比较原始的方法：

代码如下：

AsyncImageView.h:
#import <UIKit/UIKit.h>
@interface AsyncImageView : UIView
{
    NSURLConnection* connection;
    NSMutableData* data;
}
- (void)loadImageFromURL:(NSURL*)url;
@end
AsyncImageView.m:
#import "AsyncImageView.h"
@implementation AsyncImageView
- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if(self) {
        // Initialization code
    }
    returnself;
}
- (void)loadImageFromURL:(NSURL*)url {
    if(connection!=nil) { [connection release]; }
    if(data!=nil) { [data release]; }
    NSURLRequest* request = [NSURLRequest requestWithURL:url
                                             cachePolicy:NSURLRequestUseProtocolCachePolicy
                                         timeoutInterval:60.0];
    connection = [[NSURLConnection alloc]
                  initWithRequest:request delegate:self];
}
- (void)connection:(NSURLConnection *)theConnection
    didReceiveData:(NSData *)incrementalData {
    if(data==nil) {
        data =
        [[NSMutableData alloc] initWithCapacity:2048];
    }
    [data appendData:incrementalData];
}
- (void)connectionDidFinishLoading:(NSURLConnection*)theConnection {
    [connection release];
    connection=nil;
    if([[self subviews] count] > 0) {
        [[[self subviews] objectAtIndex:0] removeFromSuperview];
    }
    UIImageView *imageView = [[[UIImageView alloc] initWithImage:[UIImage imageWithData:data]] autorelease];
    imageView.contentMode = UIViewContentModeScaleAspectFit;
    imageView.autoresizingMask = ( UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight );
    [self addSubview:imageView];
    imageView.frame = self.bounds;
    [imageView setNeedsLayout];
    [self setNeedsLayout];
    [data release];
    data=nil;
}
- (UIImage*) image {
    UIImageView* iv = [[self subviews] objectAtIndex:0];
    return[iv image];
}
- (void)dealloc {
    [connection cancel];
    [connection release];
    [data release];
    [super dealloc];
}
@end

方法二：
代码如下：

@interface UIButton (AsyncImage)
//size by point
- (void)setImageFromURL:(NSString *)urlString adjustToSize:(CGSize)size completion:(void (^)(void))completion logo:(UIImage *)logoImage;
@end
@implementation UIButton (AsyncImage)
- (void)setImageFromURL:(NSString *)urlString adjustToSize:(CGSize)size completion:(void (^)(void))completion logo:(UIImage *)logoImage
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        UIImage *image = nil;
        NSURL *url = [NSURL URLWithString:urlString];
        NSData *data = [NSData dataWithContentsOfURL:url];
        image = [UIImage imageWithData:data];   
        if (image) {
            if (!CGSizeEqualToSize(size, CGSizeZero)) {
                image = [UIImage imageWithCGImage:image.CGImage scale:[self scaleImage:image adjustToSize:size] orientation:image.imageOrientation];
            }
            if (logoImage) {
                image = [self addLogoImage:logoImage toImage:image];
            }
            dispatch_async(dispatch_get_main_queue(), ^{
                [self setImage:image forState:UIControlStateNormal];
                completion();
            });
        }
        else {
            NSLog(@"async load error.");
        }
    });
}
// 缩放图片以适应按钮大小
- (CGFloat)scaleImage:(UIImage *)image adjustToSize:(CGSize)size
{
    CGFloat xScale = size.width / image.size.width;
    CGFloat yScale = size.height / image.size.height;
    return 1.0 / MIN(xScale, yScale);
}
- (UIImage *)addLogoImage:(UIImage *)logo toImage:(UIImage *)img
{
    //get image width and height
    CGFloat scale = [UIScreen mainScreen].scale;
    int w = scale * img.size.width;
    int h = scale * img.size.height;
    int logoWidth = logo.scale * logo.size.width;
    int logoHeight = logo.scale * logo.size.height;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    //create a graphic context with CGBitmapContextCreate
    CGContextRef context = CGBitmapContextCreate(NULL, w, h, 8, 4 * w, colorSpace, kCGImageAlphaPremultipliedFirst);
    CGContextDrawImage(context, CGRectMake(0, 0, w, h), img.CGImage);
    CGContextDrawImage(context, CGRectMake(w - logoWidth, 0, logoWidth, logoHeight), [logo CGImage]);
    CGImageRef imageMasked = CGBitmapContextCreateImage(context);
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    return [UIImage imageWithCGImage:imageMasked scale:scale orientation:img.imageOrientation];
}
@end

