Matt Rajca

Archimedes for Mac uses (WebKit 1) WebViews to render live previews of documents. While working on improved cache management for the next release of Archimedes, I noticed WebView allocations were not going down as documents were being closed. What’s more, the PreviewViewControllers that own the WebViews were also not being deallocated.

I had a hunch retain cycles were to blame, so I launched Instruments and ran the Allocations instrument with the Record reference counts option turned on. Immediately, it became clear what was happening. PreviewViewController, which maintains a strong reference to the WebView, was being exported to JavaScriptCore as a JSExport as such:

- (void)webView:(WebView *)webView didClearWindowObject:(WebScriptObject *)windowObject forFrame:(WebFrame *)frame {
	frame.javaScriptContext[@"archimedes"] = self;
}

Here, JavaScriptCore was creating a strong reference back to the PreviewViewController, thus creating a strong reference cycle.

I tried unloading the page right before the document closes hoping the WebView would relinquish control of the PreviewViewController, but it did not work as I expected.

Next, instead of passing the entire view controller to JavaScriptCore, I decided to create a new class whose sole purpose is to relay messages to a weakly-referenced delegate object:

@protocol PreviewViewMessengerDelegate
- (void)previewViewMessengerWantsToGoToRange:(NSRange)range;
@end


@interface PreviewViewMessenger : NSObject <PreviewViewExport>
@property (nonatomic, weak) id <PreviewViewMessengerDelegate> delegate;
@end


@implementation PreviewViewMessenger

- (void)goToRange:(int)location length:(int)length {
	[self.delegate previewViewMessengerWantsToGoToRange:NSMakeRange(location, length)];
}

@end

And the JavaScriptContext was now set up as follows:

- (void)webView:(WebView *)webView didClearWindowObject:(WebScriptObject *)windowObject forFrame:(WebFrame *)frame {
	PreviewViewMessenger *messenger = [[PreviewViewMessenger alloc] init];
	messenger.delegate = self;
	frame.javaScriptContext[@"archimedes"] = messenger;
}

Now, while JavaScriptCore still retains the proxy object, it no longer retains our PreviewViewController since it is weakly-referenced. This allows the WebView to get destroyed when the document is closed, and a moment later the PreviewViewMessenger disappears along with it. As the saying by David Wheeler goes, maybe all problems in computer science really can be solved by adding a level of indirection. 😊