创建一个写入流写入一个文件到程序的临时目录里

先获得写入的路径：

self.filePath = [[NetworkManager sharedInstance] pathForTemporaryFileWithPrefix:@"Get"];

- (NSString *)pathForTemporaryFileWithPrefix:(NSString *)prefix
{
    NSString *  result;
    CFUUIDRef   uuid;
    CFStringRef uuidStr;
    
    uuid = CFUUIDCreate(NULL);
    assert(uuid != NULL);
    
    uuidStr = CFUUIDCreateString(NULL, uuid);
    assert(uuidStr != NULL);
    
    result = [NSTemporaryDirectory() stringByAppendingPathComponent:[NSString stringWithFormat:@"%@-%@", prefix, uuidStr]];
    assert(result != nil);
    
    CFRelease(uuidStr);
    CFRelease(uuid);
    
    return result;
}

创建写入流，并打开流

    assert(self.filePath != nil);
        
    self.fileStream = [NSOutputStream outputStreamToFileAtPath:self.filePath append:NO];
    assert(self.fileStream != nil);
        
    [self.fileStream open];

发送url请求

        request = [NSURLRequest requestWithURL:url];
        assert(request != nil);
        
        self.connection = [NSURLConnection connectionWithRequest:request delegate:self];
        assert(self.connection != nil);

实现代理方法，将文件写入到临时目录

- (void)connection:(NSURLConnection *)theConnection didReceiveData:(NSData *)data
{
    #pragma unused(theConnection)
    NSInteger       dataLength;
    const uint8_t * dataBytes;
    NSInteger       bytesWritten;
    NSInteger       bytesWrittenSoFar;

    assert(theConnection == self.connection);
    
    dataLength = [data length];
    dataBytes  = [data bytes];

    bytesWrittenSoFar = 0;
    do {
        bytesWritten = [self.fileStream write:&dataBytes[bytesWrittenSoFar] maxLength:dataLength - bytesWrittenSoFar];
        assert(bytesWritten != 0);
        if (bytesWritten == -1) {
            [self stopReceiveWithStatus:@"File write error"];
            break;
        } else {
            bytesWrittenSoFar += bytesWritten;
        }
    } while (bytesWrittenSoFar != dataLength);
}

还有写代理方法也要去实现，例如：

- (void)connectionDidFinishLoading:(NSURLConnection *)theConnection

- (void)connection:(NSURLConnection *)theConnection didFailWithError:(NSError *)error

- (void)connection:(NSURLConnection *)theConnection didReceiveResponse:(NSURLResponse *)response