1. Create a NSOperationQueue to handle background downloading.

NSOperationQueue *operationQueue = [[NSOperationQueue alloc]init];

2. Now create an NSInvocationOperation for each image that you want to download and add it to operationQueue.

int noOfImages = 500;

for(int i = 0; i  noOfImages; i++)

{

NSInvocationOperation *invocationOperation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(loadDataAsychn:) object:i];

[queue addOperation:invocationOperation];

[operation release];

}

In this code, we have created an operation calling loadDataAsynchn: method which is to be implemented and added to queue. After adding created operation to the queue, it will do the rest of the downloading work for your app. You can add more operations to the same queue to be performed later.

Now we will implement loadDataAsychn method.

3. Implement loadDataAsychn method for loading data from the given URL, creating an image and setting it to desired placeholder.

-(void)loadDataAsychn:(NSString*)imageNo

{

NSString *imgStr = [NSString stringWithFormat:@"http://192.168.0.163/Test/images/%@.jpg",imageNo];

NSData *imageData=[[NSData alloc] initWithContentsOfURL: [NSURL URLWithString:imgStr]];

UIImage *image=[[UIImage alloc]initWithData:imageData];

[imgDict setValue:image forKey:@"image"];

[imgDict setValue:imageNo forKey:@"imageNo"];

[self performSelectorOnMainThread:@selector(showImg:) withObject:imgDict waitUntilDone:NO];

}

The loadDataAsychn method is used to download the information from a given URL and creating an image with downloaded data. As we all know that none of the UI related methods can be performed on background thread. So, in loadDataAsychn method, we call showImage method on the main thread to display images.

4. We will now implement showImage which is responsible for updating UI.

(void)showImage:(NSMutableDictionary *)Dict

{

int i = [[Dict objectForKey:@"imageNo"] intValue];

UIImageView *imgView = [[UIImageView alloc] initWithFrame:CGRectMake(320*(i-1), 0, 320, 460)];

[imgView setImage:[Dict objectForKey:@"image"]];

[myScrollView addSubview:imgView];

}

In this method, we create an UIImageView for displaying image, setting image on it and adding it to scroll-view dynamically.

官方的相册程序示例  http://developer.apple.com/library/ios/#samplecode/PhotoScroller/Introduction/Intro.html#//apple_ref/doc/uid/DTS40010080-Intro-DontLinkElementID_2