結(jié)論：setMaxConcurrentOperationCount 不能控制線程池中的線程數(shù)或線程數(shù)上限（或者說不是唯一因素）

摘自官方文檔對 setMaxConcurrentOperationCoun 的說明 : affects only the operations that the current queue has executing at the same time. 翻譯過來，就是影響同時執(zhí)行的 operations 數(shù)，此處的 operations 是一個任務(wù)單位，使用者將子任務(wù)以 NSOperation 對象的形式添加。
這里的 NSOperation 可以理解成一個任務(wù)，但不能理解成這個任務(wù)只對應(yīng)一個處理線程。
如: NSBlockOperation對象 是NSOperation類的一個子類, NSBlockOperation類有

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

初始化方法, 此方法可以以block的形式添加一個子任務(wù), 另外他還有

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

的對象方法, 用來繼續(xù)添加block形式的子任務(wù).

而使用了NSBlockOperation對象作為任務(wù) 添加到 NSOperationQueue時, 每個任務(wù)則可能不止對應(yīng)一個線程.

實例代碼如:

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];
    }];
//注釋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];
    }];
//注釋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];
    }];
//注釋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(@"線程總數(shù)是:%ld",dicMuThreads.count);

有注釋1 注釋2 注釋3, 三處注釋時結(jié)果如下

結(jié)果

只取消注釋2 時 結(jié)果如下:

結(jié)果2

只取消注釋 1和 2時, 結(jié)果如下:

結(jié)果3

總結(jié)實驗結(jié)果，規(guī)律比較清晰：setMaxConcurrentOperationCount 和當(dāng)前 operation 的子任務(wù)數(shù) (NSBlockOperation) 結(jié)合起來影響線程數(shù) : maxConcurrentOperationCount 屬性值為 2 時，則要開辟的線程數(shù)是，所有子任務(wù)中 NSBlockOperation 個數(shù)最多的 2 個子任務(wù)的 NSBlockOperation 個數(shù)之和；這樣設(shè)計的原因也變的很合理。

引用:
http://www.itdecent.cn/p/250807eed6ab