Initial commit.

src/EventMonitor.swift

@@ -0,0 +1,55 @@
+import Cocoa
+
+class EventMonitor {
+    fileprivate let mask: NSEvent.EventTypeMask
+    fileprivate var monitor: Any?
+    
+    fileprivate init(mask: NSEvent.EventTypeMask) {
+        self.mask = mask
+    }
+    
+    deinit {
+        stop()
+    }
+    
+    func start() {
+        fatalError("start must be implemented by a subclass of EventMonitor")
+    }
+    
+    func stop() {
+        if monitor != nil {
+            NSEvent.removeMonitor(monitor!)
+            monitor = nil
+        }
+    }
+}
+
+final class LocalEventMonitor: EventMonitor {
+    typealias Handler = (NSEvent) -> NSEvent?
+    
+    private let handler: Handler
+    
+    init(mask: NSEvent.EventTypeMask, handler: @escaping Handler) {
+        self.handler = handler
+        super.init(mask: mask)
+    }
+    
+    override func start() {
+        monitor = NSEvent.addLocalMonitorForEvents(matching: mask, handler: handler)
+    }
+}
+
+final class GlobalEventMonitor: EventMonitor {
+    typealias Handler = (NSEvent) -> Void
+    
+    private let handler: Handler
+    
+    init(mask: NSEvent.EventTypeMask, handler: @escaping Handler) {
+        self.handler = handler
+        super.init(mask: mask)
+    }
+    
+    override func start() {
+        monitor = NSEvent.addGlobalMonitorForEvents(matching: mask, handler: handler)
+    }
+}