block_copy when to use

Question

When to copy a block? The document says, blocks are "deleted when execution returns from the scope in which they are defined.This means you can’t return them directly from a function. If blocks could only be used while their defining scope was still on the call stack, they wouldn’t be nearly as useful as they actually are"

So, here is code which I tried, hoping the block will be deleted once execution is completed in viewDidLoad.

MyReaderController.h

@interface MyReaderController : UIViewController
{
    myBlockVar aBlockVar;
}
-(myBlockVar) getABlock;
@end

MyReaderController.m

@implementation MyReaderController
- (void)viewDidLoad
{
    [super viewDidLoad];
    aBlockVar=[self getABlock];
    NSLog(@"Block Result = %f",aBlockVar(1));
}
-(void) viewDidDisappear:(BOOL)animated{
    [super viewDidDisappear:animated];
    NSLog(@"Block Exists even after the execution completes=%@ %f",aBlockVar,aBlockVar(5));
}

-(myBlockVar) getABlock{
    return ^(int var){return 4.0f;};
}

@end

So, does this code require viewDidLoad to be changed to as coded below, if not then when should I use it.

- (void) viewDidLoad{
    [super viewDidLoad];
    aBlockVar=Block_copy([self getABlock]);
    NSLog(@"Block Result = %f",aBlockVar(1));
}

PART 2

Later on I tried with this following code, hoping now it will return aBlockVar as nil obj in viewDidDisappear.

- (void)viewDidLoad
{
    [super viewDidLoad];
    Blocker *blocker=[[Blocker alloc] init];
    myBlockVar myVar=[blocker getABlock];
    aBlockVar=myVar;
    NSLog(@"Block Result = %f",aBlockVar(1));
    blocker=nil;
    myVar=nil;
}

Blocker.m

#import "Blocker.h"

@implementation Blocker

-(myBlockVar) getABlock{
    return ^(int var){return 4.0f;};
}
@end

Answer 1

Are you using ARC? If so, you don't need to use Block_copy or Block_release.

If you are, then you are correct with your revised code, as Block_copy takes it off the stack and into the heap where it is has an effective retain count of 1. You would also need to call Block_release where appropriate, when finally finished with the block, to bring its balance the copy, effectively bringing the retain count back to 0.

Answer 2

use @property (nonatomic, copy) (int)(^myBlock)(void);

let the system do all right memory management for you!

initialize:

self.myBlock = ^int(void){
    return 4.0;
};

if you want to destroy your block somewhere do self.myBlock = NULL;