NSOperationQueue队列中操作依赖相关思考

• 添加依赖后，队列中网络请求任务有依赖关系时，任务结束判定以数据返回为准还是以发起请求为准？
• waitUntilFinished方法容易误解。

依赖关系

//
//  ViewController.m
//  OperationTest0108
//
//  Created by LongMa on 2020/1/8.
//

#import "ViewController.h"
#import <AFNetworking/AFNetworking.h>

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    [self testQueue];
}

- (void)testQueue{
    NSOperationQueue *lQ = [[NSOperationQueue alloc] init];
    
    //任务最大并发数，与是否开启子线程无关。
//    lQ.maxConcurrentOperationCount = 1;
    
    NSBlockOperation *lOp0 = [NSBlockOperation blockOperationWithBlock:^{
        AFHTTPSessionManager *lMng = [AFHTTPSessionManager manager];
        [lMng POST:@"https://www.baidu.com" parameters:@{
            @"mapId" : @"1"
        } progress:^(NSProgress * _Nonnull uploadProgress) {
            
        } success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
            NSLog(@"0 suc");
        } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
            NSLog(@"0 error");
        }];
        
        NSLog(@"0 %@", [NSThread currentThread]);
    }];
    
    NSBlockOperation *lOp1 = [NSBlockOperation blockOperationWithBlock:^{
        AFHTTPSessionManager *lMng = [AFHTTPSessionManager manager];
               [lMng POST:@"https://www.baidu.com" parameters:@{
                   @"mapId" : @"1"
               } progress:^(NSProgress * _Nonnull uploadProgress) {
                   
               } success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
                   NSLog(@"1 suc");
               } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
                   NSLog(@"1 error");
           }];
        
        NSLog(@"1 %@", [NSThread currentThread]);
    }];
    
    NSBlockOperation *lOp2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"2 %@", [NSThread currentThread]);
    }];
    
    [lOp0 addDependency:lOp1];
    
    NSLog(@"before add op");
    
    [lQ addOperations:@[lOp0] waitUntilFinished:NO];
    [lQ addOperations:@[lOp1] waitUntilFinished:NO];
    [lQ addOperations:@[lOp2] waitUntilFinished:NO];
}

@end

执行结果

2020-01-08 18:02:31.378260+0800 OperationTest0108[1583:527022] before add op
2020-01-08 18:02:31.378635+0800 OperationTest0108[1583:527045] 2 <NSThread: 0x283db43c0>{number = 4, name = (null)}
2020-01-08 18:02:31.379722+0800 OperationTest0108[1583:527047] 1 <NSThread: 0x283db4240>{number = 5, name = (null)}
2020-01-08 18:02:31.380265+0800 OperationTest0108[1583:527047] 0 <NSThread: 0x283db4240>{number = 5, name = (null)}
2020-01-08 18:02:31.915236+0800 OperationTest0108[1583:527022] 0 error
2020-01-08 18:02:31.921841+0800 OperationTest0108[1583:527022] 1 error

由上面log可知：任务结束判定以发起请求为准！数据返回是异步的，不受依赖关系影响！

waitUntilFinished方法

当把上面代码

[lQ addOperations:@[lOp0] waitUntilFinished:NO];

改为

[lQ addOperations:@[lOp0] waitUntilFinished:YES];

时，
log如下,没有正常执行操作：

2020-01-08 18:03:55.308276+0800 OperationTest0108[1587:527738] before add op

分析：
waitUntilFinished方法定义为：
If YES, the current thread is blocked until all of the specified operations finish executing. If NO, the operations are added to the queue and control returns immediately to the caller.
当为YES时，当前线程被阻塞，直到被添加的操作执行完毕。上面代码使线程依赖于lOp0执行完毕，而lOp0的执行依赖于lOp1执行完毕。由于lOp1比lOp0加入队列更晚。当上面代码被执行时，线程在等lOp0执行完毕，而此时lOp1还没被加入队列中，即lOp1还没开始执行，所以线程一直处于阻塞状态！当然，合理利用waitUntilFinished方法，也能实现想要的特殊效果。