Re: [xmlvm-users] Mixing GC code with Objective-C code

[Arno P. <ar...@pu...>]

that will cause a problem. The GC only knows about global variables and 
memory that has been allocated with GC_MALLOC(). The 'notes' reference 
is part of an ObjC object that was allocated via alloc/init so the GC 
does not know about it, i.e., it will think that 'notes' can be 
collected. You need to store a reference to notes in some memory area 
that the GC knows about. We do this via the 'retain' policy in 
advisor.xml. One way you can do this is to declare a global variable 
that you initialize to a cross-compiled ArrayList and then add the 
reference to notes to this ArrayList. This is essentially what the 
retain policy does.

As I mentioned, this gets pretty complicated. :)


Arno


On 2/22/13 9:40 AM, Steve Hannah wrote:
> Thanks for the reply, Arno.  The paper you sent me was quite helpful
> on this subject also.  Both here, and in the paper, you talk about
> wrapping Objective-C objects with GC managed objects.  I'm curious
> about the inverse situation, where you are wrapping GC managed objects
> with Objective-C objects.
>
> E.g. consider this class definition:
>
> //
> //  ViewController.h
> //  XcodeGUIApp
> //
> //  Created by Steve Hannah on 2013-02-20.
> //  Copyright (c) 2013 Web Lite Solutions. All rights reserved.
> //
>
> #import <UIKit/UIKit.h>
> #import "xmlvm.h"
>
> @interface NotesListController : UIViewController
> <UITableViewDataSource, UITableViewDelegate>
> @property (retain, nonatomic) IBOutlet UITableView *tableView;
> @property JAVA_OBJECT notes;
> -(void)refreshNotes;
> -(void)viewDidAppear:(BOOL)animated;
> -(void)tableView:(UITableView *)tableView
> didSelectRowAtIndexPath:(NSIndexPath *)indexPath;
> @end
>
> (In particular, notice the
> @property JAVA_OBJECT notes;
> )
>
> Then at some point the "notes" property is set like this:
>
> self.notes = ca_weblite_crossmobile_shared_Note_getNotes__();
>
> Do I need to do anything special to let the GC know that the
> JAVA_OBJECT is being "used" in this structure?  I.e. is there a chance
> that the GC will free the notes object while the Objective-C class
> still needs it?
>
> Thanks again for all your help.
>
> -Steve
>
>
>
>
> On Thu, Feb 21, 2013 at 10:30 PM, Arno Puder <ar...@pu...> wrote:
>>
>> this is an extremely tricky subject. Yes, you have to be very careful
>> when mixing the two memory management models. I can only barely scratch
>> the surface but, in essence, the cross-compiled Java version of NSObject
>> becomes a wrapper around the native instance to an NSObject. When the GC
>> reclaims the cross-compiled NSObject, it will call a finalizer
>> (essentially overriding Object.finalize()) that will delegate the
>> release to the wrapped Objective-C instance. But as I said, it gets a
>> lot more complicated: associations between objects (e.g.
>> UIView.subviews) and delegates need special attention.
>>
>> Arno
>>
>>
>> On 2/21/13 7:49 PM, Steve Hannah wrote:
>>> I'm experimenting using XMLVM to compile business logic, which I then
>>> use inside a Cocoa GUI app - built with Xcode.  I am trying to get an
>>> understanding of the implications of XMLVM's garbage collected
>>> pointers used together with objective-c objects.
>>>
>>> This page <http://www.hpl.hp.com/personal/Hans_Boehm/gc/simple_example.html>
>>> says:
>>> "It is usually best not to mix garbage-collected allocation with the
>>> system malloc-free. If you do, you need to be careful not to store
>>> pointers to the garbage-collected heap in memory allocated with the
>>> system malloc."
>>>
>>> Does this mean that I shouldn't be storing JAVA_OBJECTs as properties
>>> of objective-c classes?  Are there any best practices that should be
>>> followed here?
>>>
>>> Thanks for any tips.
>>>
>>> -Steve
>>>
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>>
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>