Open Radar

 
This is from the forums (https://forums.developer.apple.com/thread/23190), and it’s not my original discovery, but I wanted to be sure it didn’t get dropped:

When an instance calls the setter of another object of the same class from the property observer of its setter, the property observer of the second class is not called. Here's an example to make it clear:
 
import Foundation  
class TestObject {  
  
    init(name: String) {  
        self.name = name  
    }  
    let name: String  
  
    var otherObject: TestObject?  
  
    var property: String? {  
        didSet {  
            print("Set property of \(name) to \(property)")  
            otherObject?.property = property  
        }  
    }  
}  
let instance1 = TestObject(name: "Instance 1")  
let instance2 = TestObject(name: "Instance 2")  
instance1.otherObject = instance2  
instance1.property = "Test"  
 
The output on the console is
Set property of Instance 1 to Optional("Test")  
 
Though I would expect it to be
Set property of Instance 1 to Optional("Test")  
Set property of Instance 2 to Optional("Test")  
 
Both values are actually set to "Test", but the second time the didSet method is not called.