Question

I create a project as Single View Application with initial ViewController; add subViewController to project. UIImageView with pan gesture are added in both of the view controllers.
It works in viewController.m,
but when it is added to subViewController and the subview is added as subview into view controller, the program just crash with “Exc_bad_access”..

Anyone has some solution?

here is my code:

ViewController.m

#import "ViewController.h"
#import "SubViewController.h"

//#define SUB

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

#ifdef SUB
    SubViewController *sb = [[SubViewController alloc] init];
    [self.view addSubview:sb.view];
#else
    UIImageView* img_ = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"monkey_1.png"]];
    img_.userInteractionEnabled = YES;
    UIPanGestureRecognizer *stampPanGesture = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(handlePan:)];
    [stampPanGesture setMinimumNumberOfTouches:1];
    [stampPanGesture setMaximumNumberOfTouches:1];
    [img_ addGestureRecognizer:stampPanGesture];
    img_.center = self.view.center;
    [self.view addSubview:img_];
#endif
}


- (void)handlePan:(UIPanGestureRecognizer *)recognizer {
    CGPoint translation = [recognizer translationInView:self.view];
    recognizer.view.center = CGPointMake(recognizer.view.center.x + translation.x,
                                         recognizer.view.center.y + translation.y);
    [recognizer setTranslation:CGPointMake(0, 0) inView:self.view];

}

SubViewController.m

#import "SubViewController.h"
@interface SubViewController ()

@end

@implementation SubViewController

- (id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
{
    self = [super initWithNibName:nibNameOrNil bundle:nibBundleOrNil];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{
    [super viewDidLoad];


    UIImageView* img_ = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"monkey_1.png"]];
    img_.userInteractionEnabled = YES;
    UIPanGestureRecognizer *stampPanGesture = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(handlePan:)];
    [stampPanGesture setMinimumNumberOfTouches:1];
    [stampPanGesture setMaximumNumberOfTouches:1];
    [img_ addGestureRecognizer:stampPanGesture];
    img_.center = self.view.center;
    [self.view addSubview:img_];
}


- (void)handlePan:(UIPanGestureRecognizer *)recognizer {
    CGPoint translation = [recognizer translationInView:self.view];
    recognizer.view.center = CGPointMake(recognizer.view.center.x + translation.x,
                                         recognizer.view.center.y + translation.y);
    [recognizer setTranslation:CGPointMake(0, 0) inView:self.view];

}

Answer 1

Assuming this is ARC code, the problem is that you’re creating subview’s view controller, grabbing it’s view, but then letting the controller itself fall out of scope and be deallocated.

The immediate fix would be to make SubViewController *sb an instance variable of the main view controller’s class, that way it won’t go out of scope and won’t be deallocated behind you.

Thing is, while that will probably fix your crash, the bigger picture problem is that you really shouldn’t be grabbing a view controller’s view and adding it as a subview, but doing nothing with the controller itself. If you do that, for example, various things might not work properly (e.g. rotation events will never reach your subview; anything that iOS needs to communicate to view controllers might not be received by your subview’s controller; I don’t know what in the world didReceiveMemoryWarning situations would do, etc.). In short, Apple advises against this, and encourages you to keep your view hierarchy and your view controller hierarchy synchronized.

Thus, if you’re transitioning to a new view, you really should just do it via pushViewController or presentViewController. In the unlikely event that this is really a subview (e.g. parts of the prior view will stay on screen and the child view will only take up a portion of the screen), you can use View Controller Containment. You can see WWDC 2011 session 102 for more information on containment. But that’s undoubtedly overkill for what you’re trying to do. Pushing/presenting the subview is probably the logical solution.