iPhone EXC_BAD_ACCESS when using blocks - iphone

Question

Im trying to create a simple callback using blocks.
I have a MainViewController which addSubView another DatePickerViewController.view i created a block like this
typedef void(^DateChangedBlock)(NSDate*);
And i have a method on my DatePickerViewController called
setOnDateChangedCallback:(DateChangedBlock)callback
I store the callback in a property of the DatePickerViewController. The DatePickerViewController's view is a UIDatePicker instance, ive bound an IBAction to the value changed to a method that does this.
- (IBAction)dateChanged:(id)sender {
if (dateChangedCallback != nil)
{
dateChangedCallback(nil);
}
}
Here is how i register the block in the MainViewController
DatePickerViewController *dateController = [[DatePickerViewController alloc] initWithNibName:#"DatePickerView" bundle:nil];
self.datePicker = dateController;
UITextView *textView = [[UITextView alloc] initWithFrame:CGRectMake(0, 0, 100, 200)];
[self.view addSubview:textView];
DateChangedBlock myBlock = ^(NSDate *newDate) {
textView.text = #"testing";
};
[self.datePicker setOnDateChanged: myBlock];
[self.datePicker dateChanged:self]; // force trigger (this works).
When i force trigger the dateChanged method on the DatePickerViewController it works no problem. But when the datepicker itself triggers the method thru the IBAction i get a EXC_BAD_ACCESS error. The error occurs in this method on the "int retVal" line.
#import <UIKit/UIKit.h>
int main(int argc, char *argv[])
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
int retVal = UIApplicationMain(argc, argv, nil, nil); // THREAD 1: program received EXC_BAD_ACCCESS.**
[pool release];
return retVal;
}
What am i doing wrong?

Answer 1

You should copy your block when passing it to other method (such as attribute setter in your situation). So when setting the callback block do this:
[self.datePicker setOnDateChanged:[[myBlock copy] autorelease]];
You get the EXC_BAD_ACCESS cause block's variables used when creating the block don't get retained by the block itself. So when calling the block - variables do not exist anymore.

Answer 2

The accepted answer is wrong. You don't need to copy it in your MainViewController. Rather, the setOnDateChangedCallback: method which takes a block argument is responsible for copying it if it needs to store it in an instance variable (which I see it is doing, in the variable dateChangedCallback that is later used by another method). If dateChangedCallback is a synthesized property, you can accomplish this by declaring the property as copy instead of retain.

Answer 3

You can only update the UI from the UI thread. Since your block is running on another thread, you're getting the exception when updating the text of the textView. You can run code on the UI thread from within a block by using the main queue (dispatch_get_main_queue()).