PickerView Delegate Causing Crash

Question

I am dismissing a modal view controller whom is the delegate of a UIPickerView.

When I dismiss the view using

    [self dismissViewControllerAnimated:YES completion:NULL];

My app crashes, but only when the UIPickerView is showing, and currently the first responder.

I found the the cause of the crash to be a Zombie, this method, [UIPicker _updateSelectedRows] is showing in my instruments as the issue.

I am able to fix the issue by setting the UIPicker delegate and dataSource to nil prior to dismissing the view controller.

I have never needed to do this before, is there something I'm missing?

Here is a bare bones of the presenting view, presented in a modal segue. When you dismiss this using the IBAction, it will cause the crash

@interface VTSecViewController () <UIPickerViewDataSource, UIPickerViewDelegate>

@property (strong, nonatomic) UIPickerView *catPicker;
@property (strong, nonatomic) NSArray *catItems;
@property (weak, nonatomic) IBOutlet UITextField *pickerTF;

@end

@implementation VTSecViewController

- (IBAction)dismpress:(id)sender
{
    [self dismissViewControllerAnimated:YES completion:NULL];
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.catItems = @[@"one", @"two"];
    self.catPicker = [[UIPickerView alloc] init];
    self.pickerTF.inputView = self.catPicker;
    self.catPicker.delegate = self;
    self.catPicker.dataSource = self;
    [self.pickerTF becomeFirstResponder];
    // Do any additional setup after loading the view.
}

#pragma mark PickerView DataSource

- (NSInteger)numberOfComponentsInPickerView: (UIPickerView *)pickerView
{
    return 1;
}

- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component
{
    return [self.catItems count];
}

- (void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component
{
    self.pickerTF.text = self.catItems[row];
}

- (NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component
{
    return self.catItems[row];
}


@end

This post helps clear up a few things

Answer 1

delegate (in this case your view controller) is supposed to give the information to pickerview on things like titles / row count / whattodo when you tap etc.. when you dismiss the controller while picker is still on the view. pickerview does not know that your controller died & it will keep asking the delegate about those information, hence -> crash.

when you set the delegate & datasource to nil. it will let know the pickerview that your delegate does not exist anymore so you wont get the crash.

sidenote : go to the delegate & datasource property in documentation it shows

@protocol UIPickerViewDataSource, UIPickerViewDelegate;

NS_CLASS_AVAILABLE_IOS(2_0) @interface UIPickerView : UIView <NSCoding, UITableViewDataSource>

@property(nonatomic,assign) id<UIPickerViewDataSource> dataSource;                // default is nil. weak reference
@property(nonatomic,assign) id<UIPickerViewDelegate>   delegate;                  // default is nil. weak reference

you can see the properties are "assign" (weak) and not "retain" (strong). hence when you dismiss the delegate & datasource uipickerview does not make any impact on the reference count of the delegate & it deallocates. it would have been a different case if this had been retain / strong. you should probably check out memory management topic on apple documentation on how these work. https://developer.apple.com/library/mac/documentation/general/conceptual/devpedia-cocoacore/MemoryManagement.html. delegates are weak references almost all the time.