multiple return values in callback blocks - ios

Question

people I am working with don't wanna use delegate methods for a couple of reasons and they want me to come up with a solution;
basically they want a callback block with multiple return values, and when the first return value arrived block should still wait for the upcoming return values
something like
[parserobject parsefile:file withCallback {
if (started) {
//file parsing started
//this should get executed first
}
//if file parsing succesfully started also check if it
//succesfully finished or failed in the same callback
if (finished) {
//file parsing finished
}
if(failed)
{
//this represents if parsing failed any time from start to finish
//if so exit the callback
}
}];
I have seen people using structs or nsintegers to return different values but usually only one result is returned....
is that above code block possible to written in objective-c?

Answer 1

This is your function
typedef void(^VOIDCALLBACK)(void); //Just a type here
typedef void(^BOLLCALLBACK)(bool succeed);//Just a type here
-(void)parserobjectWithFile:(NSString *)file StartCallBack:(VOIDCALLBACK) startCallBack completionBlock:(BOLLCALLBACK) completion{
// When start
dispatch_async(dispatch_get_main_queue(), ^{
startCallBack();//Make call back run in main queue
});
//When finisehd set succeed to yes,otherwise set to no
BOOL succeed = YES;//Pass in to indicator if succeed
dispatch_async(dispatch_get_main_queue(), ^{
completion(succeed); //Make call back run in main queue
});
}
Then you call it like this
[self parserobjectWithFile:#""
StartCallBack:^{
//Here task is start
}
completionBlock:^(bool succeed) {
//Here task is done.Check succeed to figure out if succeed
}];
A simple example
This is the function part
#implementation ViewController
-(void)viewDidAppear:(BOOL)animated{
[self parserobjectWithFile:#"" StartCallBack:^{
NSLog(#"start");
} completionBlock:^(bool succeed) {
NSLog(#"finished");
}];
}
typedef void(^VOIDCALLBACK)(void); //Just a type here
typedef void(^BOLLCALLBACK)(bool succeed);//Just a type here
-(void)parserobjectWithFile:(NSString *)file StartCallBack: (VOIDCALLBACK) startCallBack completionBlock:(BOLLCALLBACK) completion{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
//Start
dispatch_async(dispatch_get_main_queue(), ^{
startCallBack();//Make call back run in main queue
});
sleep(2.0);
BOOL succeed = YES;//Pass in to indicator if succeed
dispatch_async(dispatch_get_main_queue(), ^{
completion(succeed); //Make call back run in main queue
});
});
}
#end
This will output
2015-06-11 23:39:25.426 OCTest[767:144615] start
2015-06-11 23:39:27.426 OCTest[767:144615] finished

Answer 2

It is possible to write a block that does different things depending on the state reached so far. In order for that to work, declare variables that represent block's state with the __block modifier:
__block BOOL started = NO;
__block BOOL finished = NO;
__block BOOL failed = NO;
[parserobject parsefile:file withCallback:^ {
if (started) {
//file parsing started
//this should get executed first
} else {
...
// Conditionally set started to YES
started = YES;
}
//if file parsing succesfully started also check if it
//succesfully finished or failed in the same callback
if (finished) {
//file parsing finished
}
if(failed) {
//this represents if parsing failed any time from start to finish
//if so exit the callback
}
}];