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

How to win big in Slots Deluxe
Cheating while gambling is illegal and morally wrong, and in some parts of the world it leads to men with names like Vinnie "Six Knuckles" Manchenzo beating you to a pulp in a dark alley. [Read more] | Read more »
How to take over the world in Dictator 2
Running a country isn't easy - especially when you're a dictator who wants to take over the world and crush everyone in your path while you do it. [Read more] | Read more »
Tips and tricks to get a higher score in...
Tank.iois - you guessed it! - another multiplayer arena battler likeAgar.io and Slither.io. It does differentiate itself by putting you in a tiny tank though, so it's not exactly the same. To help you get that all-important high score, we've got a... | Read more »
How to unlock characters in One Tap Tenn...
As the title suggests, One Tap Tennis requires only a single tap to play its particular brand of tennis, and rewards you with a ton of unlockable characters if you perform well. Fortunately for you, we at 148Apps have got a few tips and tricks to... | Read more »
Grab it now: Game Craft’s Legend of War...
The real time strategy game is now available for you to sink your teeth into, through the App Store and Google Play. Combining elements of skill, strategy and empire building, Legend of War is a real gamers’ game. [Read more] | Read more »
Skateboard Party 3 ft. Greg Lutzka (Gam...
Skateboard Party 3 ft. Greg Lutzka 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Skateboard Party is back! This third edition of the popular sports franchise features professional skater... | Read more »
Cubious (Games)
Cubious 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Cubious – How smart are you? How high is your IQube? Solve the impossible puzzles to find out, and help a lost little cube find his... | Read more »
Goat Simulator Waste of Space (Games)
Goat Simulator Waste of Space 1.1 Device: iOS Universal Category: Games Price: $4.99, Version: 1.1 (iTunes) Description: ** IMPORTANT - SUPPORTED DEVICESiPhone 4S, iPad 2, iPod Touch 5 or better.** | Read more »
Wildfulness - Unwind in nature and calm...
Wildfulness - Unwind in nature and calm your mind with nature sounds and illustrations 1.0 Device: iOS Universal Category: Healthcare & Fitness Price: $1.99, Version: 1.0 (iTunes) Description: Spending time in nature helps you to... | Read more »
Dr. Panda Racers (Education)
Dr. Panda Racers 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: STEP ON THE GAS, RACE AND WIN!Fasten your seat belts and get ready to race! Speed your way to the finish line while doing... | Read more »

Price Scanner via MacPrices.net

Enterprise Workers Pick Technology Over Perks...
New Adobe study shows surprising attitudes about office jobs and where the future of work is heading. Adobe has released survey findings revealing that a surprising 70 percent of U.S. office workers... Read more
Goal Zero and OtterBox Partner to Expand iPh...
Goal Zero, specialists in portable power, have announced a partnership with OtterBox, brand smartphone case protection, to offer the Slide and Slide Plus Batteries as modules compatible with the new... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $210 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Clearance 2015 13-inch MacBook Airs available...
B&H Photo has clearance 2015 13″ MacBook Airs available for $250 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/4GB/128GB MacBook Air (MJVE2LL/A): $799... Read more
Apple refurbished Apple TVs available for up...
Apple has Certified Refurbished 32GB and 64GB Apple TVs available for up to $30 off the cost of new models. Apple’s standard one-year warranty is included with each model, and shipping is free: -... Read more
21-inch iMacs on sale for up to $120 off MSRP
B&H Photo has 21″ iMacs on sale for up to $120 off MSRP including free shipping plus NY sales tax only: - 21″ 3.1GHz iMac 4K: $1379.99 $120 off MSRP - 21″ 2.8GHz iMac: $1189 $110 off MSRP - 21″ 1... Read more
Kanex Introduces GoPower USB-C Rechargeable B...
Kanex has announced its GoPower USB-C portable battery for the USB-C MacBook, featuring the new industry standard connector and cable used for connectivity and power. Providing users with a new... Read more
Convertible and Detachable Devices Winning Ov...
According to the latest figures published by International Data Corporation (IDC), Western European shipments of ultraslim convertibles and detachables posted positive growth (44.7%) to account for... Read more
New MacBook Pros And Will MacBook Air Be Upgr...
With my mid-2013 13-inch MacBook Air closing on its third anniversary come November, I’m in system upgrade mode. Actually the Haswell CPU equipped Air is still doing a fine job, but my good wife is... Read more
Apple’s Education discount saves up to $300 o...
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, and... Read more

Jobs Board

*Apple* Project Engineer - Smart Source Inc...
SmartSource is in need of an Apple Project Engineer for a 12 month contract opportunity in Pittsburg, PA. Role: Apple Project Engineer Location: Pittsburg, PA Read more
Automotive Sales Consultant - Apple Ford Linc...
…you. The best candidates are smart, technologically savvy and are customer focused. Apple Ford Lincoln Apple Valley is different, because: $30,000 annual salary Read more
Service Assistant - *Apple* Chevrolet *App...
Apple Automotive is one of the fastest growing dealer...and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive, we Read more
Editor, *Apple* News - APPLE (United States...
Job Summary The Apple News team is looking for a passionate and knowledgeable editor with experience covering entertainment/pop culture and experience running social Read more
*Apple* Nissan Service Technicians - Apple A...
Apple Automotive is one of the fastest growing dealer...and it shows. Consider making the switch to the Apple Automotive Group today! At Apple Automotive , Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.