请求限时-CancellationToken

被多数异步方法都青睐的参数：请求限时-CancellationToken

当请求发出去后，即使客户端关闭但服务器会继续做处理，没必要，所以CancellationToken来取消处理。
可以使用在请求超时、用户取消请求、网页关闭、断网等场景
而且很多异步方法都支持CancellationToken参数用于提前获取终止执行的信号

1、IsCancellationRequested属性：是否取消

        static async Task Main(string[] args)
        {
            CancellationTokenSource cts = new CancellationTokenSource();
  				 //指定5秒后IsCancellationRequested为true
            cts.CancelAfter(5000);
            CancellationToken cancellationToken = cts.Token;

            using (HttpClient client = new HttpClient())
            {
                for (int i = 0; i < 50; i++)
                {
                    string html = await client.GetStringAsync("https://www.baidu.com");
									//判断
                    if (cancellationToken.IsCancellationRequested)
                    {
                        Console.WriteLine("请求被取消");
                        break;
                    }
                    Console.WriteLine(html);
                }
            }
        }

5秒后真被取消了

2、ThrowIfCancellationRequested()：如果任务被取消，执行到这句话就抛异常

        static async Task Main(string[] args)
        {
            CancellationTokenSource cts = new CancellationTokenSource();
            //指定5秒后IsCancellationRequested为true
            cts.CancelAfter(5000);
            CancellationToken cancellationToken = cts.Token;

            using (HttpClient client = new HttpClient())
            {
                for (int i = 0; i < 50; i++)
                {
                    var html = await client.GetStringAsync("https://www.baidu.com");
                  
									//如果任务被取消，执行到这句话就抛异常
                    cancellationToken.ThrowIfCancellationRequested();

                    Console.WriteLine(html);
                }
            }
        }

真就异常了

3、多数异步方法都支持CancellationToken参数

        static async Task Main(string[] args)
        {
            CancellationTokenSource cts = new CancellationTokenSource();
            //指定5秒后IsCancellationRequested为true
            cts.CancelAfter(5000);
            CancellationToken cancellationToken = cts.Token;

            using (HttpClient client = new HttpClient())
            {
                for (int i = 0; i < 50; i++)
                {
                    //作为参数传递，则会自动抛出异常
                    var resp = await client.GetAsync("https://www.baidu.com", cancellationToken);
                    var html = resp.Content.ReadAsStringAsync();

                    Console.WriteLine(html);
                }
            }
        }

相当于ThrowIfCancellationRequested()的异常，但是这种更推荐，因为使用ThrowIfCancellationRequested()时，当取消的时间正好在await的时间之内，则本次await不会及时被阻止，而作为参数传递是准时阻止的。

4、也可以自定义取消的触发条件

CancellationTokenSource cts = new CancellationTokenSource();
//cts.CancelAfter(5000);
CancellationToken cancellationToken = cts.Token;
async Task A()
{
  await client.GetAsync("https://www.baidu.com", cancellationToken);
}



//触发了B方法则取消A方法中的await操作
void B(取消的条件,cts)
{
 cts.Cancel() 
}