Lecture delivered in BASTA 2018, regarding Xamarin Forms memory structure and specifics when dealing with custom renderers, effects and external plugins

1. 1© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.
Understanding memory
management in Xamarin Forms

2. About me
Tsvyatko Konov
@tsvyatco

3. Agenda
 Forms overview and scenarios
 CG in nutshell
 CG in Mono.NET – nursery, major, tracking
 Mono CG and Java CG
• Tracking
• Cases to watch out – minor can be evil
 Mono CG and IOS refcounting – framework and user pears
• Tracking
• Cases to watch out – object cycles
 Ways to identify memory leaks

4. Overview of Xamarin Forms
.NET STANDARD
Core library
INFRASTRUCTURE
Xamarin Forms
Mono class libraryBase class
library

5. Forms scenarios
 When building pure XF app we are shielded from the native platform
• We need to care mainly for memory in the managed word
• Most general .NET practices apply here
 When should we care about Xamarin internals
• Effects
• Custom (and Fast) Renderers
• Native control embedding
• Any time our app crashes with OutOfMemoryException

6. GC overview

7. GC overview
 GC help us due to its many benefits
1 Simple to work with
2 Reduces memory Leaks
3 Deals with memory reallocation under the hood

8. GC - new object allocation
 GC triggers on the current thread when there is not enough space
to satisfy new memory request
A B C
Current pointer
of free space
var D = new D();

9. GC - new object allocation
 GC triggers on the current thread when there is not enough space
to satisfy new memory request
A B C D
Current pointer of free
space
var D = new D();

10. GC - reclaiming memory
 GC triggers on the current thread when there is not enough space
to satisfy new memory request
A B C D
E
Current pointer
of free space
var E = new E();

11. GC overview - traverse
 During collection, GC locates the roots and traverses the live
reference graph
A
B
C
D
Current thread

12. GC overview – what is root
 Static property
 Reference on the stack of a managed thread
 Objects passed into the native platform and pinned*

13. GC – reclaiming memory
 GC removes the dead objects and eventually moves rest to
reduce fragmentation and to open space for new objects
A B C D
var E = new E();

14. GC – reclaiming memory
 GC removes the dead objects and eventually moves rest to
reduce fragmentation and to open space for new objects
A B D
var E = new E();

15. GC – reclaiming memory
 GC removes the dead objects and eventually moves rest to
reduce fragmentation and to open space for new objects
A B D
var E = new E();

16. GC – reclaiming memory
 GC removes the dead objects and eventually moves rest to
reduce fragmentation and to open space for new objects
A B D E
var E = new E();

17. GC in Mono

18. GC overview
 Mono uses GC similar to .NET one - SGen
• Some differences
– Partial API implementation
– Less generations

19. GC in Mono
 Offers partial implementation
• Collect
 Missing implementation or exceptions
• AddMemoryPressure
• RemoveMemoryPressure
• WaitForFullGCApproach
• WaitForFullGCComplete
• CancelFullGCNotification
• RegisterForFullGCNotification

20. Mono Generations
 Nursery
• Tend to hold primarily transient or temporary objects – return values,
strings, temp objects from methods
 Major
• Object that have survived collection – static or long live objects

21. Mono Generations
D E F G
HNursery
A C
Major

22. Mono Generations
E G
H
Nursery
A C
Major

23. Pause Time
 Each time the GC runs the app pauses
 The duration (pause time) it takes to do a collection depends on
• Type or generation of collection
• The size of the live graph
 Our target would be to minimize pause time and the numbers of
major GC

24. GC in Mono – how to track it
 Xamarin.Android – automatically in debug mode
 Xamarin.IOS – requires environment variables
• MONO_LOG_LEVEL
• MONO_LOG_MASK

25. 25© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.
Demo – tracking GC

26. Objects in Xamarin
 Some objects live in both the managed and the native world and
must be treated properly to ensure that they are freed from both
worlds.
Xamarin
Managed
Android.Graphics.
Image
IOS/Android
Native
class [UIButton]
Class
android.Graphics.
Image
UIKit.UIButtonBinding layer

27. Mono GC and Android GC

28. Xamarin.Android objects
 When using native objects Mono holds a strong reference (root)
to the object (peer) in native heap
public interface IJavaObject : IDisposable
{
IntPtr Handle { get; }
}
Mono
Managed
IJavaObject
JVM
Native
Java Object
Handle
(GC Root in Android)

