One of the new features introduced in Mac OS X 10.5 Leopard was Spaces, a virtual desktop manager for Mac OS X. Unfortunately, it has a number of problems, including a complete lack of consistency when it comes to determining which space you will end up on when switching between applications with windows on multiple spaces.

As an example, try the following (from my bug report to Apple, rdar://5596671):

1. Make sure that Spaces is enabled.
2. Make sure that only the Finder is running, and that no windows are open in the Finder.
3. Open Text Edit. A new document should be created on the current space (Space 1).
4. Create a second document.
5. Move the second document to the next space (Space 2). Note that the second document is TextEdit's frontmost window.
6. While in Space 2, click in the Finder.
7. Using Command-Tab, switch back to TextEdit.

When switching back to TextEdit, I would expect that the last document that was open gets focus. What actually happens is that Spaces decides that now is a good time to switch back to space 1. When this happens hundreds of times every day, it soon gets very annoying. I'm not the only one who thinks so either; Henry Story has his take and Dave Dribin also shares his views. John Gruber then summarized by saing: "In short, Spaces seems designed for app partitioning, not task partitioning."

So this is the reason I put together my new application called Spaces.. Spaces.. Spaces.. (or TripleS for short). To make it work, I had to reverse engineer the Dock, figure out how it switches spaces and figure out how to make the most annoying aspect of the above behaviour disappear. My initial idea was to patch the CGSSetWorkspace function, but that didn't work very well as the Dock maintains quite a bit of state outside the window server (one crucial bit is which workspace is the current workspace). Instead, I decided to move one level up, and prevent the entire workspace switch from ever happening. This turned out to be quite successful, but the downside is that it ties the TripleS binary to a very specific version of Mac OS X (the current build, 10.5.0/9A581).

With the target to patch identified, I needed a way of determining when to inhibit workspace-switches. My desired behaviour was to prevent the Dock from switching space whenever I had just Command-Tab'd to a different application. After some digging, I discovered that I could override the __CPSSetFrontProcessWithOptions function to discover when the Dock changed the front process. Then I could simply note down the current time, and later when the patch to the Dock's workspace-switch function is called calculate the delta. If that delta is below some threshold (currently 0.2 seconds), the workspace switch inhibited.

I have made the code for Spaces.. Spaces.. Spaces.. available under the BSD license, if you're interested in taking a look. I'm indebted to the work of several others, including Jonathan 'Wolf' Rentzsch for his work developing mach_star, as well as Infinite Labs for their work in making mach_star work on Leopard.