異步下載兩個任務（利用協議）

#import "ViewController.h"

@interface ViewController ()<NSURLConnectionDataDelegate>
{
    NSMutableData *csdnData;
    NSMutableData *baiduData;
    NSURLConnection *conn1;
    NSURLConnection *conn2;
}

@end

@implementation ViewController
            
- (void)viewDidLoad {
    [super viewDidLoad];
    
    NSURL *url1 = [NSURL URLWithString:@"http://www.csdn.net"];
    NSURLRequest *request1 = [NSURLRequest requestWithURL:url1];
    conn1 = [NSURLConnection connectionWithRequest:request1 delegate:self];
    [conn1 start];
    
    NSURL *url2 = [NSURL URLWithString:@"http://www.ifeng.com"];
    NSURLRequest *request2 = [NSURLRequest requestWithURL:url2];
    conn2 = [NSURLConnection connectionWithRequest:request2 delegate:self];
    [conn2 start];
    
    csdnData = [NSMutableData data];
    baiduData = [NSMutableData data];
}

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
{
    NSLog(@"failed");
}

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    NSHTTPURLResponse *httpUrl = (NSHTTPURLResponse *)response;
    if (connection == conn1)
    {
         //狀態碼爲200，說明纔有數據
         if (httpUrl.statusCode == 200)
         {
              NSLog(@"Response-------->%p: %@,expectedContentLength=%lld,---%@",connection,httpUrl,response.expectedContentLength,response.URL);//response.expectedContentLength只對於文本，視頻等具體的東西它的值纔有用，對於網頁等動態的是得不到它的具體長度的，得到的也是無意義的。
         }
    }
    else
    {
        //狀態碼爲200，說明纔有數據
        if (httpUrl.statusCode == 200)
        {
            NSLog(@"Response-------->%p: %@,expectedContentLength=%lld,---%@",connection,httpUrl,response.expectedContentLength,response.URL);//response.expectedContentLength只對於文本，視頻等具體的東西它的值纔有用，對於網頁等動態的是得不到它的具體長度的
        }
    }

}


//只有-(void)connection: didReceiveData: 纔有可能多次調用，因爲當文件很大時需要下載多次
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
     NSLog(@"Received-------->%p: %d",connection,data.length);
    ////多個下載需要判斷是哪個connection，否則會引起數據污染
    if (connection == conn1) {
        [csdnData appendData:data];
    }
    else
    {
        [baiduData appendData:data];
    }
    
}

- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    //多個下載需要判斷是哪個connection，否則會引起數據污染
    NSLog(@"finish--------->%p:",connection);
    if (connection == conn1) //通過比較兩個指針的地址，來判斷是否爲同一個NSURLconnection對象
    {
        NSString *str = [[NSString alloc] initWithData:csdnData encoding:NSUTF8StringEncoding];
        NSLog(@"str=%@",str);
    }
    else
    {
        NSString *str1 = [[NSString alloc] initWithData:baiduData encoding:NSUTF8StringEncoding];
        NSLog(@"str1=%@",str1);
    }
    
    
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

@end

異步下載步驟：

1，創建NSURL對象

2，創建NSURLRequest對象
3，創建NSURLConnection對象

4，掛出NSURLConnectionDataDelegate協議，並實現協議裏的方法。

@protocol NSURLConnectionDataDelegate<NSURLConnectionDelegate>

協議裏有四個方法用的最多：

//Fail方法聲明在協議NSURLConnectionDelegate裏，其他三個在NSURLConnectionDataDelegate裏聲明

- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
- (void)connectionDidFinishLoading:(NSURLConnection *)connection

其他
1，response.expectedContentLength只對於文本，視頻等具體的東西它的值纔有用，對於網頁等動態的是得不到它的具體長度的。得到的也是無意義的。
2，if (connection == conn1) //通過比較兩個指針的地址，來判斷是否爲同一個NSURLConnection對象。
3，多個下載需要判斷是哪個connection，否則會引起數據污染。