Question

I am having a very, very strange error, probably related to memory management (even though I’m using ARC).

I have a my AppDelegate, Foo, and SubFoo (which is a subclass of Foo).

Foo.h

@protocol FooDelegate <NSObject>

- (void)didReceiveDownloadRequest:(NSURLRequest *)downloadRequest;

@end

@interface Foo : NSObject {
    __weak id <FooDelegate> delegate;
}

- (void)performRequest;

@property (nonatomic, weak) id <FooDelegate> delegate;
@property (nonatomic, retain) NSString *fileIdentifier;

Foo.m

@implementation Foo

@synthesize delegate, fileIdentifier;

- (id)init {
    if ((self = [super init])) {
        self.delegate = nil; // I tried leaving this line out, same result.
        NSLog(@"I am %p.", self);
    }

    return self;
}

- (void)performRequest {
    // Bah.
}

@end

SubFoo.h

@interface SubFoo : Foo {
    WebView *aWebView;
}

SubFoo.m

- (void)performRequest {
    if (self.fileIdentifier) {
        aWebView = [[WebView alloc] init];
        [aWebView setFrameLoadDelegate:self];
        [[aWebView mainFrame] loadRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:@"theURL"]];
    }
}

- (void)webView:(WebView *)sender didFinishLoadForFrame:(WebFrame *)frame {
    NSLog(@"Finished loading.");

    // ...

    NSLog(@"Class Name: %@", NSStringFromClass([self class]));
    NSLog(@"Memory Location of delegate: %p", self.delegate);

    // ...
}

Sometimes, the class name on webView:didFinishLoadForFrame: returns a completely different class (instead of SubFoo, it returns random classes, like NSSet, NSArray, it even sometimes returns CFXPreferencesSearchListSource), other times it just crashes there with an EXC_BAD_ACCESS, and when it returns a random class on Class Name: it returns that [randomClassName delegate] is an unrecognized selector.

EDIT: When self gets set to another thing, it gets set RIGHT on webView:didFinishLoadForFrame:, and on performRequest it is ALWAYS SubFoo.

Any help here would be appreciated.

Answer 1

First, even though you are using ARC zeroing weak references in your project (@property (weak)), other projects and frameworks may not be (and are probably not) using zeroing weak references.

In other words, assume that all delegates in frameworks are __unsafe_unretained unless:

1. The delegate property is declared weak in a header
2. The documentation/header explicitly states otherwise

---

That said, let’s talk about your example. Your object ownership chart looks something like this:

(Note: I’m not entirely sure which class in your project uses SubFoo. Based on common practice, I’m assuming that you have a class with a strong reference to SubFoo, and that class is also set up to be a SubFooDelegate)

Ultimately, your instance of SubFoo is losing its last strong reference and is deallocating. In a perfect ARC-enabled world, the WebView’s pointer to SubFoo would nil out at this time. However, it’s not a perfect world yet, and WebView’s frameLoadDelegate is __unsafe_unretained. Due to run loop interaction, the WebView is outliving SubFoo. The web request completes, and a dead pointer is dereferenced.

To fix this, you need to call [aWebView setFrameLoadDelegate:nil]; in SubFoo’s dealloc method. You also need to call it when you reassign aWebView, as you are losing track of the old aWebView:

SubFoo.m

@implementation SubFoo

- (void)dealloc {
    [aWebView setFrameLoadDelegate:nil];
    // Also nil out any other unsafe-unretained references
}

- (void)performRequest {
    if (self.fileIdentifier) {
        [aWebView setFrameLoadDelegate:nil]; // Protects us if performRequest is called twice.  Is a no-op if aWebView is nil
        aWebView = [[WebView alloc] init];
        [aWebView setFrameLoadDelegate:self];
        [[aWebView mainFrame] loadRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:@"theURL"]];
    }
}

- (void)webView:(WebView *)sender didFinishLoadForFrame:(WebFrame *)frame {
    // ...
}