Iphone 目标-C从Web问题下载图像

我使用以下代码从服务器下载图像：

- (void) loadDataWithOperation {

//Connection test
NSURL *url = [NSURL URLWithString:@"http://myurl.com/testconnection.php"];
NSError* error; 
NSString* connected = [NSString stringWithContentsOfURL:url encoding:NSASCIIStringEncoding error:&error];

//If the string Connected has NOT manged to initialise itself with the contents of the URL:
if (connected == NULL) {
    //Display error picture:

    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error!" message:@"Error downloading gallery, please check network connection and try again."  delegate:self cancelButtonTitle:@"OK" otherButtonTitles: nil];
    [alert show];
    [alert release];

    [self createBackButton];

} else {
    //Load Data
    NSLog(@"Connected - %@",connected);

    NSLog(@"loadDataWithOperartion");
    //Create an array to hold the URLS
    NSMutableArray *myURLS;

    //Initialize the array with nil
    myURLS = [[NSMutableArray alloc] init];

    NSLog(@"Here1");
    //Add all the URLs from the server to the array
    for (int i = 0; i <= 4; i++){
        NSString *tempString = [[NSString alloc] initWithFormat : @"http://myurl.com/GalleryImages/%djack1.jpg", i];
        [myURLS addObject: [NSURL URLWithString:tempString]];
        [tempString release];
    }

    //Array to hold the image data
    NSMutableArray *myData;

    //Initialize the array with nil
    myData = [[NSMutableArray alloc] init];

    NSLog(@"Here2");
    //Add all the URLs from the server to the array
    for (int i = 0; i <= 4; i++){
        [myData addObject: [[NSData alloc] initWithContentsOfURL: [myURLS objectAtIndex:i]]];
    }

    //Array to hold the image data
    NSMutableArray *myImages;

    //Initialize the array with nil
    myImages = [[NSMutableArray alloc] init];

    NSLog(@"Here3");
    //Add all the URLs from the server to the array
    for (int i = 0; i <= 4; i++){
        [myImages addObject: [UIImage imageWithData: [myData objectAtIndex:i]]];
    }

    // Load an array of images into the page view controller
    //Initialising them with the data stored above
    NSArray *array = [[NSArray alloc] initWithArray:myImages];
    [self setImages:array];

    //Release the image data
    [myURLS release];
    [myData release];
    [myImages release];
    [array release];
}

}

-（void）loadDataWithOperation{
//连接测试
NSURL*url=[NSURL URLWithString:@”http://myurl.com/testconnection.php"];
n错误*错误；
NSString*connected=[NSString stringWithContentsOfURL:url编码：NSASCISTRINGEncoding错误：&错误]；
//如果连接的字符串未使用URL的内容初始化自身：
if（connected==NULL）{
//显示错误图片：
UIAlertView*alert=[[UIAlertView alloc]initWithTitle:@“错误！”消息：@“下载多媒体资料时出错，请检查网络连接并重试。”代理：自取消按钮：@“确定”其他按钮：无]；
[警报显示]；
[警报发布]；
[自创建BackButton]；
}否则{
//加载数据
NSLog（@“已连接-%@”，已连接）；
NSLog（@“LoadDataWithOperation”）；
//创建一个数组来保存URL
NSMutableArray*myurl；
//用nil初始化数组
myURL=[[NSMutableArray alloc]init]；
NSLog（“此处1”）；
//将服务器中的所有URL添加到阵列中
对于（int i=0；i在后台下载图像：

-(void)imageDownloadStart
{
[self performSelectorInBackground:@selector(downloadImages) withObject:nil];
}



-(void)downloadImages
{
NSAutoreleasePool* pool = [[NSAutoreleasePool alloc] init];

//WRITE YOU DOWNLOADING CODE HERE

if(yourCondition)
  {
  [self performSelectorOnMainThread:@selector(imageDownloadStart) withObject:nil waitUntilDone:YES];
  } 
[pool release];
}

以及调整大小：

-(UIImage *)resizeImage:(UIImage *)image withSize:(CGSize)newSize
{
float actualHeight = image.size.height;
float actualWidth = image.size.width;
float imgRatio = actualWidth/actualHeight;
float maxRatio = newSize.width/newSize.height;

if(imgRatio!=maxRatio)
{
    if(imgRatio < maxRatio){
        imgRatio = newSize.width / actualHeight;
        actualWidth = round(imgRatio * actualWidth);
        actualHeight = newSize.width;
    }
    else{
        imgRatio = newSize.height / actualWidth;
        actualHeight = round(imgRatio * actualHeight);
        actualWidth = newSize.height;
    }
}
CGRect rect = CGRectMake(0.0, 0.0, actualWidth, actualHeight);
UIGraphicsBeginImageContext(rect.size);
[image drawInRect:rect];
UIImage *resizedImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
//[resizedImage release];
return resizedImage;
}

-（UIImage*）大小图像：（UIImage*）图像大小：（CGSize）新闻大小
{
浮动实际高度=image.size.height；
浮动实际宽度=image.size.width；
浮动高度=实际宽度/实际高度；
float maxRatio=newSize.width/newSize.height；
如果（imgRatio！=最大比率）
{
if（imgRatio

然而，我的问题是，如果我是
要增加要显示的图像数量，请执行以下操作：
比如说，20，会有很长的一段时间
正在下载图像，请稍候，然后重试
然后显示

NSURLConnection
只允许同时连接6个服务器。您应该使用
NSOperationQueue
并调用
setMaxConcurrentOperationCount
将值设置为小于6

库
ASIHTTPRequest
通过最多4个并发操作实现此方法（以防代码中的其他内容发出请求）。请参阅中的示例