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在日常开发中我们总是会和网络打交道,从服务端拿数据渲染UI、上传数据到服务器、登陆等,那么就会遇到一些问题。eg:当用户登陆完毕后才获取数据渲染UI或者是多个网络请求从服务端拿到多个数据后,才进行下一步的操作,那么对网络请求之间顺序的控制是十分重要的,本文对这两种情况进行总结,如有不足之处,请多多指教。同时本文只提供了部分截图,其他运行效果可自行尝试。
原文:https://juejin.cn/post/7078892935093157902
示例代码中用到的宏定义:
#define GlobalQueue dispatch_get_global_queue(0, 0)
#define MainQueue dispatch_get_main_queue()
思路:group其实就是管理指定queue中任务的。在这里通过调用dispatch_group_notify方法,等待group中管理queue的任务执行完毕后,会在该group指定的队列中执行block内部的代码。
- (void)multipleRequest_NoOrder_after_executeOtherTask_byGroupNotify {
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, GlobalQueue, ^{
sleep(1);
NSLog(@"网络请求1");
});
dispatch_group_async(group, GlobalQueue, ^{
sleep(1);
NSLog(@"网络请求2");
});
dispatch_group_async(group, GlobalQueue, ^{
sleep(1);
NSLog(@"网络请求3");
});
dispatch_group_async(group, GlobalQueue, ^{
sleep(1);
NSLog(@"网络请求4");
});
//当group中的任务执行完后,会调用
dispatch_group_notify(group, MainQueue, ^{
NSLog(@"更新UI");
});
}
运行截图: 第一次运行:
第二次运行:
思路:通过NSBlockOperation创建多个任务,将任务添加到NSBlockOperationQueue中.通过调用addBarrierBlock方法,会等到将任务添加到NSBlockOperationQueue中所有的任务执行完毕后,才会执行block中的代码。
- (void)multipleRequest_NoOrder_after_executeOtherTask_byOperation {
NSBlockOperation *block1 = [NSBlockOperation blockOperationWithBlock:^{
sleep(1);
NSLog(@"网络请求1");
}];
NSBlockOperation *block2 = [NSBlockOperation blockOperationWithBlock:^{
sleep(1);
NSLog(@"网络请求2");
}];
NSBlockOperation *block3 = [NSBlockOperation blockOperationWithBlock:^{
sleep(1);
NSLog(@"网络请求3");
}];
NSBlockOperation *block4 = [NSBlockOperation blockOperationWithBlock:^{
sleep(1);
NSLog(@"网络请求4");
}];
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
//设置最大并发数
queue.maxConcurrentOperationCount = [NSProcessInfo processInfo].activeProcessorCount;
//waitUntilFinished:是否等待queue中的任务执行完后,才执行后面的代码。会阻塞当前线程
[queue addOperations:@[block1,block2,block3,block4] waitUntilFinished:NO];
//当queue中的所有任务执行完后,会调用
[queue addBarrierBlock:^{
NSLog(@"更新UI");
}];
}
思路:栅栏函数其实和NSOperationQueue的addBarrierBlock方法相似,都是等待队列中的任务执行完毕后,才执行block中的代码。但是需要注意提到的坑点。
- (void)multipleRequest_NoOrder_after_executeOtherTask_byBarrier {
dispatch_queue_t queue = dispatch_queue_create("ConcurrentQueue", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
sleep(1);
NSLog(@"网络请求1");
});
dispatch_async(queue, ^{
sleep(1);
NSLog(@"网络请求2");
});
dispatch_async(queue, ^{
sleep(1);
NSLog(@"网络请求3");
});
dispatch_async(queue, ^{
sleep(1);
NSLog(@"网络请求4");
});
//坑点:不能用全局并发队列,栅栏函数会失效并且栅栏函数只能用于自定义创建的并发队列,如果传递dispatch_get_global_queue(0, 0),那么dispatch_barrier_async等价于dispatch_async
dispatch_barrier_async(queue, ^{
NSLog(@"更新UI");
});
}
思路:很简单,enter代表一个block任务进入,leave代表一个block任务离开。
- (void)multipleRequest_InOrder_after_executeOtherTask_byGroup {
dispatch_group_t group = dispatch_group_create();
dispatch_group_enter(group);
[self request:@"1" finished:^{
dispatch_group_leave(group);
}];
dispatch_group_enter(group);
[self request:@"2" finished:^{
dispatch_group_leave(group);
}];
dispatch_group_enter(group);
[self request:@"3" finished:^{
dispatch_group_leave(group);
}];
dispatch_group_enter(group);
[self request:@"4" finished:^{
dispatch_group_leave(group);
}];
dispatch_group_notify(group, MainQueue, ^{
NSLog(@"更新UI");
});
}
- (void)request:(NSString *)parama finished:(void(^)(void))finished {
dispatch_async(GlobalQueue, ^{
NSLog(@"网络请求%@",parama);
if(finished) { finished(); }
});
}
原理:异步开启子线程,但是由于是串行队列,任务的执行按照FIFO的原则,那么先进入队列的网络请求任务,会被子线程优先从队列中取出来执行。最终在网络请求4完成后,在主线程刷新UI。
- (void)multipleRequest_InOrder_after_executeOtherTask_byAsyncSerialQueue{
dispatch_queue_t asyncSerialQueue = dispatch_queue_create("AsyncSerialQueue", DISPATCH_QUEUE_SERIAL);
dispatch_async(asyncSerialQueue, ^{
sleep(1);
NSLog(@"网络请求1");
});
dispatch_async(asyncSerialQueue, ^{
sleep(1);
NSLog(@"网络请求2");
});
dispatch_async(asyncSerialQueue, ^{
sleep(1);
NSLog(@"网络请求3");
});
dispatch_async(asyncSerialQueue, ^{
sleep(1);
NSLog(@"网络请求4");
dispatch_async(MainQueue, ^{
NSLog(@"更新UI");
});
});
运行截图:
- (void)multipleRequest_InOrder_after_executeOtherTask_byOperation {
NSBlockOperation *block1 = [NSBlockOperation blockOperationWithBlock:^{
sleep(1);
NSLog(@"网络请求1");
}];
NSBlockOperation *block2 = [NSBlockOperation blockOperationWithBlock:^{
sleep(1);
NSLog(@"网络请求2");
}];
NSBlockOperation *block3 = [NSBlockOperation blockOperationWithBlock:^{
sleep(1);
NSLog(@"网络请求3");
}];
NSBlockOperation *block4 = [NSBlockOperation blockOperationWithBlock:^{
NSLog(@"网络请求4");
sleep(1);
}];
[block4 addDependency:block3];
[block3 addDependency:block2];
[block2 addDependency:block1];
NSOperationQueue *operationQueue = [[NSOperationQueue alloc] init];
[operationQueue addOperations:@[block1,block2,block3,block4] waitUntilFinished:NO];
//会等到queue中的任务执行完后才会调用
[operationQueue addBarrierBlock:^{
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(@"刷新UI");
});
}];
}