线程池NSOperationQueue的 setMaxConcurrentOperationCount函数

结论：setMaxConcurrentOperationCount 不能控制线程池中的线程数或线程数上限（或者说不是唯一因素）

摘自官方文档对 setMaxConcurrentOperationCoun 的说明 : affects only the operations that the current queue has executing at the same time. 翻译过来，就是影响同时执行的 operations 数，此处的 operations 是一个任务单位，使用者将子任务以 NSOperation 对象的形式添加。
这里的 NSOperation 可以理解成一个任务，但不能理解成这个任务只对应一个处理线程。
如: NSBlockOperation对象是NSOperation类的一个子类, NSBlockOperation类有

+ (instancetype)blockOperationWithBlock:(void (^)(void))block;

初始化方法, 此方法可以以block的形式添加一个子任务, 另外他还有

- (void)addExecutionBlock:(void (^)(void))block;

的对象方法, 用来继续添加block形式的子任务.

而使用了NSBlockOperation对象作为任务添加到 NSOperationQueue时, 每个任务则可能不止对应一个线程.

实例代码如:

NSMutableDictionary *dicMuThreads = [NSMutableDictionary dictionary];
    NSLock *lock = [[NSLock alloc] init];
    
    NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
        [lock lock];
        NSThread *current = [NSThread currentThread];
        NSString *strThread = [NSString stringWithFormat: @"%@",current];
        [dicMuThreads setObject:@"a" forKey:strThread];
        NSLog(@"a--%@",strThread);
        [lock unlock];
    }];
    [op1 addExecutionBlock:^{
        [lock lock];
        NSThread *current = [NSThread currentThread];
        NSString *strThread = [NSString stringWithFormat: @"%@",current];
        [dicMuThreads setObject:@"a" forKey:strThread];
        NSLog(@"a--%@",strThread);
        [lock unlock];
    }];
    [op1 addExecutionBlock:^{
        [lock lock];
        NSThread *current = [NSThread currentThread];
        NSString *strThread = [NSString stringWithFormat: @"%@",current];
        [dicMuThreads setObject:@"a" forKey:strThread];
        NSLog(@"a--%@",strThread);
        [lock unlock];
    }];
//注释1
//    [op1 addExecutionBlock:^{
//        [lock lock];
//        NSThread *current = [NSThread currentThread];
//        NSString *strThread = [NSString stringWithFormat: @"%@",current];
//        [dicMuThreads setObject:@"a" forKey:strThread];
//        NSLog(@"a--%@",strThread);
//        [lock unlock];
//    }];
    
    
    NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
        [lock lock];
        NSThread *current = [NSThread currentThread];
        NSString *strThread = [NSString stringWithFormat: @"%@",current];
        [dicMuThreads setObject:@"b" forKey:strThread];
        NSLog(@"b--%@",strThread);
        [lock unlock];
    }];
    [op2 addExecutionBlock:^{
        [lock lock];
        NSThread *current = [NSThread currentThread];
        NSString *strThread = [NSString stringWithFormat: @"%@",current];
        [dicMuThreads setObject:@"b" forKey:strThread];
        NSLog(@"b--%@",strThread);
        [lock unlock];
    }];
//注释2
//    [op2 addExecutionBlock:^{
//        [lock lock];
//        NSThread *current = [NSThread currentThread];
//        NSString *strThread = [NSString stringWithFormat: @"%@",current];
//        [dicMuThreads setObject:@"b" forKey:strThread];
//        NSLog(@"b--%@",strThread);
//        [lock unlock];
//    }];
    
    
    NSBlockOperation *op3 = [NSBlockOperation blockOperationWithBlock:^{
        [lock lock];
        NSThread *current = [NSThread currentThread];
        NSString *strThread = [NSString stringWithFormat: @"%@",current];
        [dicMuThreads setObject:@"c" forKey:strThread];
        NSLog(@"c--%@",strThread);
        [lock unlock];
    }];
    [op3 addExecutionBlock:^{
        [lock lock];
        NSThread *current = [NSThread currentThread];
        NSString *strThread = [NSString stringWithFormat: @"%@",current];
        [dicMuThreads setObject:@"c" forKey:strThread];
        NSLog(@"c--%@",strThread);
        [lock unlock];
    }];
//注释3
//    [op3 addExecutionBlock:^{
//        [lock lock];
//        NSThread *current = [NSThread currentThread];
//        NSString *strThread = [NSString stringWithFormat: @"%@",current];
//        [dicMuThreads setObject:@"c" forKey:strThread];
//        NSLog(@"c--%@",strThread);
//        [lock unlock];
//    }];
    
    NSOperationQueue *opeQueue = [[NSOperationQueue alloc]init];
    [opeQueue setMaxConcurrentOperationCount:2];
    [opeQueue addOperation:op1];
    [opeQueue addOperation:op2];
    [opeQueue addOperation:op3];
    [opeQueue setSuspended: NO];
    [opeQueue waitUntilAllOperationsAreFinished];
    
    NSLog(@"线程总数是:%ld",dicMuThreads.count);

有注释1 注释2 注释3, 三处注释时结果如下

结果

只取消注释2 时结果如下:

结果2

只取消注释 1和 2时, 结果如下:

结果3

总结实验结果，规律比较清晰：setMaxConcurrentOperationCount 和当前 operation 的子任务数 (NSBlockOperation) 结合起来影响线程数 : maxConcurrentOperationCount 属性值为 2 时，则要开辟的线程数是，所有子任务中 NSBlockOperation 个数最多的 2 个子任务的 NSBlockOperation 个数之和；这样设计的原因也变的很合理。

引用:
https://www.jianshu.com/p/250807eed6ab