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: { })
}

func foo() -> Any {
    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(_ notify: Any) {
   obs.append(notify)
}
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: { })
]

names.map { self.notificationCenter.addObserver(forName: $0, object: object, queue: queue, using: block) }

f { return nc.addObserver(forName: $0, object: object, queue: queue, using: block) }

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: { })
}

class C {
    var i: Int {
        set { ↓notificationCenter.addObserver(forName: $0, object: object, queue: queue, using: block) }
        get {
            ↓notificationCenter.addObserver(forName: $0, object: object, queue: queue, using: block)
            return 2
        }
    }
}

f {
    ↓nc.addObserver(forName: $0, object: object, queue: queue) {}
    return 2
}

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