从网络下载图片，保存，并用UIImageView从保存中显示

代码：

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.

    UIImageView *img=[[UIImageView alloc]initWithFrame:CGRectMake(50, 100, 200, 200)];
    img.backgroundColor=[UIColor redColor];
    [self.view addSubview:img];

    //从网络下载图片，保存，并用 UIImageView 从保存中显示
    NSString * documentsDirectoryPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
    NSLog(@"保存路径:%@",documentsDirectoryPath);

    //Get Image From URL
    UIImage * imageFromURL = [self getImageFromURL:@"http://file.duteba.com/phone/2009/04/5/ftGxL8kcUL.jpg"];

    //Save Image to Directory
    [self saveImage:imageFromURL withFileName:@"MyImage" ofType:@"jpg" inDirectory:documentsDirectoryPath];

    //Load Image From Directory
    UIImage * imageFromWeb = [self loadImage:@"MyImage" ofType:@"jpg" inDirectory:documentsDirectoryPath];
    [img setImage:imageFromWeb];

    //取得目录下所有文件名
    NSArray *file = [[[NSFileManager alloc] init] subpathsAtPath:documentsDirectoryPath];
    NSLog(@"%d",[file count]);
    NSLog(@"%@",file);

}
//从网络下载图片
-(UIImage *) getImageFromURL:(NSString *)fileURL {
    NSLog(@"执行图片下载函数");
    UIImage * result;

    NSData * data = [NSData dataWithContentsOfURL:[NSURL URLWithString:fileURL]];
    result = [UIImage imageWithData:data];

    return result;
}

//将所下载的图片保存到本地
-(void) saveImage:(UIImage *)image withFileName:(NSString *)imageName ofType:(NSString *)extension inDirectory:(NSString *)directoryPath {
    if ([[extension lowercaseString] isEqualToString:@"png"]) {
        [UIImagePNGRepresentation(image) writeToFile:[directoryPath stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.%@", imageName, @"png"]] options:NSAtomicWrite error:nil];
    } else if ([[extension lowercaseString] isEqualToString:@"jpg"] || [[extension lowercaseString] isEqualToString:@"jpeg"]) {
        [UIImageJPEGRepresentation(image, 1.0) writeToFile:[directoryPath stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.%@", imageName, @"jpg"]] options:NSAtomicWrite error:nil];
    } else {
        //ALog(@"Image Save Failed\nExtension: (%@) is not recognized, use (PNG/JPG)", extension);
        NSLog(@"文件后缀不认识");
    }
}
//读取本地保存的图片
-(UIImage *) loadImage:(NSString *)fileName ofType:(NSString *)extension inDirectory:(NSString *)directoryPath {
    UIImage * result = [UIImage imageWithContentsOfFile:[NSString stringWithFormat:@"%@/%@.%@", directoryPath, fileName, extension]];

    return result;
}