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Cross-platform Controls

From Windows to Linux, and Back

These are exciting times for Borland. Not since the first whisper of Delphi has there been this much excitement about a Borland product. I'm talking, of course, about Kylix, the project to bring C++Builder and Delphi to the Linux operating system. The Delphi version will be available first, so for the rest of this article, Kylix refers to Delphi for Linux.

We're developing a new VCL that will work with the Windows and Linux versions of Delphi. This means you can write an application in Windows, then move the source to a Linux box and recompile it - or vice versa. This new VCL is named CLX, for Component Library Cross-Platform. CLX encompasses the entire cross-platform library distributed with Kylix. There are a few sub-categories, which, as of this writing, break down as follows:

• BaseCLX is the RTL, up to, and including, Classes.pas.
• VisualCLX includes the user interface classes, i.e. the usual controls.
• DataCLX comprises the cross-platform database components.
• NetCLX includes the Internet stuff, e.g. Apache, etc.

At the time of this writing [early May 2000], the first Field Test for Kylix is just beginning. By the time you read this, there will be a big difference between the Kylix I'm using and working on, and the version you'll see when it's available. This makes my job all that more difficult. It would be easy to talk in generalities, waxing eloquent about the underlying architecture. I'd much rather discuss the details, however, so you can get a head start producing CLX controls. Just keep in mind that it's likely some of the particulars discussed in this article will have changed by the time you read it.

No One Else Comes Close

This article is a primer on writing custom VisualCLX controls. Essentially, the VisualCLX is what you know and love about the VCL. When you think about it, Visual Component Library is a bit of a misnomer; there's a lot more to it than the visual components. In this article, however, I'm only going to write about the visual controls. The Button, Edit, ListBox, PageControl, StatusBar, ProgressBar, etc. controls, have all been re-implemented to be cross-platform. How did we do this when the current VCL relies so much on Windows? In brief, we ripped out all the Windows stuff, and replaced it with another toolkit.

In Linux, there are a number of toolkits that contain the standard windowing controls, such as Buttons. They're called widgets, and GTK and Qt (pronounced "cute") are two of the more popular. Qt is a Linux widget toolkit that works on Windows and Linux. Because it aligned most closely with our goals, Qt was chosen as the basis for CLX. In other words, Qt is to CLX what the Windows API and common controls are to the VCL. Qt has some definite positives for the Delphi custom component developer on Linux:

• It's a prevalent Linux widget set, used by the popular KDE desktop.
• It's similar to the Windows API style of development.
• Its graphics model is close to the VCL's graphics model.
• Its classes look very much like VCL components.
• It introduces many standard widgets, and handles the message loop.

This begs two questions: Does this mean that Kylix supports only KDE, and no other desktops, such as Gnome? And how does using Qt as the basis of CLX affect me? The answer to the first question is that Kylix applications will run under any Linux desktop - particularly Gnome and KDE. The rest of this article answers the second question.

Don't Want You Back

The goal is to make it easy for developers to port their applications to Linux with the least amount of trouble. Most of the component names are the same, and most of the properties are the same. Although a few properties will be missing from some components, and a few new ones will be added, for the most part, it should be fairly painless to port your applications.

For component writers, it's a different story. For starters, there is no Windows.pas, nor Windows API (see Figure 1). You can say good-bye to the message directive, and all of the CN and CM notifications. These have been changed to dynamics. There is also no docking, bi-directional (BiDi) methods/properties, input method editor (IME), or Asian support in the first release. Of course, there's no ActiveX, COM, or OLE support. Windows 3.1 components are also out, as I write this.

Figure 1: Methods and properties missing from TWidgetControl (formerly known as TWinControl).

By now I bet you're thinking, "That's not so bad; porting my components doesn't sound too difficult." But wait - there's more. At the time of this writing, the CLX unit names have all been changed to include "Q" as a prefix. So StdCtrls is now QStdCtrls, some classes have shuffled around a bit, and there are some subtle differences in the hierarchy (see Figure 2).

Figure 2: The differences in the class hierarchy are subtle.

The CLX prefix of "Q" may or may not end up as the permanent prefix in the final release. TWinControl is now a TWidgetControl, but to ease the pain we added a TWinControl alias to TWidgetControl. TWidgetControl and descendants all have a Handle property that is an opaque reference to the Qt object; and a Hooks property, which is a reference to the hook objects that handle the event mechanism. (Hooks are part of a complex topic that is outside the scope of this article.)

OwnerDraw will be replaced with a new idea called Styles. Styles are basically a mechanism whereby a widget or application can take on a whole new look, similar to skins in Windows. This is something that's still in development, so I'm not going to discuss it further in this article. I will say this: It's way cool.

Is anything the same? Sure. TCanvas is the same as you remember, with its collection of Pens and Brushes. As I mentioned, the class hierarchy is basically the same, and events, such as OnMouseDown, OnMouseMove, OnClick, etc., are still there.

Show Me the Meaning ...

Let's move on to the meat of CLX, and see how it works. Qt is a C++ toolkit, so all of its widgets are C++ objects. On the other hand, CLX is written in Object Pascal, and Object Pascal can't talk directly to C++ objects. To make matters a little more difficult, Qt uses multiple inheritance in a few places. So we created an interface layer that takes all of the Qt classes and reduces them to a series of straight C functions. These are then wrapped up in a DLL in Windows and a shared object in Linux.

Every TWidgetControl has CreateWidget, InitWidget, and HookEvents virtual methods that almost always have to be overridden. CreateWidget creates the Qt widget, and assigns the Handle to the FHandle private field variable. InitWidget gets called after the widget is constructed, and the Handle is valid. Some of your property assignments will move from the Create constructor to InitWidget. This will allow delayed construction of the Qt object until it's really needed. For example, say you have a property named Color. In SetColor, you can check with HandleAllocated to see if you have a Qt handle. If the Handle is allocated, you can make the proper call to Qt to set the color. If not, you can store the value in a private field variable, and, in InitWidget, you set the property.

There are two types of events: Widget and System. HookEvents is a virtual method that hooks the CLX controls event methods to a special hook object that communicates with the Qt object. (At least that's how I like to look at it.) The hook object is really just a set of method pointers. System events now go through EventHandler, which is basically a replacement for WndProc.

Larger Than Life

All of this is just background information, because you really don't need to know it in order to write cross-platform custom controls. It helps, but with CLX, writing cross-platform controls is a snap. Just as you didn't have to understand the complexities of the Windows API to write a VCL control, the same goes for CLX and Qt. Listing One  shows a custom control written with CLX. [It's available for download; see end of article for details.]

The project file, CalcTest.dpr, is shown in Figure 3. The calculator control running in Windows (shown in Figure 4), and in Linux (shown in Figure 5) looks much like the standard Microsoft Windows calculator.

program CalcTest;