passing ManagedObjectContext via prepareforsegue

问题

First of all, this is my basic setup. I'm trying to pass a NSManagedObjectContext (MOC) from my AppDelegate to the selected custom ViewController.

First, in "AppDelegate.m", I do:

UINavigationController *navigationController = (UINavigationController *)self.window.rootViewController;
FirstTableViewController *tableVC = (FirstTableViewController *)navigationController.topViewController;
tableVC.managedObjectContext = self.managedObjectContext;

to pass the MOC to the tableViewController which is in between the navigationController and the custom ViewController.

This causes no errors so far.

However, in the tableViewController "FirstTableViewController.m", I then want to pass the MOC onto the custom ViewController using prepareforsegue:

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([[segue identifier] isEqualToString:@"mapClicked"]) {
        CustomScrollViewController *customSVC = [segue destinationViewController];
        NSManagedObjectContext *context = self.managedObjectContext;

        [customSVC setManagedObjectContext:context];
    }
}

This then calls the following method in the custom ViewController "CustomScrollViewController.m":

- (void)setManagedObjectContext:(NSManagedObjectContext *)context
{
    self.managedObjectContext = context;
}

and this is where it gets stuck. It seems to perform the method over and over again, ( see here ) and then crashes.

If you need to look at more code, here is the github repository

Any help is appreciated!

回答1:

You probably don't need the custom setter method setManagedObjectContext at all, because property accessor methods are created automatically by the compiler, if necessary.

But if you use a custom setter, it must access the instance variable directly inside the setter:

- (void)setManagedObjectContext:(NSManagedObjectContext *)context
{
    _managedObjectContext = context;
}

The reason is that

self.managedObjectContext = context;

is translated by the compiler to

[self setManagedObjectContext:context];

and there you have the recursion.

回答2:

This code contains your problem:

- (void)setManagedObjectContext:(NSManagedObjectContext *)context
{
    self.managedObjectContext = context;
}

You should simply synthesize your properties. This code will in fact result in this:

- (void)setManagedObjectContext:(NSManagedObjectContext *)context
{
    [self setManagedObjectContext:context];
}

So you se the recursion? So either synthesise, or, if you really want to implement this yourself: (I am assuming you use ARC, and that there is an instance variable called _context.

- (void)setManagedObjectContext:(NSManagedObjectContext *)context
{
    _context = context;
}

Also, if you are implementing you own getter, this should be it:

- (NSManagedObjectContext *) managedObjectContext{
    return _context;
}

来源：https://stackoverflow.com/questions/19831394/passing-managedobjectcontext-via-prepareforsegue

标签

ios

objective-c

segue

nsmanagedobjectcontext