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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1.    UIImageView *imageView = [[UIImageView alloc] initWithFrame:[self.view bounds]];  
2.    imageView.image = [self loadImageFromUrl:@"http://storage.live.com/items/72A00BF5A838647C!1616?filename=meinv004.jpg"];  
3.    [self.view addSubview:imageView];  
4.      
5.    -(UIImage*)loadImageFromUrl: (NSString*)url  
6. {  
7.     NSURL  *imageUrl = [NSURL URLWithString:url];  
8.     NSData *imageData = [NSData dataWithContentsOfURL:imageUrl];  
9.     UIImage *image = [UIImage imageWithData:imageData];  
10.     return image;  
11. }  

    
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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1.    // set imageview  
2.    UIImageView *imageView = [[UIImageView alloc] initWithFrame:[self.view bounds]];  
3.    imageView.backgroundColor = [UIColor yellowColor];  
4.    imageView.tag = imageView_tag;  
5.    [self.view addSubview:imageView];  
6.      
7.    // load image in background  
8.    NSString *url = IMAGE_URL;  
9.    [self performSelectorInBackground:@selector(loadImageFromUrl:) withObject:url];  
10.      
11.   
12.   
13. -(void)loadImageFromUrl: (NSString*)url {  
14.     NSURL  *imageUrl = [NSURL URLWithString:url];  
15.     NSData *imageData = [NSData dataWithContentsOfURL:imageUrl];  
16.     [self performSelectorOnMainThread:@selector(updateImageView:) withObject:imageData waitUntilDone:NO];  
17. }  
18. -(void) updateImageView:(NSData*) data {  
19.     UIImageView *imageView = (UIImageView *)[self.view viewWithTag:imageView_tag];  
20.     imageView.image = [UIImage imageWithData:data];  
21. }  

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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1. #import <SDWebImage/UIImageView+WebCache.h>  
2. [imageView setImageWithURL:[NSURL URLWithString:[_objects objectAtIndex:indexPath.row]]  
3.                   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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1. // 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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1. @interface c:NSOperation  
2.   
3. @property NSString *url;  
4. @property NSString *imageName;  
5. @property UIImage *image;  
6. @property UIImageView *delegate;  
7.   
8. -(void) main;  
9. -(id) initWith:(NSString *)url imageName:(NSString *)imageName delegate:(UIImageView *)delegate;  
10.   
11. @end  
12.   
13. @implementation c:NSOperation  
14. @synthesize url = _url,imageName=_imageName, image=_image, delegate=_delegate;  
15.   
16. -(id) initWith:(NSString *)url imageName:(NSString *)imageName delegate:(UIImageView *)delegate{  
17.     if (self = [super init]) {  
18.         self.url = url;  
19.         self.imageName = imageName;  
20.         self.delegate = delegate;  
21.     }  
22.     return self;  
23. }  
24.   
25. -(void) main{  
26.     //  
27.     NSString *cachefile = [NSTemporaryDirectory() stringByAppendingPathComponent: self.imageName];  
28.     NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:self.url]];  
29.     [data writeToFile:cachefile atomically:YES];  
30.       
31.     //  
32.     self.image = [UIImage imageWithData:data];  
33.     [self performSelectorOnMainThread:@selector(u) withObject:nil waitUntilDone:NO];  
34. }  
35. -(void)u{  
36.     [self.delegate setImage:self.image];  
37. }  

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1. 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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1. NSString *filename = [NSString stringWithFormat:@"%d", indexPath.row];  
2. NSString *cachefile = [NSTemporaryDirectory() stringByAppendingPathComponent: filename];  
3. UIImage *image = [UIImage imageWithContentsOfFile:cachefile];  
4. if (image) {  
5.     cell.imageView.image = image;  
6. } else {  
7.     c *o = [[c alloc] initWith:[_objects objectAtIndex:indexPath.row] imageName:[NSString stringWithFormat:@"%d",indexPath.row] delegate:cell.imageView];  
8.     [queue addOperation:o];  
9.     cell.imageView.image= [UIImage imageNamed:@"placeholder.png"];  
10. }