swift playground / deinit {}中的内存泄漏并未始终如一地调用

Question

ARC object deletion seems to be inconsistent in the Swift Playground. Is it a bug or by design?

Consider this class:

class Test {
  var name: String
  init(name:String){
    self.name = name
    println("\(name) initialized")
  }
  deinit{
    println("\(name) deinitialized")
  }
}

When I call it from the playground (not the command line REPL, see comment below):

var t1 = Test(name: "t1")
var t2 : Test? = Test(name: "t2")
t2 = nil

I see only initialization messages in the console:

t1 initialized
t2 initialized

What's missing is deinit of t2.

When I run it in an app (eg entry of app delegate), I the output is consistent with ARC deletion (ie t1 init, then t2, and t2 deinit and then t1, since the entire invocation block goes out of scope):

t1 initialized
t2 initialized
t2 deinitialized
t1 deinitialized

Finally, in the command line REPL (see comment below for accessing the REPL), the results are consisent, ie: t1 is alive due to its top level scope, but t2 is ARC deleted, as one would expect.

  1> class Test {
  2.       var name: String
  3.       init(name:String){
  4.               self.name = name
  5.               println("\(name) initialized")
  6.       }
  7.       deinit{
  8.               println("\(name) deinitialized")
  9.       }
 10. }
 11> var t1 = Test(name: "t1")
t1 initialized
t1 initialized
t1 deinitialized
t1: Test = {
  name = "t1"
}
 12> var t2 : Test? = Test(name: "t2")
t2 initialized
t2 initialized
t2 deinitialized
t2: Test? = (name = "t2")
 13> t2 = nil
t2 deinitialized
 14> t1
$R2: Test = {
  name = "t1"
}
 15> t2
$R3: Test? = nil

Answer 1

We compared ARC/deletion of objection in an app scenario and within the Playground. Our test code used object created inside and outside a particular scope. We also nested the tester object to test multi-nested scoping.

We saw that the app scenario deletes objects right on cue (zeroth reference) whereas the Playground scenario deletes a few, but holds on to a majority of objects (irrespective of scope, but apparently consistent over multiple runs).

The Playground probably holds on to objects in order to service its assistant GUI output (and/or its playback feature).

See blog post here .

Answer 2

I found that I could trigger the call to deinit{} in a playground (Xcode version 6.0 (6A313)) by adding an outer layer of optional:

struct MetaTest {
   var t3: Test? = Test(name: "t3")
}

var mt3: MetaTest? = MetaTest()
mt3!.t3 = nil

which creates this output:

t3 initialized
t3 deinitialized