---
title: "addLocalMonitorForEvents(matching:handler:)"
framework: appkit
role: symbol
role_heading: Type Method
path: "appkit/nsevent/addlocalmonitorforevents(matching:handler:)"
---

# addLocalMonitorForEvents(matching:handler:)

Installs an event monitor that receives copies of events the system posts to this app prior to their dispatch.

## Declaration

```swift
class func addLocalMonitorForEvents(matching mask: NSEvent.EventTypeMask, handler block: @escaping (NSEvent) -> NSEvent?) -> Any?
```

## Parameters

- `mask`: An event mask specifying which events you wish to monitor. See doc://com.apple.appkit/documentation/AppKit/NSEvent/EventTypeMask for possible values.
- `block`: The event handler block object. It is passed the event to monitor. You can return the event unmodified, create and return a new NSEvent object, or return nil to stop the dispatching of the event.

## Return Value

Return Value An event handler object.

## Discussion

Discussion Your handler will not be called for events that are consumed by nested event-tracking loops such as control tracking, menu tracking, or window dragging; only events that are dispatched through the applications sendEvent(_:) method will be passed to your handler. note: The monitor Block is called for all future events that match mask. You must call removeMonitor(_:) to stop the monitor. Special Considerations In OS X v 10.6, event monitors are only able to monitor the following event types: NSFlagsChanged NSLeftMouseDragged NSRightMouseDragged NSOtherMouseDragged NSLeftMouseUp NSRightMouseUp NSOtherMouseUp NSLeftMouseDown NSRightMouseDown NSOtherMouseDown NSMouseMoved NSFlagsChanged NSScrollWheel NSTabletPoint NSTabletProximity NSKeyDown (Key repeats are determined using the isARepeat property.)

## See Also

### Monitoring app events

- [addGlobalMonitorForEvents(matching:handler:)](appkit/nsevent/addglobalmonitorforevents(matching:handler:).md)
- [removeMonitor(_:)](appkit/nsevent/removemonitor(_:).md)