理解iOS中的线程池

_opQueue = [[NSOperationQueue alloc]init];
[_opQueue addOperationWithBlock:^{
        NSData* data = [NSData dataWithContentsOfURL:[NSURL URLWithString:url3]];
        //_imgv3.image = [UIImage imageWithData:data];

        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            _imgv3.image = [UIImage imageWithData:data];
        }];
    }];

- (void)downloadWithGCD
{
    dispatch_async(_gcdQueue, ^{
        NSData* data = [NSData dataWithContentsOfURL:[NSURL URLWithString:url4]];
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imgv4.image = [UIImage imageWithData:data];
             NSLog(@"done downloading 3rdimage ");
        });
    });
}

- (void)downloadWithNSOperationDependency
{
    ETOperation* op1 = [ETOperation new];
    op1.url = [NSURL URLWithString:url3];
    __weak ETOperation* _op1 = op1;
    [op1 setCompletionBlock:^{
        _imgv3.image = _op1.image;
    }];
    [op1 start];

    ETOperation* op2 = [ETOperation new];
    op2.url = [NSURL URLWithString:url4];
    __weak ETOperation* _op2 = op2;
    [op2 setCompletionBlock:^{
        _imgv4.image = _op2.image;
    }];
    [op2 addDependency:op1];
    [op2 start];
}

- (void)downloadWithGCDGroups
{
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_group_async(group, queue, ^(){
        NSData* data = [NSData dataWithContentsOfURL:[NSURL URLWithString:url3]];
        dispatch_group_async(group, dispatch_get_main_queue(), ^(){
            self.imgv3.image = [UIImage imageWithData:data];
        });
    });
    // This block will run once everything above is done:
    dispatch_group_notify(group, dispatch_get_main_queue(), ^(){
        dispatch_async(_gcdQueue, ^{
            NSData* data = [NSData dataWithContentsOfURL:[NSURL URLWithString:url4]];
            dispatch_async(dispatch_get_main_queue(), ^{
                self.imgv4.image = [UIImage imageWithData:data];
            });
        });
    });
}

dispatch_queue_t queue = dispatch_get_global_queue( 0, 0 );
dispatch_group_t group = dispatch_group_create();

//run task #1
dispatch_group_enter(group);
dispatch_async( queue, ^{
    NSLog( @"task 1 finished: %@", [NSThread currentThread] );
    dispatch_group_leave(group);
} );

//run task #2
dispatch_group_enter(group);
dispatch_async( queue, ^{
    NSLog( @"task 2 finished: %@", [NSThread currentThread] );
    dispatch_group_leave(group);
} );

//sychronization point
dispatch_group_notify( group, queue, ^{
    NSLog( @"all task done: %@", [NSThread currentThread] );
} );

@interface MXOperation : NSOperation
{
    NSString*   _threadName;
    NSString*   _url;
    BOOL        executing;
    BOOL        finished;
}

@end

@implementation MXOperation

- (id)initWithUrl:(NSString*)url name:(NSString*)name;
{
    self = [super init];

    if (self) {
        if (name!=nil)
        _threadName = name;
        _url = url;
        executing = NO;
        finished = NO;

    }
    return self;
}

- (BOOL)isConcurrent {
    return YES;
}

- (BOOL)isExecuting {
    return executing;
}

- (BOOL)isFinished {
    return finished;
}

- (void)start
{
    [NSThread currentThread].name = _threadName;
    currentThreadInfo(@"start");

    if ([self isCancelled])
    {
        // Must move the operation to the finished state if it is canceled.
        [self willChangeValueForKey:@"isFinished"];
        finished = YES;
        [self didChangeValueForKey:@"isFinished"];
        return;
    }

    // If the operation is not canceled, begin executing the task.
    [self willChangeValueForKey:@"isExecuting"];

    executing = YES;

    //下载图片
    [NSData dataWithContentsOfURL:[NSURL URLWithString:_url]];

    //完成下载
    [self completeOperation];

    [self didChangeValueForKey:@"isExecuting"];
}

- (void)completeOperation {
    [self willChangeValueForKey:@"isFinished"];
    [self willChangeValueForKey:@"isExecuting"];

    executing = NO;
    finished = YES;

    [self didChangeValueForKey:@"isExecuting"];
    [self didChangeValueForKey:@"isFinished"];
}

- (void)dealloc
{
    dumpThreads(@"dealloc");
}

@end

static inline void currentThreadInfo(NSString* str)
{
    if (str)
        NSLog(@"---------%@----------",str);

    NSThread* thread = [NSThread currentThread];
    mach_port_t machTID = pthread_mach_thread_np(pthread_self());
    NSLog(@"current thread num: %x thread name:%@", machTID,thread.name);

    if (str)
        NSLog(@"-------------------");
}


static inline void dumpThreads(NSString* str) {

    NSLog(@"---------%@----------",str);
    currentThreadInfo(nil);
    char name[256];
    thread_act_array_t threads = NULL;
    mach_msg_type_number_t thread_count = 0;
    task_threads(mach_task_self(), &threads, &thread_count);
    for (mach_msg_type_number_t i = 0; i < thread_count; i++) {
        thread_t thread = threads[i];
        pthread_t pthread = pthread_from_mach_thread_np(thread);
        pthread_getname_np(pthread, name, sizeof name);
        NSLog(@"mach thread %x: getname: %s", pthread_mach_thread_np(pthread), name);
    }
    NSLog(@"-------------------");
}

// Do any additional setup after loading the view, typically from a nib.
   NSArray* urls = @[@"http://www.collegedj.net/wp-content/uploads/2010/10/6.jpg",
                     @"http://www.collegedj.net/wp-content/uploads/2010/10/Rihanna.jpg",
                     @"http://www.collegedj.net/wp-content/uploads/2010/10/chris-brown.jpg",
                     @"http://www.collegedj.net/wp-content/uploads/2010/10/dj_scary.jpg",
                   ];
   _queue = [NSOperationQueue new];
   for (int i=0; i<urls.count; i++)
   {
       MXOperation* operation = [[MXOperation alloc]initWithUrl:urls[i] name:[NSString stringWithFormat:@"%d",i]];
       [_queue addOperation:operation];
   }

_queue.maxConcurrentOperationCount = 3;

NSArray* urls = @[    @"http://www.collegedj.net/wp-content/uploads/2010/10/1-150x150.jpg",
                      @"http://www.collegedj.net/wp-content/uploads/2010/10/Rihanna.jpg",
                      @"http://www.collegedj.net/wp-content/uploads/2010/10/chris-brown.jpg",
                      @"http://www.collegedj.net/wp-content/uploads/2010/10/dj_scary.jpg",
                      @"http://www.collegedj.net/wp-content/uploads/2010/10/3-150x150.jpg",
                      @"http://www.collegedj.net/wp-content/uploads/2010/10/3-300x199.jpg"
                    ];

[_queue cancelAllOperations];
    _queue = nil;
    dumpThreads(@"finish");