iOS NSURLConnection和异步网络请求

在日常应用中，我们往往使用AFNetworking等第三方库来实现网络请求部分。这篇文章会简要地介绍一下如何使用NSURLConnection来进行异步的网络请求。

我们先看一个小demo

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    NSString *urlStr = @"http://www.baidu.com";
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:urlStr]];

    //从这个试验可以看出，connection的函数在 mainrunloop运行for循环时根本无法被调用，由此可见，这里的connection是在mainThread中运行的。
    NSURLConnection *con = [[NSURLConnection alloc] initWithRequest:request delegate:self];

    for(int i =  ;i<;i++)
    {
        NSLog(@"%d",i);
    }

    NSLog(@"for end==========");
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    NSLog(@"good!!!!!!!!");
}

看下输入

-- ::18.861 UrlConnectionASyncTest[:60b]
-- ::18.862 UrlConnectionASyncTest[:60b]
-- ::18.862 UrlConnectionASyncTest[:60b]
-- ::18.862 UrlConnectionASyncTest[:60b] for end==========
-- ::18.865 UrlConnectionASyncTest[:60b] good!!!!!!!!

查看苹果的文档会发现如下的解释

By default, a connection is scheduled on the current thread in the default mode when it is created. If you create a connection with the initWithRequest:delegate:startImmediately: method and provide NO for the startImmediately parameter, you can instead schedule the connection on an operation queue before starting it with the start method.

You cannot reschedule a connection after it has started.

这就是说，如果你仅仅通过在主线程中使用initWithRequest:delegate:方法创建一个connection对象，它会默认地加入到mainThread中，这样当数据返回时，会在main thread中执行，这就会影响UI的刷新。这种connection就是不是同步connection，不同于同步网络请求函数sendSynchronousRequest，但是它的回调函数会在main thread中执行，会影响main thread中的其他函数执行。

下面是官方文档对NSURLConnection请求数据的3中方法的总结：

To retrieve the contents of a URL synchronously: In code that runs exclusively on a background thread, you can call sendSynchronousRequest:returningResponse:error: to perform an HTTP request. This call returns when the request completes or an error occurs. For more details, see Retrieving Data Synchronously.

To retrieve the contents of a URL using a completion handler block: If you do not need to monitor the status of a request, but merely need to perform some operation when the data has been fully received, you can call sendAsynchronousRequest:queue:completionHandler:, passing a block to handle the results. For more details, see Retrieving Data Using a Completion Handler Block.

To retrieve the contents of a URL using a delegate object: Create a delegate class that implements at least the following delegate methods: connection:didReceiveResponse:, connection:didReceiveData:, connection:didFailWithError:, and connectionDidFinishLoading:. The supported delegate methods are defined in the NSURLConnectionDelegate, NSURLConnectionDownloadDelegate, and NSURLConnectionDataDelegate protocols.

那么如何创建一个在其他thread中执行回调函数的connection呢？系统提供了2套方法，

第一套是使用类方法，sendAsynchronousRequest:queue:completionHandler:

第二套是使用几个对象方法，顺序如下

.使用initWithRequest:delegate:startImmediately:生成一个不立即开始的connnection

.通过scheduleInRunLoop:forMode: 或者 setDelegateQueue: 设置回调方法运行的thread，推荐使用第二个。

.调用start开始connection请求。

下面给出一个demo

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    NSString *urlStr = @"http://www.baidu.com";
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:urlStr]];

    //从这个试验可以看出，connection的函数在 mainrunloop运行for循环时根本无法被调用，由此可见，这里的connection是在mainThread中运行的。
    // NSURLConnection *con = [[NSURLConnection alloc] initWithRequest:request delegate:self];

    NSURLConnection *con = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:NO];

    [con setDelegateQueue:[[NSOperationQueue alloc] init]];
    [con start];

    for(int i =  ;i<;i++)
    {
        NSLog(@"%d",i);
    }

    NSLog(@"for end==========");
}

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
  //  NSLog(@"data is %@",[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding]);
    NSLog(@"good!!!!!!!!");
}

结果如下

-- ::15.129 UrlConnectionASyncTest[:60b]
-- ::15.129 UrlConnectionASyncTest[:60b]
-- ::15.129 UrlConnectionASyncTest[:60b]
-- ::15.129 UrlConnectionASyncTest[:60b]
-- ::15.129 UrlConnectionASyncTest[:60b]
-- ::15.129 UrlConnectionASyncTest[:60b]
-- ::15.130 UrlConnectionASyncTest[:60b]
-- ::15.134 UrlConnectionASyncTest[:] good!!!!!!!!
-- ::15.143 UrlConnectionASyncTest[:60b]
-- ::15.144 UrlConnectionASyncTest[:60b]
-- ::15.144 UrlConnectionASyncTest[:] good!!!!!!!!
-- ::15.144 UrlConnectionASyncTest[:60b]
-- ::15.144 UrlConnectionASyncTest[:60b]
-- ::15.144 UrlConnectionASyncTest[:60b]
-- ::15.145 UrlConnectionASyncTest[:60b] 

可以看出connection的回调函数已经不再main thread中执行了！