iOS 图片加载方法

(1)最简单的下载,显示图片的方法: 

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   UIImageView *imageView = [[UIImageView alloc] initWithFrame:[self.view bounds]];  

   imageView.image = [self loadImageFromUrl:@"http://storage.live.com/items/72A00BF5A838647C!1616?filename=meinv004.jpg"];  

   [self.view addSubview:imageView];  

     

   -(UIImage*)loadImageFromUrl: (NSString*)url  

{  

    NSURL  *imageUrl = [NSURL URLWithString:url];  

    NSData *imageData = [NSData dataWithContentsOfURL:imageUrl];  

    UIImage *image = [UIImage imageWithData:imageData];  

    return image;  

}  

    
这种最简单的图片加载方式阻塞了main线程. 使得流程不能流畅进行.

(2)开辟线程来解决这个问题.

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   // set imageview  

   UIImageView *imageView = [[UIImageView alloc] initWithFrame:[self.view bounds]];  

   imageView.backgroundColor = [UIColor yellowColor];  

   imageView.tag = imageView_tag;  

   [self.view addSubview:imageView];  

     

   // load image in background  

   NSString *url = IMAGE_URL;  

   [self performSelectorInBackground:@selector(loadImageFromUrl:) withObject:url];  

     

  

  

-(void)loadImageFromUrl: (NSString*)url {  

    NSURL  *imageUrl = [NSURL URLWithString:url];  

    NSData *imageData = [NSData dataWithContentsOfURL:imageUrl];  

    [self performSelectorOnMainThread:@selector(updateImageView:) withObject:imageData waitUntilDone:NO];  

}  

-(void) updateImageView:(NSData*) data {  

    UIImageView *imageView = (UIImageView *)[self.view viewWithTag:imageView_tag];  

    imageView.image = [UIImage imageWithData:data];  

}  

并且只能在main线程中设置UI的内容, 所以代码量增加了较多. 代码量暂且不管, 这里还有一个比较严重的问题就是每次都要加载图片, 

使用SDWebImage: 

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#import <SDWebImage/UIImageView+WebCache.h>  

[imageView setImageWithURL:[NSURL URLWithString:[_objects objectAtIndex:indexPath.row]]  

                  placeholderImage:[UIImage imageNamed:@"placeholder.png"]];  

SDWebImage可以实现: 
*下载和缓存图片.
*相同的url不会被重复下载.

*坏的url不会一直请求.

使用HJCache:

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// 目前HJCache不支持ARC, 所以这是个问题.  

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(二)多线程初步实现TableView的图片显示(之前用第三库老是不稳定) 这个算是比较满意的.

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@interface c:NSOperation  

  

@property NSString *url;  

@property NSString *imageName;  

@property UIImage *image;  

@property UIImageView *delegate;  

  

-(void) main;  

-(id) initWith:(NSString *)url imageName:(NSString *)imageName delegate:(UIImageView *)delegate;  

  

@end  

  

@implementation c:NSOperation  

@synthesize url = _url,imageName=_imageName, image=_image, delegate=_delegate;  

  

-(id) initWith:(NSString *)url imageName:(NSString *)imageName delegate:(UIImageView *)delegate{  

    if (self = [super init]) {  

        self.url = url;  

        self.imageName = imageName;  

        self.delegate = delegate;  

    }  

    return self;  

}  

  

-(void) main{  

    //  

    NSString *cachefile = [NSTemporaryDirectory() stringByAppendingPathComponent: self.imageName];  

    NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:self.url]];  

    [data writeToFile:cachefile atomically:YES];  

      

    //  

    self.image = [UIImage imageWithData:data];  

    [self performSelectorOnMainThread:@selector(u) withObject:nil waitUntilDone:NO];  

}  

-(void)u{  

    [self.delegate setImage:self.image];  

}  

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queue = [[NSOperationQueue alloc] init];//这是成员队列的实例化  

设置TableView cell中的图片:

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NSString *filename = [NSString stringWithFormat:@"%d", indexPath.row];  

NSString *cachefile = [NSTemporaryDirectory() stringByAppendingPathComponent: filename];  

UIImage *image = [UIImage imageWithContentsOfFile:cachefile];  

if (image) {  

    cell.imageView.image = image;  

} else {  

    c *o = [[c alloc] initWith:[_objects objectAtIndex:indexPath.row] imageName:[NSString stringWithFormat:@"%d",indexPath.row] delegate:cell.imageView];  

    [queue addOperation:o];  

    cell.imageView.image= [UIImage imageNamed:@"placeholder.png"];  

}