29. Mono GC an Android GC
Mono
Managed
IJavaObject
JVM
Native
Java Object
Handle
(GC Root in Android)
Candidate for collection
Mono GCAndroid GC

30. Mono GC an Android GC
Mono
Managed
IJavaObject
JVM
Native
Java Object
WeakReference
Candidate for collection
Mono GCAndroid GC
Force native GC

31. Mono GC an Android GC
Mono
Managed
IJavaObject
WeakReference
Mono GCAndroid GC
IsAlive - NO

32. Mono GC an Android GC
Mono
Managed
IJavaObject
JVM
Native
Java Object
Handle
(GC Root in Android)
Mono GCAndroid GC
IsAlive YES

33. Pros/Cons
 Pros
• Resolves cyclical references automatically
 Cons
• More memory pressure
• Time consuming GC

34. Android Peers - samples
 Properties of peers
• Caches
• Additional functionality
 Overridden methods/Properties

35. Tips
 Reduce the graph of objects living in both worlds, especially the
short lived ones. (wrap or remove them).
 Avoid passing non-peer into Java (as peers are created)
 Since Mono is orchestrating the peer objects, consider calling
GC.Collect when you finish using small C# object that holds big
Java one
Mono
Managed
ImageDrawable
JVM
Native
ImageDrawable
Handle
(GC Root in Android)
~20 b> 1000 kb

36. 36© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.
Demo – peer objects

37. Mono GC and IOS

38. IOS – retain count
 IOS uses ARC
A
B
A A A
CB C
RC = 1 RC = 2 RC = 1 RC = 0

39. Peer objects
 Similar to Android, however increase the RC of a native object
instead
Mono
Managed
INativeObject
iOS
Native
iOS Object
Handle
(RC + 1)
public interface INativeObject
{
IntPtr Handle { get; }
}

40. IOS + Mono – framework peers
 All ios objects live as framework peers in mono.
Mono
Managed
UIKit.UIButton
iOS
Native
[UIButton]
Handle
(RC + 1)
PeerNative

41. IOS + Mono – framework peers
 All ios objects live as framework peers in mono.
• Built-in
• Stateless
• Created each time we create or access native object
• No Guaranteed that will keep one peer instance for the same native object
 RC is decremented if peer is disposed or finalized

42. IOS + Mono – User peers
 Also called derived objects
Mono
UIKit.UIButton
iOS
[UIButton] Handle
(RC + 1)
Mono
MyButton

43. IOS + Mono – User peers (2)
 Holds managed state (iOS does not know about them)
public class MyButton : UIButton
{
string IconPath { get; set; }
}

44. IOS + Mono – User peers + GC
 Holding managed reference to User Peer creates reference
cycles that cannot be broken automatically by GC
Mono
MyParentView
iOS
[UIView] Handle
(RC + 1)
Mono
MyButton
iOS
[UIButton]
(RC + 1)
Handle
(RC + 1)
Parent property

45. IOS + Mono – User peers + GC
 We need to manually break the cycle
Mono
MyParentView
iOS
[UIView] Handle
(RC + 1)
Mono
MyButton
iOS
[UIButton]
(RC + 1)
Handle
(RC + 1)
Parent property
Parent=null;
Dispose()

46. Promoting User peers
 Adding a state (e.g. event handler) promote framework peer to
user peer.
• We need to manually detach from the event
• Use WeakEventListener

47. IOS + Mono – User peers + GC
 Xamarin.IOS keeps user peers alive to preserve state
public class CustomRenderer: EntryRenderer
{
protected override void Dispose(bool disposing)
{
base.Dispose(disposing);
if(disposing && this.Control!=null)
{
this.Control.TouchDown -= OnTouchDown;
this.Control.Dispose();
}
}
}

48. Techniques for memory tracking

49. Memory Tracking tools
 VS profiler
 Android monitor
 Apple instruments
 Xamarin profiler

50. 50© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.
Demo - profiler

51. Memory Tracking – alternatives
 Weak references (native and Mono)
 Finalizers
 Do make sure the code is only executed in Debug

52. 52© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.© 2017 Progress Software Corporation and/or its subsidiaries or affiliates. All rights reserved.
Demo

53. UI for Xamarin
 Leave dealing with these native problems to us while focusing on
your app
 Save time and effort with ready to go toolkit
 Covering various scenarios – 20+ components
• Grid
• Chart
• ListView
• Calendar
• DataForm
• TabView

54. UI for Xamarin
 Visit our booth in the expo area