项目中集成了环信，点击联系客服时需要调取环信的接口，现在有个要求，如果调取环信的登录接口失败了，就要重新登录，但是这个操作不能影响主线程的操作，登录次数达到一定的数量后停止登录。
首先：环信的登录时同步的，需要我们开启一个线程，不然当环信登录失败时会很容易造成界面卡死的情况。
+ (void)loginWithSuccessBlock:(void(^)())success FailureBlock:(void(^)())failure{   // 自己封装的一个公共类

// 开启一个线程防止登录失败时造成主线程卡死

NSOperationQueue *q = [[NSOperationQueue alloc]init];

[q addOperationWithBlock:^{

EMError *error = [[EMClient sharedClient] loginWithUsername:USER_NAME password:@"123456"];

// 主线程更新UI

[[NSOperationQueue mainQueue] addOperationWithBlock:^{

if (!error)

{

[[EMClient sharedClient].options setIsAutoLogin:YES];

success();

}else{

failure();   // 登录失败调取另一个方法继续尝试登录，登录5次停止登录

[[[self alloc] init] HXLoginFailed];

}

}];

}];

}

然后就是登录失败  开启一个线程继续登录，登录5次后停止登录

- (void)HXLoginFailed
{
    [self.thread cancel];
    self.thread = nil;
    NSString* countStr = [SKUserDefaults valueForKey:@"HXCount"];
    if ([countStr intValue]>5) {
        return;
    }
    DLog(@"========%@=====%d",[NSThread currentThread],[countStr intValue]);
    self.thread=[[NSThread alloc]initWithTarget:self selector:@selector(HXLoginFailed2) object:nil];
    [self.thread start];
    self.requestCount++;
}
- (void)HXLoginFailed2
{
    EMError *error = [[EMClient sharedClient] loginWithUsername:USER_NAME password:@"123456"];
    if (error) {
        NSString* countStr = [SKUserDefaults valueForKey:@"HXCount"];
        countStr = [NSString stringWithFormat:@"%d",[countStr intValue]+1];
        [SKUserDefaults setValue:countStr forKey:@"HXCount"];
        [self HXLoginFailed];
    } else {
        
    }
}
------------------------------------------------------------------------华丽分割线---------------------------------------------------------------------------------------------
最后看看NSOperation怎么实现

原理：只要将一个NSOperation（实际开中需要使用其子类NSInvocationOperation、NSBlockOperation）放到NSOperationQueue这个队列中线程就会依次启动。

NSOperation有两个常用子类用于创建线程操作：NSInvocationOperation和NSBlockOperation，两种方式本质没有区别，但是是后者使用Block形式进行代码组织，使用相对方便。
- (instancetype)init
{
    self = [super init];
    if (self) {
        self.OperationQueue = [[NSOperationQueue alloc]init];
        self.OperationQueue.maxConcurrentOperationCount = 1;
    }
    return self;
}

+ (void)loginWithSuccessBlock:(void(^)())success FailureBlock:(void(^)())failure{
    // 开启一个线程防止登录失败时造成主线程卡死
    NSOperationQueue *q = [[NSOperationQueue alloc]init];
    [q addOperationWithBlock:^{
        EMError *error = [[EMClient sharedClient] loginWithUsername:USER_NAME password:@"jimaijie654321"];
        // 主线程更新UI
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            if (!error)
            {
                [[EMClient sharedClient].options setIsAutoLogin:YES];
                success();
            }else{
                failure();
                [SKUserDefaults setValue:@"0" forKey:@"HXCount"];
                [[[self alloc] init] HXLoginFailed];
            }
        }];
    }];
}

- (void)HXLoginFailed
{
    NSString* countStr = [SKUserDefaults valueForKey:@"HXCount"];
    if ([countStr intValue]>5) {
        self.OperationQueue = nil;
        return;
    }
    DLog(@"========%@=====%d",[NSThread currentThread],[countStr intValue]);
    [self.OperationQueue addOperationWithBlock:^{
        DLog(@"+++++++++++++%@=====%d",[NSThread currentThread],[countStr intValue]);
        EMError *error = [[EMClient sharedClient] loginWithUsername:USER_NAME password:@"jimaijie654321"];
        // 主线程更新UI
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            if (!error)
            {
                [[EMClient sharedClient].options setIsAutoLogin:YES];
            }else{
                NSString* countStr = [SKUserDefaults valueForKey:@"HXCount"];
                countStr = [NSString stringWithFormat:@"%d",[countStr intValue]+1];
                [SKUserDefaults setValue:countStr forKey:@"HXCount"];
                [self HXLoginFailed];
            }
        }];
    }];

}