iPhone程序中图片延时加载

从网上加载图片,当网速慢或是图片较大时,你会发现程序可能会失去对用户的响应.这样你可以用多线程:

Java代码  

1. -(void) buildData {   
2.     NSOperationQueue *queue = [NSOperationQueue new];   
3.        
4.     [queue setMaxConcurrentOperationCount:NSOperationQueueDefaultMaxConcurrentOperationCount];   
5.        
6.     NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self    
7.                                                                             selector:@selector(downloadImage)    
8.                                                                               object:nil];   
9.     [queue addOperation:operation];   
10.     [operation release];   
11. }  

-(void) buildData {
        NSOperationQueue *queue = [NSOperationQueue new];
        
        [queue setMaxConcurrentOperationCount:NSOperationQueueDefaultMaxConcurrentOperationCount];
        
        NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self 
                                                                                                                                                        selector:@selector(downloadImage) 
                                                                                                                                                          object:nil];
        [queue addOperation:operation];
        [operation release];
}

解决的方法是从网上down下来一次后就将图片缓存起来,再次显示的时候就不用去下载。
假设有多张图片,用循环存多个路径:

Java代码  

1. - (void)downloadImage {   
2.     NSString *imagePath;   
3.     for (...)   
4.         imagePath = [GetImage saveImage:imageUrlPath withCache:@""];   
5. }  

- (void)downloadImage {
        NSString *imagePath;
        for (...)
                imagePath = [GetImage saveImage:imageUrlPath withCache:@""];
}

需要写GetImage类,实现刚才的方法.
GetImage.h文件如下:

Java代码  

1. #import    
2.   
3.   
4. @interface GetImage : NSObject {   
5.        
6. }   
7. +(NSString *) saveImage:(NSString *)urlpath withCache:(NSString *)filename;   
8.   
9. @end  

#import 


@interface GetImage : NSObject {
        
}
+(NSString *) saveImage:(NSString *)urlpath withCache:(NSString *)filename;

@end

GetImage.m文件如下:

Java代码  

1. @implementation GetImage   
2. +(NSString *) saveImage:(NSString *)urlpath withCache:(NSString *)filename   
3. {   
4.     NSData *retureData=nil;   
5.     NSAutoreleasePool *pool=[[NSAutoreleasePool alloc] init];   
6.     NSFileManager *fileManager = [NSFileManager defaultManager];   
7.     NSArray *cache = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);   
8.     NSString *cachePath = [cache objectAtIndex:0] ;    
9.     filename=[filename stringByAppendingFormat:@"%@",[urlpath lastPathComponent]];   
10.     NSString *filepath = [cachePath stringByAppendingString:@"/"];             
11.     filepath=[filepath stringByAppendingString:filename];   
12.        
13.     NSLog(@"filepath=%@",filepath);   
14.     BOOL success;   
15.     success = [fileManager fileExistsAtPath:filepath];   
16.     if (success)    
17.     {   
18.         return   filepath;   
19.            
20.     }   
21.     else  
22.     {   
23.         NSMutableURLRequest *request = [[[NSMutableURLRequest alloc] init] autorelease];     
24.         [request setURL:[NSURL URLWithString:urlpath]];     
25.         [request setHTTPMethod:@"GET"];     
26.            
27.         NSURLResponse *response;   
28.         NSError *error;   
29.            
30.         retureData=[NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];   
31.            
32.         if ([fileManager createDirectoryAtPath:cachePath attributes:nil]==NO){   
33.             ////NSLog(@"fileManager createDirectoryAtPath:cachePath attributes:nil");   
34.         }   
35.         if ([retureData writeToFile:filepath atomically:YES]){   
36.             NSLog(@"save Image Success");   
37.         }   
38.         else  
39.         {   
40.             NSLog(@"save Image Fail");   
41.         }   
42.     }   
43.        
44.     if (retureData !=nil && [fileManager fileExistsAtPath:filepath]){   
45.            
46.         return   filepath;   
47.     }   
48.     [pool release];   
49.        
50.     NSLog(@" Image return nil");   
51.     return nil;   
52.        
53.        
54.        
55. }  

@implementation GetImage
+(NSString *) saveImage:(NSString *)urlpath withCache:(NSString *)filename
{
        NSData *retureData=nil;
        NSAutoreleasePool *pool=[[NSAutoreleasePool alloc] init];
        NSFileManager *fileManager = [NSFileManager defaultManager];
        NSArray *cache = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES);
        NSString *cachePath = [cache objectAtIndex:0] ; 
        filename=[filename stringByAppendingFormat:@"%@",[urlpath lastPathComponent]];
        NSString *filepath = [cachePath stringByAppendingString:@"/"];                  
        filepath=[filepath stringByAppendingString:filename];
        
        NSLog(@"filepath=%@",filepath);
        BOOL success;
        success = [fileManager fileExistsAtPath:filepath];
    if (success) 
        {
                return   filepath;
                
        }
        else
        {
                NSMutableURLRequest *request = [[[NSMutableURLRequest alloc] init] autorelease];  
                [request setURL:[NSURL URLWithString:urlpath]];  
                [request setHTTPMethod:@"GET"];  
                
                NSURLResponse *response;
                NSError *error;
                
                retureData=[NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error];
                
                if ([fileManager createDirectoryAtPath:cachePath attributes:nil]==NO){
                        ////NSLog(@"fileManager createDirectoryAtPath:cachePath attributes:nil");
                }
                if ([retureData writeToFile:filepath atomically:YES]){
                        NSLog(@"save Image Success");
                }
                else
                {
                        NSLog(@"save Image Fail");
                }
        }
        
        if (retureData !=nil && [fileManager fileExistsAtPath:filepath]){
                
                return   filepath;
        }
        [pool release];
        
        NSLog(@" Image return nil");
        return nil;
        
        
        
}

至此,存储完毕,在用的时候调用刚才存的路径就可以了,可用方法[[UIImage alloc] initWithContentsOfFile:imagePath]