event/MaskMonitor: add variant with bound method

src/event/MaskMonitor.hxx

@@ -22,6 +22,7 @@
 
 #include "check.h"
 #include "DeferredMonitor.hxx"
+#include "util/BoundMethod.hxx"
 
 #include <atomic>
 
@@ -50,4 +51,22 @@ protected:
 	void RunDeferred() override;
 };
 
+/**
+ * A variant of #MaskMonitor which invokes a bound method.
+ */
+template<typename T>
+class CallbackMaskMonitor final : public MaskMonitor {
+	BoundMethod<T, void, unsigned> callback;
+
+public:
+	template<typename... Args>
+	explicit CallbackMaskMonitor(EventLoop &_loop, Args&&... args)
+		:MaskMonitor(_loop), callback(std::forward<Args>(args)...) {}
+
+protected:
+	void HandleMask(unsigned mask) override {
+		callback(mask);
+	}
+};
+
 #endif