NSOperationQueue隊列中操做依賴相關思考

• 添加依賴後，隊列中網絡請求任務有依賴關係時，任務結束斷定以數據返回爲準仍是以發起請求爲準？
• waitUntilFinished方法容易誤解。

依賴關係

//
//  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];
    
    //任務最大併發數，與是否開啓子線程無關。
//    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

執行結果

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：任務結束斷定以發起請求爲準！數據返回是異步的，不受依賴關係影響！

waitUntilFinished方法

當把上面代碼

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

改成

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

時，
log以下,沒有正常執行操做：

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

分析：
waitUntilFinished方法定義爲：
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.
當爲YES時，當前線程被阻塞，直到被添加的操做執行完畢。上面代碼使線程依賴於lOp0執行完畢，而lOp0的執行依賴於lOp1執行完畢。因爲lOp1比lOp0加入隊列更晚。當上面代碼被執行時，線程在等lOp0執行完畢，而此時lOp1還沒被加入隊列中，即lOp1還沒開始執行，因此線程一直處於阻塞狀態！固然，合理利用waitUntilFinished方法，也能實現想要的特殊效果。