A Swift window management framework for OS X

In the past few years, many developers formerly on Linux and Windows have migrated to Mac for their excellent hardware and UNIX-based OS that "just works".

But along the way we gave up something dear to us: control over our desktop environment.

The goal of Swindler is to help us take back that control, and give us the best of both worlds.

Writing window managers for OS X is hard. There are a lot of systemic challenges, including limited and poorly-documented APIs. All window managers on OS X must use the C-based accessibility APIs, which are difficult to use and are surprisingly buggy themselves.

As a result, the selection of window managers is pretty limited, and many of the ones out there have annoying bugs, like freezes, race conditions, "phantom windows", and not "seeing" windows that are actually there. The more sophisticated the window manager is, the more it relies on these APIs and the more these bugs start to show up.

Swindler's job is to make it easy to write powerful window managers using a well-documented Swift API and abstraction layer. It addresses the problems of the accessibility API with these features:

Swindler's API is fully documented and type-safe thanks to Swift. It's much easier and safer to use than the C-based accessibility APIs. (See the example below.)

Window managers on OS X rely on IPC: you ask an application for a window's position, wait for it to respond, request that it be moved or focused, then wait for the application to comply (or not). Most of the time this works okay, but it works at the mercy of the remote application's event loop, which can lead to long, multi-second delays.

Swindler maintains a model of all applications and window states, so your code knows everything about the windows on the screen. Reads are instantaneous, because all state is cached within your application's process and stays up to date. Swindler is extensively tested to ensure it stays consistent with the system in any situation.

If you need to resize a lot of windows simultaneously, for example, you can do so without fear of one unresponsive application holding everything else up. Write requests are dispatched asynchronously and concurrently, and Swindler's promise-based API makes it easy to keep up with the state of operations.

More sophisticated window managers have to observe events on windows, but the observer API is not well documented and often leaves out events you might expect, or delivers them in the wrong order. For example, the following situation is common when a new window pops up:

1. MainWindowChanged on com.google.chrome to <window1>
2. WindowCreated on com.google.chrome: <window1>

See the problem? With Swindler, all events are emitted in the expected order, and missing ones are filled in. Swindler's in-memory state will always be consistent with itself and with the events you receive, avoiding many bugs that are difficult to diagnose.

As a bonus, events caused by your code are marked as such, so you don't respond to them as user actions. This feature alone makes a whole new level of sophistication possible.

The following code assigns all windows on the screen to a grid. Note the simplicity and power of the promise-based API. Requests are dispatched concurrently and in the background, not serially.

let screen = Swindler.state.screens.first!
let allPlacedOnGrid = screen.knownWindows.enumerate().map { index, window in
  let rect = gridRect(screen, index)
  return window.position.set(rect.origin).then { window.size.set(rect.size) }
}

when(allPlacedOnGrid) { _ in
  print("all done!")
}

func gridRect(screen: Swindler.Screen, index: Int) -> CGRect {
  let gridSteps = 3
  let position  = CGSize(width: screen.width / gridSteps, height: screen.height / gridSteps)
  let size      = CGPoint(x: gridSize.width * (index % gridSteps), y: gridSize.height * (index / gridSteps))
  return CGRect(origin: position, size: size)
}

Watching for events is simple. Here's how you would implement snap-to-grid:

swindler.on { (event: WindowMovedEvent) in
  guard event.external == true else {
    // Ignore events that were caused by us.
    return
  }
  let snapped = closestGridPosition(event.window.position.value)
  event.window.position.set(snapped)
}

Swindler is in development and is in alpha. Here is the state of its major features:

• Asynchronous property system: 100% complete
• Event system: 100% complete
• Window API: 80% complete
• Application API: 80% complete
• Screen API: 0% complete
• Spaces API: 0% complete

You can see the entire planned API here.

Follow me on Twitter: @tmandry

• Silica
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• Hammerspoon, a fork of Mjolnir