多線程

1. GCD

• 同步/異步 串行/併發
• dispatch_barrier_async
• dispatch_group

主隊列引發的循環等待（主隊列當中提交的任務，不管是同步/異步，都會在主線程中執行）

-(void)viewDidLoad {
    dispatch_sync(dispatch_get_main_queue(), ^{
        [self doSomething];
    });
}
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-(void)viewDidLoad {
    dispatch_sync(serialQueue, ^{
        [self doSomething];
    });
}
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子線程默認不開啓RunLoop，performSelector:withObject:afterDelay:不執行

-(void)viewDidLoad {
    dispatch_async(global_queue, ^{
        NSLoag(@"1");
        [self performSelector:@selector(printLog)
              withObjec:nil
              afterDelay:0];
        NSLog(@"3")
    });
}

-(void)printLog {
    NSLog(@"2")
}
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dispatch_barrier_async()

利用GCD實現多讀單寫

@interface UserCenter()
{
    dispatch_queue_t concurrent_queue;
    NSMutableDictionary *userCenterDic;
}
@end

@implementation UserCenter 

- (id)init 
{
    self = [super init];
    if (self) {
        concurrent_queue = dispatch_queue_create("read_write_queue", DISPATCH_QUEUE_CONCURRENT);
        userCenterDic = [NSMutableDictionary dictionary];
    }
    
    return self;
}

- (id)objectForKey:(NSString *)key 
{
    __block id obj;
    dispatch_sync(concurrent_queue, ^{
        obj = [userCeneterDic objectForKey:key];
    });
    
    return obj;
}

- (void)setObject:(id)obj forKey:(NSString *)key 
{
    dispatch_barrier_async(concurrent_queue, ^{
        [userCenterDic setObject:obj, forKey:key];
    });
}

@end
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dispatch_group_async()

@interface GroupObject()
{
    dispatch_queue_t concurrent_queue;
    NSMutableArray <NSURL *> *urlArr;
}
@end

@implementation GroupObject

- (id)init 
{
    self = [super init];
    if (self) {
        concurrent_queue = dispatch_queue_create("concurrent_queue", DISPATCH_QUEUE_CONCURRENT);
        urlArr = [NSMutableArray array];
    }
    
    return self;
}

- (void)handle
{
    dispatch_group_t group = dispatch_group_create();
    
    for (NSURL *url in urlArr) {
        dispatch_group_async(group, concurrent_queue, ^{
            NSLog(@"url is %@", url);
        });
    }
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"完成");
    })
}

@end
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二、NSOperation

• 添加任務依賴
• 任務執行狀態的控制
• 最大併發量

任務執行狀態

• isReady
• isExecuting
• isFinished
• isCancelled

狀態控制

重寫main方法，地城控制變動任務及執行完成狀態，以及任務退出 重寫start方法，自行控制任務狀態

系統經過KVO移除isFinished = YES的NSOperation

三、NSThread

四、多線程與鎖

• NSRecursiveLock
• NSLock
• dispatch_semaphore_t

1. @synchronized 通常在建立單例對象的時候使用
2. atomic 屬性關鍵字，對被修飾對象進行原子操做
3. OSSpinLock 自旋鎖 循環等待，不釋放當前資源；輕量級數據訪問

dispatch_semaphore_t

• dispatch_semaphore_create(1);
• dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
• dispatch_semaphore_signal(semaphore);

dispatch_semaphore_create()
struct semaphore {
    int value;
    List<thread>;
}

dispatch_semaphore_wait()
{
    S.value = S.value - 1;
    if S.value < 0 then Block(S.List); => 阻塞線程
}

dispatch_semaphore_signal()
{
    S.value = S.value + 1;
    if S.value <= 0 then wakeup(S.List);
}
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