Microsoft Corpora
+0.18
GOOG
$507.91
Google Inc.
-3.19

MacTech Search:
Community Search:

Software Updates via MacUpdate

Mellel 3.3.7 - Powerful word processor w...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
ScreenFlow 5.0.1 - Create screen recordi...
Save 10% with the exclusive MacUpdate coupon code: AFMacUpdate10 Buy now! ScreenFlow is powerful, easy-to-use screencasting software for the Mac. With ScreenFlow you can record the contents of your... Read more
Simon 4.0 - Monitor changes and crashes...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
BBEdit 11.0.2 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
ExpanDrive 4.2.1 - Access cloud storage...
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
Adobe After Effects CC 2014 13.2 - Creat...
After Effects CC 2014 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous After Effects customer). After Effects CS6 is still available... Read more
Command-C 1.1.7 - Clipboard sharing tool...
Command-C is a revolutionary app which makes easy to share your clipboard between iOS and OS X using your local WiFi network, even if the app is not currently opened. Copy anything (text, pictures,... Read more
Tidy Up 4.0.2 - Find duplicate files and...
Tidy Up is a complete duplicate finder and disk-tidiness utility. With Tidy Up you can search for duplicate files and packages by the owner application, content, type, creator, extension, time... Read more
Typinator 6.3 - Speedy and reliable text...
Typinator turbo-charges your typing productivity. Type a little. Typinator does the rest. We've all faced projects that require repetitive typing tasks. With Typinator, you can store commonly used... Read more
GraphicConverter 9.5 - Graphics editor w...
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

Latest Forum Discussions

See All

Shift - Photo Filters Designed By You (...
Shift - Photo Filters Designed By You 1.0 Device: iOS Universal Category: Photography Price: $.99, Version: 1.0 (iTunes) Description: | Read more »
Elastic Drums (Music)
Elastic Drums 1.0 Device: iOS iPhone Category: Music Price: $3.99, Version: 1.0 (iTunes) Description: *** Introduction price 3,99$ instead of 7,99$ *** Elastic Drums is a music app with 6 channels of synthesized drum sounds, a step... | Read more »
Fireworks Simulator (Games)
Fireworks Simulator 1.0.8 Device: iOS Universal Category: Games Price: $.99, Version: 1.0.8 (iTunes) Description: *** 50% discount – For a short time only *** You can play Fireworks Simulator on these devices: - iPhone 5, 5s, 5c, 6,... | Read more »
Nicky's Gift (Games)
Nicky's Gift 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Everybody! Merry Christmas! There's 48 levels in the game. Let's go! Nicky's Gift | Read more »
The Hit List — Simply Powerful Tasks, To...
The Hit List — Simply Powerful Tasks, To-Dos, Projects, & Reminders 2.0 Device: iOS iPhone Category: Productivity Price: $9.99, Version: 2.0 (iTunes) Description: >> LAUNCH SPECIAL: The Hit List 2 for iPhone is ONLY $9.99... | Read more »
Mahjong Journey Review
Mahjong Journey Review By Jennifer Allen on December 18th, 2014 Our Rating: :: STEADY MATCHINGiPad Only App - Designed for the iPad Aimed at the more laid back gamer, Mahjong Journey isn’t for everyone, but those looking for some... | Read more »
Emoji Type - custom keyboard with predic...
Emoji Type - custom keyboard with predictive emojis 0.4.0 Device: iOS iPhone Category: Utilities Price: $.99, Version: 0.4.0 (iTunes) Description: Emoji Type is custom keyboard for iOS 8 that auto suggests emojis as you type. ABOUT... | Read more »
Game of the Year 2014 – 148Apps Staff Pi...
The end of 2014 is almost here, which can only mean one thing. Okay it can mean a lot of things, but in this specific context it means Game of the Year lists! Which is why the 148Apps staff have all picked their favorites from the past year. And why... | Read more »
UponPixels Review
UponPixels Review By Jennifer Allen on December 18th, 2014 Our Rating: :: CREATIVE TYPOGRAPHYUniversal App - Designed for iPhone and iPad Add cool typography and objects to your photos with the easy to use UponPixels.   | Read more »
The Vikings are Coming! CastleStorm’s Ne...
The Vikings are Coming! CastleStorm’s New Update Adds a Survival Mode Posted by Jessica Fisher on December 18th, 2014 [ permalink ] | Read more »

Price Scanner via MacPrices.net

Holiday sales this weekend: Mac minis availab...
B&H Photo has new 2014 Mac minis on sale for up to $80 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 1.4GHz Mac mini: $459 $40 off MSRP - 2.6GHz Mac mini: $629 $70 off MSRP... Read more
Holiday sales this weekend: Mac Pros for up t...
B&H Photo has Mac Pros on sale for up to $500 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.7GHz 4-core Mac Pro: $2599, $400 off MSRP - 3.5GHz 6-core Mac Pro: $3499, $... Read more
Save up to $400 on MacBooks with Apple Certif...
The Apple Store has Apple Certified Refurbished 2014 MacBook Pros and MacBook Airs available for up to $400 off the cost of new models. An Apple one-year warranty is included with each model, and... Read more
Save up to $300 on Macs, $30 on iPads with Ap...
Purchase a new Mac or iPad at The Apple Store for Education 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
iOS and Android OS Targeted by Man-in-the-Mid...
Cloud services security provider Akamai Technologies, Inc. has released, through the company’s Prolexic Security Engineering & Research Team (PLXsert), a new cybersecurity threat advisory. The... Read more
KMI MIDI K-Board Great Gift for Amateur &...
The K-Board is a MIDI Nano keyboard for music creation for iPad, Android, And computers; the easiest way to make music with iPads & Android tablets, and Mac, Windows, or Linux computers. Ultra-... Read more
Amazon offers 15-inch 2.2GHz Retina MacBook P...
 Amazon.com has the 15″ 2.2GHz Retina MacBook Pro on sale for $1699 including free shipping. Their price is $300 off MSRP. Stock is limited, so act now if you’re interested. Read more
Holiday sales continue: MacBook Pros for up t...
 B&H Photo has new MacBook Pros on sale for up to $300 off MSRP as part of their Holiday pricing. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.2GHz Retina MacBook Pro: $1699... Read more
Holiday sale: Mac minis available for up to $...
 B&H Photo has new 2014 Mac minis on sale for up to $80 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 1.4GHz Mac mini: $459 $40 off MSRP - 2.6GHz Mac mini: $629 $70 off... Read more
Google Search App For iOS Gets A Major Makeov...
Google has given iOS users an early Christmas present with a substantial update of it’s not-very-often-upgraded Google Search app. Google Search has been my go-to tool for Web searches since it was... Read more

Jobs Board

Project Manager, *Apple* Financial Services...
**Job Summary** Apple Financial Services (AFS) offers consumers, businesses and educational institutions ways to finance Apple purchases. We work with national and 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
*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
*Apple* Retail - Multiple Positions (US) - A...
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* 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
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.