WC3 JASS.com
"The Jass Vault" plus vJASS and Zinc

It does two things:
1) Try to read the file, if "Allow Local Files" is enabled then also searches in the game folder
2) Append filename to preload buffer

The game only reads these files, does not load them. The reading is done in a separate thread and does not freeze the game. One file is not read twice, no matter how often you call Preload().

Trick: It does not escape double-quotes " on purpose. It is possible to inject custom code in Preload files this way:
Results in the following preload file code:


Game folder:
Reforged: Warcraft III\_retail_\somefile.txt, instead of _retail_ there's also a _ptr_ game version currently.
Classic: ?

Mini tutorial:
What are Preload files?
Preload files instruct the game to pre-read a file/resources to avoid freezes/stutter during gameplay. It's done to move the file into OS cache. Blizzard used preload files to load all required files at map init. See blizzard.j or campaign maps.

Create a preload file

How to run a preload file
This must be done manually:

Lua code in preload files?
It is possible although in a very hacky way, described here. You need to use "//! beginusercode" to start a section containing Lua code and end it using "//! endusercode". It works because the code is transpiled on the fly with Jass2Lua.

See: PreloadEnd, PreloadStart, PreloadRefresh, PreloadEndEx, PreloadGenClear, PreloadGenStart, PreloadGenEnd, Preloader