讲述如何开发一个控件,很有价值(三)(3)

In this case the Sender object is the RichEdit being changed. The code basically uses the RichEdit''''s Tag variable (initially 0) as a handy Control specific variable. Everytime the [OnChange] event is called, it increases the Tag by 1, and display its value in an Edit Control as Text. You should pre-set the RichEdit control with some text in it, otherwise the following may be confusing!

1. Compile and Run...
2. Click in the Control. Nothing...
3. Move around in it using CursorKeys... Nothing...
4. Click outside the control.. and then back inside.. Nothing...
5. Press the [Space Bar].. Tag=1...
6. Press [Backspace].. Tag=2...
7. Press return.. Tag=3..
8. Select some text.. No change..
9. CTRL-C some text.. No change..
10. CTRL-X some text.. Tag=4..
11. CTRL-V some text.. Tag=5..

As it looked good so far I then added to the Form1.OnShow event:

RichEdit1.Lines.LoadFromFile(''''c:\winzip.log''''); {Just a plain text file hanging around }

to see what happened. And guess what - an [OnChange] event was called sometime and "Tag=1" was displayed in the Edit control as the proof when the Form appears for the first time. So we can see that procedures do call Events that apply to what they are doing.

5. What happens in Syntax Highlighting anyhow?

Watch carefully in the Delphi Editor. Now try and reproduce it. Open a WordPad (since WordPad is a souped up RichEdit basically). Read in a source file (e.g any Unit1.pas) and do syntax highlighting manually:

1. Select a token
2. Manipulate it using the buttons provided to change Font, Size, Color, and Bold
3. Move onto the next token
4. Goto 1

So therefore in [OnChange] we''''ll try and write code to reproduce what we have done manually.

6. Which text do I want.. and where do I get it ?

Hunting through the Delphi Helpfile on RichEdit controls we find that the actual text information in the RichEdit control is stored (or rather can be accessed from) either:

Now Lines seemed to be what I wanted - after all I wanted the Syntax highlighting to work on a line by line basis. So let’s have a look at whats been changed.

Oh.. look at what? How can I tell which line is the one that is changed?

Unlike some Events, the [OnChange] isn''''t passed any variable''''s save the identity of the RichEdit control affected. The RichEdit Control doesn''''t have a runtime variable that tells us either. The only variables are the SelStart and SelLength - but their about selecting text aren''''t they? I just want to know what line I''''m on :-(

It was about then that I re-read the information on the Sel??? properties, and recalled my "concept" code. Selection - I realised - was the name of the game. By manipulating these variables I could reproduce what I was doing manually - selecting text - as program code. Once selected you can then manipulate the attributes of the selected text through the SelAttributes structure.

Let’s get familiar with these variables (in Summary)
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