Discarded Notification Center Observer

When registering for a notification using a block, the opaque observer that is returned should be stored so it can be removed later

• Identifier: discarded_notification_center_observer
• Enabled by default: No
• Supports autocorrection: No
• Kind: lint
• Analyzer rule: No
• Minimum Swift compiler version: 5.0.0
• Default configuration:

  Key	Value
  severity	warning

Non Triggering Examples

let foo = nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil) { }

let foo = nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { })

func foo() -> Any {
    return nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { })
}

var obs: [Any?] = []
obs.append(nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { }))

var obs: [String: Any?] = []
obs["foo"] = nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { })

var obs: [Any?] = []
obs.append(nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { }))

func foo(_ notif: Any) {
   obs.append(notif)
}
foo(nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { }))

var obs: [NSObjectProtocol] = [
   nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { }),
   nc.addObserver(forName: .CKAccountChanged, object: nil, queue: nil, using: { })
]

Triggering Examples

↓nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil) { }

_ = ↓nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil) { }

↓nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { })

@discardableResult func foo() -> Any {
   return ↓nc.addObserver(forName: .NSSystemTimeZoneDidChange, object: nil, queue: nil, using: { })
}