iPhone程序中圖片延時加載

轉自：http://wonderzl.iteye.com/blog/696160

從網上加載圖片,當網速慢或是圖片較大時,你會發現程序可能會失去對用戶的響應.這樣你可以用多線程: 

-(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];
}

解決的方法是從網上down下來一次後就將圖片緩存起來,再次顯示的時候就不用去下載。 
假設有多張圖片,用循環存多個路徑: 

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

需要寫GetImage類,實現剛纔的方法. 
GetImage.h文件如下: 

#import <Foundation/Foundation.h>


@interface GetImage : NSObject {

}
+(NSString *) saveImage:(NSString *)urlpath withCache:(NSString *)filename;

@end

GetImage.m文件如下: 

@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,存儲完畢,在用的時候調用剛纔存的路徑就可以了,可用方法[[UIImage alloc] initWithContentsOfFile:imagePath]