CODEWITHSUNDEEP
swiftrealm
Said Sikira
Said Sikira

Reputation: 4533

How Realm handles dynamic dispatch with List<T> and RealmOptional<T>?

Realm relies on dynamic dispatch to access ObjectiveC runtime for some KVC mechanisms. Model properties should be marked with dynamic keyword to enable KVC that is used by Realm objects to fill them with exact values. So you can have model defined like this:

class Car: Object {
    dynamic var color = ""
    dynamic var age = 0
}

At some level you would be able to set properties like this:

var car = Car()
car["color"] = "white"
car["age"] = 20

This works for all basic types like Int or String but doesn't work with generic classes and structures since they can't be represented in Objective C. Realm uses two of those types:

  1. List<T>
  2. RealmOptional<T>

When adding properties of these types to the Realm models, you add them without dynamic keyword:

let cars = List<Car>
let registration = RealmOptional<String>

What mechanism Realm uses to set and read data of those types without using dynamic dispatch?

Update 1

Upon some inspection, I've found that Realm uses some Objective C runtime methods like class_copyPropertyList() and property_getName() for introspection of property names. What I haven't found yet is how properties are populated with correct data when you read from Realm? Is that part of the ObjectStore C++ framework?

Upvotes: 2

Views: 348

Answers (1)

kishikawa katsumi
kishikawa katsumi

Reputation: 10573

Generic properties such like List<T> or RealmOptional<T> cannot be appeared from Objective-C. Realm uses Swift's reflection (Mirror()) to inspect those.

(See https://github.com/realm/realm-cocoa/blob/b71daecd0f4cf7a89fcb30178be02f506d9b3124/RealmSwift/Object.swift#L310-L316)

Then directly access their ivar: https://github.com/realm/realm-cocoa/blob/76d1018b32ba98babce31afbefd31863075cde8c/Realm/RLMObjectSchema.mm#L217-L223

These generic properties are dispatched statically. Realm cannot hook accessing those. That's why those properties must be declared as let. Realm cannot see re-assign the properties.

Upvotes: 1

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