GCD queues and dispatch method were extracted to properties to facilitate unit testing

Dispatcher.m

#import <XCTest/XCTest.h>
#import "Dispatcher.h"
#import <ReactiveObjC/ReactiveObjC.h>

@interface Dispatcher ()

@property(nonatomic, assign) void (*dispatchMethod)(dispatch_queue_t, dispatch_block_t);
@property(nonatomic, strong) dispatch_queue_t queue;

@end

@implementation Dispatcher

- (instancetype)initWithQueue:(dispatch_queue_t)queue dispatchMethod:(void(*)(dispatch_queue_t, dispatch_block_t))dispatchMethod {
    self = [super init];
    if (self) {
        _queue = queue;
        _dispatchMethod = dispatchMethod;
    }
    return self;
}

- (void)dispatch:(void(^)(void))block {
    self.dispatchMethod(self.queue, block);
}

@end
  
@interface DispatcherTest : XCTestCase

@property (nonatomic, strong) Dispatcher *dispatcher;
@property (nonatomic, strong) dispatch_queue_t queue;
@property (nonatomic, assign) void (*dispatchMethod)(dispatch_queue_t, dispatch_block_t);

@end

@implementation DispatcherTest

static BOOL dispatchGetsCalled = NO;
void dispatchMethod(dispatch_queue_t queue, DISPATCH_NOESCAPE dispatch_block_t block) {
    dispatchGetsCalled = YES;
    block();
}

- (void)setUp {
    [super setUp];
    _queue = dispatch_queue_create("mock queue", NULL);
    XCTAssertFalse(dispatchGetsCalled);
}

- (void)tearDown {
    _dispatcher = nil;
    _queue = nil;
    _dispatchMethod = nil;
    dispatchGetsCalled = NO;
    [super tearDown];
}

- (void)testDispatchSync {
    _dispatchMethod = &dispatch_sync;
    _dispatcher = [[Dispatcher alloc] initWithQueue:self.queue dispatchMethod:self.dispatchMethod];
    
    XCTestExpectation *expectation = [self expectationWithDescription:NSStringFromSelector(_cmd)];

    @weakify(self);
    [self.dispatcher dispatch:^{
        @strongify(self);
        [self assertQueue];
        [expectation fulfill];
    }];
    [self waitForExpectations:@[expectation] timeout:0.1];
}

- (void)testDispatchAsync {
    _dispatchMethod = &dispatch_async;
    _dispatcher = [[Dispatcher alloc] initWithQueue:self.queue dispatchMethod:self.dispatchMethod];
    XCTestExpectation *expectation = [self expectationWithDescription:NSStringFromSelector(_cmd)];
    
    @weakify(self);
    [self.dispatcher dispatch:^{
        @strongify(self);
        [self assertQueue];
        [expectation fulfill];
    }];
    [self waitForExpectations:@[expectation] timeout:0.1];
}

- (void)testDispatcherUsesDispatchMethod {
    _dispatchMethod = &dispatchMethod;
    _dispatcher = [[Dispatcher alloc] initWithQueue:self.queue dispatchMethod:self.dispatchMethod];
    XCTestExpectation *expectation = [self expectationWithDescription:NSStringFromSelector(_cmd)];
    
    @weakify(self);
    [self.dispatcher dispatch:^{
        @strongify(self);
        XCTAssertTrue(dispatchGetsCalled);
        [expectation fulfill];
    }];
    [self waitForExpectations:@[expectation] timeout:0.1];
}

#pragma mark - Privates

- (void)assertQueue {
    const char *dispatchQueueName = dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL);
    const char *queueName = dispatch_queue_get_label(self.queue);
    int comparison = 1; //default int value is 0
    comparison = strcmp(dispatchQueueName, queueName);
    XCTAssertTrue(comparison == 0);
}