方法三：

#import <Foundation/Foundation.h>#import "StringUtils.h"@interface ImageManager : NSObject{  NSMutableDictionary *_imageDict;  NSMutableArray *_imageArr;}@property(nonatomic, strong) NSString *httpUrl;@property(nonatomic, strong) NSMutableDictionary *imageDict;@property(nonatomic, assign) dispatch_queue_t networkQueue;+ (ImageManager *) sharedInstance;- (void)asyncImage:(NSString *)imageName imageView:(UIImageView *)imageView;//插队- (void)asyncImageInsert:(NSString *)imageName imageView:(UIImageView *)imageView insert:(BOOL)insert;//不要在下载之前的数据- (void)asyncImageCleanOld:(NSString *)imageName imageView:(UIImageView *)imageView cleanOld:(BOOL)cleanOld;@end

实现文件：

//// ImageManager.m// myb-ios//// Created by warrior gao on 13-6-5.// Copyright (c) 2013年 51myb. All rights reserved.//#import "ImageManager.h"@interface ImageManager()@end@implementation ImageManager//缓存图片的最大数量static int counter = 0;@synthesize imageDict = _imageDict;//Singleton + (ImageManager *)sharedInstance{  static id instance;  static dispatch_once_t onceToken;  dispatch_once(&onceToken, ^{    instance = self.new;  });  return instance;}- (id)init{  if((self = [super init]))  {    self.networkQueue = dispatch_queue_create("com.warrior.network.image", nil);    _imageDict = [[NSMutableDictionary alloc] init];    _imageArr = [[NSMutableArray alloc] init];  }  return self;}- (NSString *) fileFullPath:(NSString *)fileName{  NSString *cachePath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) objectAtIndex:0];    NSString *fileFullPath = [NSString stringWithFormat:@"%@/%@",cachePath,fileName];    return fileFullPath;}//不要在下载之前的数据- (void)asyncImageCleanOld:(NSString *)imageName imageView:(UIImageView *)imageView cleanOld:(BOOL)cleanOld{  if(cleanOld)  {    [_imageArr removeAllObjects];  }    [self asyncImage:imageName imageView:imageView];}//插队，优先- (void)asyncImageInsert:(NSString *)imageName imageView:(UIImageView *)imageView insert:(BOOL)insert{  if([StringUtils isEmpty:imageName]){    return;  }    NSData *data = [NSData dataWithContentsOfFile:[self fileFullPath:[imageName stringByReplacingOccurrencesOfString:@"/" withString:@"-"]]];  if(data == nil){    [_imageDict setValue:imageView forKey:imageName];    if(insert)    {      [_imageArr insertObject:imageName atIndex:0];    }    else    {      [_imageArr addObject:imageName];    }        [self cacheImage];  } else {    [imageView setImage:[UIImage imageWithData:data]];  }}//正常，附加到后面- (void)asyncImage:(NSString *)imageName imageView:(UIImageView *)imageView{  [self asyncImageInsert:imageName imageView:imageView insert:NO];}//异步缓存图片到本地，最多有两个线程-(void)cacheImage{  for (; counter < 2 && _imageArr.count > 0; counter++)  {    NSString *imageName = nil;    @synchronized(self){      imageName = [[_imageArr objectAtIndex:0] copy];      [_imageArr removeObjectAtIndex:0];    }        if(imageName == nil) continue;        dispatch_async(self.networkQueue, ^{            NSLog(@"Starting: %@", imageName);      UIImage *avatarImage = nil;      NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:@"%@%@",self.httpUrl, imageName]];      NSData *responseData = [NSData dataWithContentsOfURL:url];      if(responseData.length > 0)      {        [responseData writeToFile:[self fileFullPath:[imageName stringByReplacingOccurrencesOfString:@"/" withString:@"-"]] atomically:NO];        avatarImage = [UIImage imageWithData:responseData];        NSLog(@"Finishing: %@", imageName);                if (avatarImage) {          dispatch_async(dispatch_get_main_queue(), ^{            UIImageView *imageView = [_imageDict objectForKey:imageName];            if(imageView != nil && avatarImage != nil){              [imageView setImage:avatarImage];            }                        [_imageDict removeObjectForKey:imageName];            [imageName release];          });        }      }      counter--;      [self cacheImage];    });      }}@end

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