未在 realm 属性上调用 WillSet/DidSet

Question

I have a problem because willSet and didSet are not being called with dynamic realm object.

Code sample:

try! realm.write {
    sut = Backup()
    realm.add(sut) // here willSet and didSet are invoked with nil object
}

XCTAssertFalse(sut.didEditPatient) // ok
try! realm.write {
    print("CHECKING: will add the patient")
    let patient = Patient()
    realm.add(patient)
    sut.patient = patient // nothing gets printed here!
    print("CHECKING: added the patient")
}

XCTAssertTrue(sut.didEditPatient) // fails
XCTAssertNotNil(sut.patient) // ok

Where Backup class is defined this way:

final class Backup: Object {
    @objc dynamic var patient: Patient? {
        willSet {
            print("CHECKING: willSet: \(String(describing: newValue))")
            if newValue != patient {
                didEditPatient = true
            }
        }
        didSet { print("CHECKING: didSet: \(String(describing: patient))") }
    }
    @objc dynamic var didEditPatient: Bool = false

Output in the console is:

• CHECKING: willSet: nil
• CHECKING: didSet: nil
• CHECKING: will add the patient
• CHECKING: added the patient

While I'd rather expect that between will add the patient and added the patient I should get willSet and didSet with patient object. Obviously, patient is not nil.

Answer 1

There is one issue described about this in realm repo:

---

I would recommend using a private persisted property which has no logic, along with a non-persisted computed property which has the willSet/didSet functionality:

class Model : RLMObject {
    private dynamic var backingProp = 0

    var prop : Int {
        get {
            return backingProp
        }
        set(newValue) {
            // do willSet stuff
            backingProp = newValue
            // do didSet stuff
        }
    }

    override class func ignoredProperties() -> [AnyObject]! {
        return ["prop"]
    }
}

This is a bit verbose, but gives you identical behavior for objects in a realm and standalone objects.

---

Source: https://github.com/realm/realm-cocoa/issues/870#issuecomment-54543539

Answer 2

This is easy to fix. Just update the objects properties before writing to Realm. willSet and didSet are Swift constructs, not Objc.

let p = Patient()
try! realm.write {
    print("CHECKING: will add the patient")
    sut.patient = p // moved before add to realm.
    realm.add(p)
    print("CHECKING: added the patient")
}

Will print

CHECKING: will add the patient
willSet
didSet
CHECKING: added the patient

Which is the order you want.