AGEnqueueCoalescedBlock.m

//
//  AGEnqueueCoalescedBlock.h
//  EnqueuedBlocks
//
//  Created by Seth Willits on 2/26/15.
//  Copyright (c) 2015 Araelium Group. All rights reserved.
//

#import <Foundation/Foundation.h>

//! Executes the block on the given queue immediately if no block with 'key' is running. Otherwise, this block is enqeued,
//! replacing any previously-enqueued block with the same 'key'. Once the running block finishes, the enqueued block executes (if there is one).
void AGEnqueueCoalescedBlock(void * key, dispatch_queue_t queue, void (^block)(void));

//! Removes an enqueued block with the given key, preventing it from running.
void AGCancelEnqueuedBlock(void * key);





//
//  AGEnqueueCoalescedBlock.m
//  EnqueuedBlocks
//
//  Created by Seth Willits on 2/26/15.
//  Copyright (c) 2015 Araelium Group. All rights reserved.
//

#import "AGEnqueueCoalescedBlock.h"


static void AGEnqueueCoalescedBlock_Init(void);
static void AGEnqueue_runEnqueuedBlockForKey(void * key);
static void AGEnqueue_doneRunningEnqueuedBlockForKey(void * key);

static dispatch_queue_t       __AGEnqueueQueue;
static NSMutableDictionary *  __AGEnqueueTable;
static NSMutableSet *         __AGEnqueueRunningSet;




#pragma mark -
#pragma mark Public

void AGEnqueueCoalescedBlock(void * key, dispatch_queue_t queue, void (^block)(void))
{
    if (key == NULL)  [NSException raise:NSInvalidArgumentException format:@"AGEnqueueCoalescedBlock key cannot be AGAllEnqueuedBlocks"];
    if (queue == nil) [NSException raise:NSInvalidArgumentException format:@"AGEnqueueCoalescedBlock queue cannot be nil"];
    if (block == nil) [NSException raise:NSInvalidArgumentException format:@"AGEnqueueCoalescedBlock block cannot be nil"];
    
    AGEnqueueCoalescedBlock_Init();
    dispatch_sync(__AGEnqueueQueue, ^{
        
        // Insert/Update entry
        [__AGEnqueueTable setObject:@[[[block copy] autorelease, (id)queue]] forKey:[NSValue valueWithPointer:key]];
    
        // Run if needed
        BOOL running = [__AGEnqueueRunningSet containsObject:[NSValue valueWithPointer:key]];
        if (!running) {
            AGEnqueue_runEnqueuedBlockForKey(key);
        }
    });
}


void AGCancelEnqueuedBlock(void * key)
{
    AGEnqueueCoalescedBlock_Init();
    dispatch_sync(__AGEnqueueQueue, ^{
        [__AGEnqueueTable removeObjectForKey:[NSValue valueWithPointer:key]];
    });
}




#pragma mark -
#pragma mark Private

static void AGEnqueueCoalescedBlock_Init(void)
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        __AGEnqueueQueue = dispatch_queue_create(NULL, DISPATCH_QUEUE_SERIAL);
        __AGEnqueueTable = [[NSMutableDictionary alloc] init];
        __AGEnqueueRunningSet = [[NSMutableSet alloc] init];
    }); 
}



// Can only be called from __AGEnqueueQueue and assumes
// there is no block already running for the given tag
static void AGEnqueue_runEnqueuedBlockForKey(void * key)
{
    NSArray * info = [[__AGEnqueueTable objectForKey:[NSValue valueWithPointer:key]] retain];
    if (info) {
        void (^block)(void) = [info objectAtIndex:0];
        dispatch_queue_t queue = (dispatch_queue_t)[info objectAtIndex:1];
        
        [__AGEnqueueTable removeObjectForKey:[NSValue valueWithPointer:key]];
        [__AGEnqueueRunningSet addObject:[NSValue valueWithPointer:key]];
        dispatch_async(queue, ^{
            block();
            AGEnqueue_doneRunningEnqueuedBlockForKey(key);
        });
        [info release];
    }
}



// Could be called from any thread
static void AGEnqueue_doneRunningEnqueuedBlockForKey(void * key)
{
    dispatch_sync(__AGEnqueueQueue, ^{
        [__AGEnqueueRunningSet removeObject:[NSValue valueWithPointer:key]];
        AGEnqueue_runEnqueuedBlockForKey(key);
    });
}