Example of Blocks/Dispatch Queues in Obj-C

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Cower	Created : 11 October 2009 Edited : 11 October 2009 System : Mac Language : C# Example of Blocks/Dispatch Queues in Obj-C A simple implementation of a Range type in Obj-C that uses blocks and dispatch queues.
Some code to, hopefully, illustrate the use of blocks and dispatch queues (libdispatch) together in Objective-C. You should be able to use blocks in C if you grab Clang under Linux. I don't know if it works under Windows, I wouldn't be surprised if it did. However, libdispatch currently only works under Mac OS (since it's Apple tech), but given that it's open source, Linux may still get support for it. At any rate, this isn't code that I'd use in any sort of real-world situation, but it works pretty well for illustrating the use of blocks and at least one asynchronous method of libdispatch. Range.h #import <Cocoa/Cocoa.h> #import <dispatch/dispatch.h> @interface CRange : NSObject { NSInteger _begin; NSInteger _end; } #pragma mark Initialization -(id)initWithRange:(NSRange)range; -(id)initWithBeginning:(NSInteger)begin end:(NSInteger)end; #pragma mark - #pragma mark Properties @property(readonly) NSInteger begin; @property(readonly) NSInteger end; @property(readonly) NSUInteger length; #pragma mark - #pragma mark Iteration // iterate over each value between begin/end inclusive -(void)each:(void (^)(NSInteger index))iterator; // iterate over each value asynchronously (stride of 1 - each value will be queued separately) // uses the default global queue -(void)each_async:(void (^)(NSInteger))iterator; // iterate over each value asynchronously with a stride (every <stride> values will be queued separately) // uses the default global queue // stride allows for some optimization in the use of queues, so that you can // fine-tune how many jobs are queued rather than queueing a ton of them for // a large range -(void)each_async:(void (^)(NSInteger))iterator stride:(NSUInteger)stride; // iterate over each value asynchronously in a specific queue (stride of 1) -(void)each_async:(void (^)(NSInteger))iterator queue:(dispatch_queue_t)queue; // iterate over each value asynchronously with a stride in a specific queue -(void)each_async:(void (^)(NSInteger))iterator stride:(NSUInteger)stride queue:(dispatch_queue_t)queue; @end --v Range.m #import "Range.h" @implementation CRange #pragma mark - #pragma mark Properties @synthesize begin=_begin; @synthesize end=_end; @dynamic length; -(NSUInteger)length { return (_begin<_end ? _end-_begin : _begin-_end); } #pragma mark Initialization -(id)init { return [self initWithBeginning:0 end:0]; } -(id)initWithRange:(NSRange)range { // check for undefined ranges if (range.location == NSNotFound) return nil; self = [super init]; if ( self ) { _begin=(NSInteger)range.location; _end=_begin+(NSInteger)range.length; } return self; } -(id)initWithBeginning:(NSInteger)begin end:(NSInteger)end { self = [super init]; if ( self ) { _begin = begin; _end = end; } return self; } #pragma mark - #pragma mark Iteration -(void)each:(void (^)(NSInteger index))iterator { NSInteger i; if ( _begin <= _end ) for (i = _begin; i <= _end; ++i) iterator(i); else for (i = _begin; _end <= i; --i) iterator(i); } -(void)each_async:(void (^)(NSInteger))iterator { [self each_async:iterator stride:1 queue:dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)]; } -(void)each_async:(void (^)(NSInteger))iterator queue:(dispatch_queue_t)queue { [self each_async:iterator stride:1 queue:queue]; } -(void)each_async:(void (^)(NSInteger))iterator stride:(NSUInteger)stride { [self each_async:iterator stride:stride queue:dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)]; } // auxiliary function to get the number of iterations needed for an asynchronous, strided 'each' op size_t get_iterations(NSInteger begin, NSInteger end, NSInteger stride) { size_t iterations = begin < end ? end - begin : begin - end; NSUInteger abstride = (NSUInteger)stride; if ( end < begin ) stride = -1; iterations = (iterations+1)/abstride; if ((begin-end)%abstride) ++iterations; return iterations; } -(void)each_async:(void (^)(NSInteger))iterator stride:(NSUInteger)stride queue:(dispatch_queue_t)queue { if ( stride == 0 ) @throw [NSException exceptionWithName:NSInvalidArgumentException reason:@"Stride must be greater than zero" userInfo:nil]; // set some variables used by the blocks send to dispatch_apply NSInteger block_begin = _begin; NSInteger s_stride = stride; // signed stride (used to get reverse enumeration) // get the number of iterations needed for dispatch_apply size_t iterations = get_iterations(_begin, _end, &s_stride); if ( stride == 1 ) { // if stride is 1, just call the iterator block in the following block, // no need to account for stride except in multiplying the index. dispatch_apply(iterations, queue, ^(size_t i) { iterator(block_begin + is_stride); }); } else { // a value used by the following block NSInteger block_end = _end; // direction of the iteration (-1 starts at a higher number, 1 starts at a lower number) NSInteger dir = s_stride/(NSInteger)stride; dispatch_apply(iterations, queue, ^(size_t i) { // determine the startind value of this iteration NSInteger val = block_begin + is_stride; NSInteger x = 0; while (x++ < stride) { if ((dir == 1) ? (val > block_end) : (block_end > val)) break; iterator(val); val += dir; } }); } } @end --v main.m #import <Cocoa/Cocoa.h> #import "Range.h" int main( int argc, char ** argv ) { // setup the autorelease pool NSAutoreleasePool pool = [NSAutoreleasePool new]; // create and initialize a range that goes from 5 to 10 (and set it to be autoreleased) CRange range = [[[CRange alloc] initWithBeginning:5 end:10] autorelease]; // note: __block makes the variable mutable in blocks __block int someValueInTheFunction = 5; // set up a block void (^printNum)(NSInteger) = ^(NSInteger x) { // this block increments the value in the function and prints the index as well as that value NSLog(@"index:%d somevalue:%d", x, ++someValueInTheFunction); }; NSLog(@"Synchronous - Forwards"); // iterate over the values designated by the range synchronously [range each:printNum]; NSLog(@"Asynchronous - Backwards"); // iterate over the values designated by the range asynchronously // -- asynchronous iteration is a little more obvious when you use a // larger stride and decent sized range... range = [[[CRange alloc] initWithBeginning:25 end:1] autorelease]; [range each_async:printNum stride:7]; // cleanup [pool drain]; return 0; } --v

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Example of Blocks/Dispatch Queues in Obj-C

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