比较原始的方法：

复制代码 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：
复制代码 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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