Nesting blocks in iOS - ios

Question

I'm trying to be able to call a 'completionHandler' block from inside another completionHandler block (called after an asynchronous URL request). This however results in the application crashing with the following message (I'm using Zombie objects):
*** -[CFRunLoopTimer hash]: message sent to deallocated instance
Using instruments I was able to find out that the problem is due to the block being deallocated but I can't figure out how to keep it retained for long enough. Is it because I'm calling the block from another asynchronous block? My code is below (MyCompletionHandler returns void and takes void):
-(void)requestWithRequest:(NSURLRequest*)request withCompletionHandler:(MyCompletionHandler)serverCompletionHandler{
[NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
// do stuff…
if (serverCompletionHandler) {
dispatch_async(dispatch_get_main_queue(), ^{
serverCompletionHandler();
});
}];
}
However this code is called through another method which supplies the serverCompletionHandler parameter (could this be the problem?).
So for example, the above method would be called by this method:
-(void)createAndSendRequestWithCompletionHandler:(MyCompletionHandler)serverCompletionHandler{
NSMutableURLRequest* request = //..
[self requestWithRequest:request withCompletionHandler:serverCompletionHandler];
}
Instruments shows that a block is either released or deleted (I assume the block I am calling) which would explain the deallocated object being called.
Any help would be appreciated.
EDIT
The code for the timer (which seems to be deallocated) is:
if ([timer isValid]) {
[timer invalidate];
}
timer = [NSTimer scheduledTimerWithTimeInterval:10 target:self selector:#selector(createAndSendRequestWithCompletionHandler:) userInfo:nil repeats:YES];
The confusing thing is the code worked fine until I added the completionHandlers.

Answer 1

No thats not a problem you can acess the block within another block.I think problem is that you are already on mainQueue ([NSOperationQueue mainQueue]) and you again try to getMainQueue on mainQueue.As sendAsynchronousRequest uses NSRunloop of queue it gets deallocated when you again ask for main queue as you are already on main queue.You can check if you are already on main queue just call serverCompletionHandler else dispatch on mainqueue.You can skip this check in this case as you are sure your are main queue and can just call serverCompletionHandler()
[NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
// do stuff…
if([[NSOperationQueue currentQueue] isEqual:[NSOperationQueue mainQueue]]){ //check for main queue
if (serverCompletionHandler) {
serverCompletionHandler();
}
}
else{
if (serverCompletionHandler) { if not than dispatch on main queue
dispatch_async(dispatch_get_main_queue(), ^{ //No need to do that
serverCompletionHandler();
});
}
}];
EDIT:Thanks of edited code.As you are using
timer = [NSTimer scheduledTimerWithTimeInterval:10 target:self selector:#selector(createAndSendRequestWithCompletionHandler:) userInfo:nil repeats:YES];
Now as you are not passing completionhandler so by doing this createAndSendRequestWithCompletionHandler: timer passes itself to your serverCompletionHandler.
So serverCompletionHandler itself contains timer object not any block object.If you try to NSLog serverCompletionHandler in requestWithRequest you will find it is timer object.Now when dispatch_async tries to call serverCompletionHandler as it is not block it will crash.
Write these two lines in createAndSendRequestWithCompletionHandler
NSLog(#"serverCompletionHandler obnj %#",serverCompletionHandler );
NSLog(#"class %#",NSStringFromClass([serverCompletionHandler class] ));
EDIT 2
if you really want to pass the completion handler than pass in userInfo of timer object.Use below code
#import "YourViewController.h"
typedef void (^MyCompletionHandler)(void);
#interface YourViewController ()
{
NSTimer *timer;
}
#end
#implementation YourViewController
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
if ([timer isValid]) {
[timer invalidate];
}
MyCompletionHandler com = ^{
NSLog(#"Hi this is completion handler");
};
timer = [NSTimer scheduledTimerWithTimeInterval:10 target:self selector:#selector(createAndSendRequestWithCompletionHandler:) userInfo:#{#"serverCompletionHandler":com} repeats:YES];
}
- (void)didReceiveMemoryWarning
{
[super didReceiveMemoryWarning];
// Dispose of any resources that can be recreated.
}
-(void)requestWithRequest:(NSURLRequest*)request withCompletionHandler:(MyCompletionHandler)serverCompletionHandler{
[NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
// do stuff…
if (serverCompletionHandler) {
dispatch_async(dispatch_get_main_queue(), ^{
serverCompletionHandler();
});
}
}];
}
-(void)createAndSendRequestWithCompletionHandler:(NSTimer *)timerObj{
// NSLog(#"serverCompletionHandler obnj %#",serverCompletionHandler );
// NSLog(#"class %#",NSStringFromClass([serverCompletionHandler class] ));
//get completion handler from `userInfo`
NSMutableURLRequest* request = [[NSMutableURLRequest alloc] init];
[self requestWithRequest:request withCompletionHandler:timerObj.userInfo[#"serverCompletionHandler"]];
}
#end