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

// 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];
   }
    

NSArray* urls = @[ @"http://www.collegedj.net/wp-content/uploads/2010/10/3-150x150.jpg",
                       @"http://www.collegedj.net/wp-content/uploads/2010/10/3-300x199.jpg"];
    
 for (NSString* url in urls)
 {
     MXOperation* operation = [[MXOperation alloc]initWithUrl:url name:nil];
     [_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");

- (void)downloadImageWithPthread
{
    //线程参数结构体
    struct threadInfo * const info = (struct threadInfo *) malloc(sizeof(*info));
    info->url = (unsigned char*)url1.UTF8String;
    
    //
    pthread_t tid;
    int err_create = pthread_create(&tid, NULL, &downloadImage, info);
    NSCAssert(err_create == 0, @"pthread_create() failed: %d", err_create);
    
    // Wait for the threads to exit:
    struct threadResult * results;
    
    int err_join = pthread_join(tid, (void**)(&results));
    NSCAssert(err_join == 0, @"pthread_join() failed: %d", err_join);
    
    NSData* imgData = [NSData dataWithBytes:results->imageRawData length:results->imageLength];
    _imgv1.image = [UIImage imageWithData: imgData];
  
    free(results);
    results = NULL;
}