Bug 29418 - Garbage Collector issue when subclassing native objects
Summary: Garbage Collector issue when subclassing native objects
Alias: None
Product: iOS
Classification: Xamarin
Component: XI runtime ()
Version: XI 8.10
Hardware: Macintosh Mac OS
: --- normal
Target Milestone: Untriaged
Assignee: Rolf Bjarne Kvinge [MSFT]
Depends on:
Reported: 2015-04-27 06:40 UTC by René
Modified: 2015-04-27 08:15 UTC (History)
3 users (show)

Is this bug a regression?: ---
Last known good build:

Demo project (34.62 KB, application/zip)
2015-04-27 06:40 UTC, René

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Description René 2015-04-27 06:40:27 UTC
Created attachment 10906 [details]
Demo project

Please refer to the attached video and the sample project.

* "btn" is an instance of a UIButton subclass that is created in ViewDidLoad(). Tapping the button removes it from its superview. The button will get collected.
* "imageView" is an instance of a UIImageView subclass that is created in ViewDidLoad(). There is a "removeBtn" of type UIButton which removed the image view from its superview. The image view will _not_ be collected.
* As soon as the "removeBtn" is added, the "btn" instance won't be collected either even though they are completely unrelated.


* Should "btn" really be collected?
* If yes, why is "imageView" not collected?
* Why is "btn" no longer collected if another independent button is added?

=== Xamarin Studio ===

Version 5.9 (build 431)
Installation UUID: 7a127b8e-364a-4453-8b18-e87f614b7d80
	Mono 4.0.0 ((detached/d136b79)
	GTK+ 2.24.23 (Raleigh theme)

	Package version: 400000143

=== Apple Developer Tools ===

Xcode 6.3 (7569)
Build 6D570

=== Xamarin.iOS ===

Version: (Business Edition)
Hash: c8648f0
Branch: master
Build date: 2015-04-21 12:22:24-0400

=== Xamarin.Android ===

Version: (Business Edition)
Android SDK: /Users/rene/Library/Developer/Xamarin/android-sdk-macosx
	Supported Android versions:
		2.3 (API level 10)
		4.2 (API level 17)
		4.4 (API level 19)
		5.0 (API level 21)
Java SDK: /usr
java version "1.6.0_65"
Java(TM) SE Runtime Environment (build 1.6.0_65-b14-466.1-11M4716)
Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-466.1, mixed mode)

=== Xamarin Android Player ===

Version: Unknown version
Location: /Applications/Xamarin Android Player.app

=== Xamarin.Mac ===

Version: (Business Edition)

=== Build Information ===

Release ID: 509000431
Git revision: 7560726734fc7267de2fa9abed2509968deefaa8
Build date: 2015-04-17 19:25:48-04
Xamarin addins: 2e772c734ab3148054eae7bf8949f340fdeb5e5e

=== Operating System ===

Mac OS X 10.10.3
Darwin iMacRR.local 14.3.0 Darwin Kernel Version 14.3.0
    Mon Mar 23 11:59:05 PDT 2015
    root:xnu-2782.20.48~5/RELEASE_X86_64 x86_64
Comment 1 René 2015-04-27 06:41:13 UTC
Recording of the issue: https://dl.dropboxusercontent.com/u/111612273/GCConfusion.mp4
Comment 2 Rolf Bjarne Kvinge [MSFT] 2015-04-27 06:59:41 UTC
First: the imgView isn't collected because it's kept alive by removeImgBtn's event handler. If I remove removeImgBtn when it's clicked, then the imgView is collected:

	removeImgBtn.TouchUpInside += (sender, e) => {
		removeImgBtn.RemoveFromSuperview ();

the second part (btn keeping removeImgBtn alive (and removeImgBtn keeps btn alive as well)) is still somewhat puzzling though.
Comment 3 Rolf Bjarne Kvinge [MSFT] 2015-04-27 07:01:07 UTC
So in the last comment I answered your question #2.

Question #1: yes, btn should be collected when it's removed.
Comment 4 Rolf Bjarne Kvinge [MSFT] 2015-04-27 07:41:27 UTC
Question #3: it's managed code keeping the button alive:

> xamarin_release_trampoline (MemoryTest_CustomButton Handle=0x796d5590) retainCount=2; HasManagedRef=1 GCHandle=35
> Switched object 0x796d5590 to weak GCHandle = 18

This shows that Xamarin.iOS only keeps a weak reference to the CustomButton instance.
Comment 5 Rolf Bjarne Kvinge [MSFT] 2015-04-27 08:15:46 UTC
> René Ruppert: @rolf: Xamarin Profiler shows that there is an anonymous storey that is still alive. This storey is used for both buttons. Since button 2 is still there, button 1 is kept alive.

Closing since this is not a Xamarin.iOS bug.