Question

This is my first time using IB, but after spending a one or two intimate days with it I believe I’m beginning to understand it. That’s just my way of saying I might be overlooking something simple here:

I’ve set up a UIPickerView and joined it to its DataSource and Delegate object in IB (both different Classes in my case). This allows the picker to show up when I run the app, which is very encouraging when it hasn’t been showing up in any previous test runs. 😉 However, when I scroll the UIPickerView, the program crashes, and I can’t find any of my code referenced in the backtrace. After quite a bit of troubleshooting, I think I’ve narrowed down the crash to two distinct cases, as far as the backtrace is concerned:

the return value of -pickerView:numberOfRowsInComponent: > the number of rows displayed

• The app crashes as soon as a motion is begun to select a new row
• The app crashes if I try to use -selectRow:inComponent:animated:

backtrace (ignoring main):

#0  0x955e8688 in objc_msgSend ()
#1  0x0167bea8 in -[UIPickerView table:cellForRow:column:reusing:] ()
#2  0x016773c1 in -[UIPickerView table:cellForRow:column:] ()
#3  0x017fef53 in -[UITable createPreparedCellForRow:column:] ()
#4  0x018077c8 in -[UITable _updateVisibleCellsNow] ()
#5  0x018027cf in -[UITable layoutSubviews] ()
#6  0x03ac42b0 in -[CALayer layoutSublayers] ()
#7  0x03ac406f in CALayerLayoutIfNeeded ()
#8  0x03ac38c6 in CA::Context::commit_transaction ()
#9  0x03ac353a in CA::Transaction::commit ()
#10 0x03acb838 in CA::Transaction::observer_callback ()
#11 0x007b8252 in __CFRunLoopDoObservers ()
#12 0x007b765f in CFRunLoopRunSpecific ()
#13 0x007b6c48 in CFRunLoopRunInMode ()
#14 0x000147ad in GSEventRunModal ()
#15 0x00014872 in GSEventRun ()
#16 0x0168a003 in UIApplicationMain ()

the return value of -pickerView:numberOfRowsInComponent: < the number of rows displayed

• The app crashes after the motion ceases and the row is selected
• The app does not crash if I try to use -selectRow:inComponent:animated:

backtrace (ignoring main):

#0  0x955e8688 in objc_msgSend ()
#1  0x0167700d in -[UIPickerView _sendSelectionChangedForComponent:] ()
#2  0x017f4187 in -[UIScroller _scrollAnimationEnded] ()
#3  0x016f732c in -[UIAnimator stopAnimation:] ()
#4  0x016f7154 in -[UIAnimator(Static) _advance:] ()
#5  0x00017739 in HeartbeatTimerCallback ()
#6  0x007b7ac0 in CFRunLoopRunSpecific ()
#7  0x007b6c48 in CFRunLoopRunInMode ()
#8  0x000147ad in GSEventRunModal ()
#9  0x00014872 in GSEventRun ()
#10 0x0168a003 in UIApplicationMain ()

My delegate and datasource implementations follow:

- (NSInteger)numberOfComponentsInPickerView:(UIPickerView *)pickerView {
    return (NSInteger)3;
}

- (NSInteger)pickerView:(UIPickerView *)pickerView numberOfRowsInComponent:(NSInteger)component {
    return (NSInteger)4;
}

- (NSString *)pickerView:(UIPickerView *)pickerView titleForRow:(NSInteger)row forComponent:(NSInteger)component {
  //it will probably be better to use the method following when creating the rows, so I can better customize it 
    return @"strings";
}

- (void)pickerView:(UIPickerView *)pickerView didSelectRow:(NSInteger)row inComponent:(NSInteger)component {
    NSLog(@"selected a row");
}

Answer 1

Investigated Apple documentation a bit and it proved my previous guess. From Resource Programming Guide:

Objects in the nib file are created
with a retain count of 1 and then
autoreleased. As it rebuilds the
object hierarchy, however, UIKit
reestablishes connections between the
objects using the setValue:forKey:
method, which uses the available
setter method or retains the object by
default if no setter method is
available. If you define outlets for
nib-file objects, you should also
define a setter method for accessing
that outlet. Setter methods for
outlets should retain their values,
and setter methods for outlets
containing top-level objects must
retain their values to prevent them
from being deallocated. If you do not
store the top-level objects in
outlets, you must retain either the
array returned by the
loadNibNamed:owner:options: method or
the objects inside the array to
prevent those objects from being
released prematurely.

So the top level objects are created autoreleased and you must retain them in your code. There’s also described recommended way to handle that:

For both Mac OS X and UIKit, the
recommended way to manage the
top-level objects in a nib file is to
create outlets for them in the File’s
Owner object and then define setter
methods to retain and release those
objects as needed. Setter methods give
you an appropriate place to include
your memory-management code, even in
situations where your application uses
garbage collection. One easy way to
implement your setter methods is to
use the @property syntax and let the
compiler create them for you.

I’ve tested this approach in a sample code – defined outlets for delegate and data source objects in file owner class and connected them in IB. And in file owner class defined a property for those outlets:

@property (nonatomic, retain) NSObject<UIPickerViewDelegate>* myDelegate;
@property (nonatomic, retain) NSObject<UIPickerViewDataSource>* mySource;

Worked fine.