How Transmission Does Cross-Platform

John Gruber went out of his way to mention that Transmission is a cross-platform app which is actually very good on the Mac. In other words, it feels like it was designed for the platform. I was curious about this, so I looked at the source. I found a pleasant surprise.



Transmission doesn't just give the appearance of being a good Mac app, it actually is. There's real, solid Cocoa goodness here. Based on a quick survey of the project (so don't hold me to the details), it looks like the developers did this by separating the core application logic out into a library called libtransmission, which I assume is shared across the different platforms.

The Mac application project imports libtransmission and uses Objective-C's natural integration with ansi C to layer a Mac UI on top of the core library. The Torrent Objective-C class wraps the low-level C structs, acting as a bridge for the data model.

For example, the -checkForFiles: implementation calls tr_torrentGetFileDL() with the tr_torrent struct as an argument (edited slightly for formatting here):

- (int) checkForFiles: (NSIndexSet *) indexSet

{

    BOOL onState = NO;

    BOOL offState = NO;

    int index;

    for ( index = [indexSet firstIndex];

          index != NSNotFound;

          index = [indexSet indexGreaterThanIndex: index] )

    {

        if (tr_torrentGetFileDL(fHandle, index)

          || ![self canChangeDownloadCheckForFile: index])

            onState = YES;

        else

            offState = YES;

        

        if (onState && offState)

            return NSMixedState;

    }

    return onState ? NSOnState : NSOffState;

}

(I believe I'm allowed to post snippets without inclding the entire copyright, but this chunk of code is essentially under an MIT license. See the project for details.)

As if the concept wasn't enough, even the actual code for Transmission is beautiful. Very well written and easy to understand. It even has a great icon. If you're wondering how to go about writing a cross-platform app that takes advange of all the Mac has to offer, this is a working model of how to do it. All of this in a 2.5MB download.

Also interesting is that the project is covered under two separate open source licenses, thanks to a special exemption.

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