Fast Text Highlighting

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Problem Description

Here's a program to rapidly apply highlighting to selected  words in a TRichedit (Rich edit) control..

Background & Techniques

Applying syntax formatting to a sizable Richedit file is quite slow.  The technique used here is to intercept the WindowProc procedure used by TRichedit and let it paint the window which we then scan for words to by highlighted and rewrite them in the specified color.   Although my demo only highlights single words, more sophisticated highlighting, as for example performed by the Delphi IDE for strings, comments, numbers, etc. is possible.  The Basic code to do this was sent to me by Emile Tredoux and when someone else had the problem recently, I finally got around to converting the guts of the process to Delphi. 

I believe that there are enough comments in the code to make it usable to others.  The main constraint of the technique is that the highlighted text must fit into the space used for the text being replaced since TRichedit has already painted the window and we are just repainting selected portions.  The safest way to ensure this, used here, is to  use the TRichedit font for our Textout operations.  If the font has has fixed pitch as used by Delphi IDE, then highlighted  text font style could be set Bold or Italic as well.   

The code uses a number of Windows API calls and took a bit of fiddling to get the parameters defined correctly but it seems to be working fine for now.

Running/Exploring the Program 

bullet Download source
bullet Download  executable

Suggestions for Further Explorations

Lot's of room for variations of this technique. For example::

bullet Detecting and highlighting phrases.
bullet Highlighting color based on the word or phrase
bullet  Syntax highlighting as in the Delphi IDE by detecting comments, string constants, numbers, reserved words, etc.



Original Date: mmm dd, yyyy 

Modified: May 15, 2018


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