Joke基本项目构建3 添加缓存图片

糗事百科的阅读方式跟微博很想，它是按时间线排布的。所以过几分钟刷新出来的内容会是全新的。那这种情况其实做缓存的意义不是很大。这边只是个实例而已，演示如何把图片缓存到本地，在需要时优先加载本地图片，如果不存在再从网络获取，获取成功后缓存起来。

所以整个流程是：

所有的操作均在UIImageView+Util这个UIImageView扩展类中实现，具体代码如下：

 
(void)setImageWithURL:(NSString *)url placehold:(UIImage *)image
 {
 self.image = image;
 UIImage * cacheImage = [self imageFromCacheWithURLString:url];
 if (cacheImage) {
 self.image = cacheImage;
 return;
 }
 
if (url == nil) {
 return;
 }
 
NSURL * requestURL = [NSURL URLWithString:url];
 NSURLRequest * request = [NSURLRequest requestWithURL:requestURL];
 NSOperationQueue * queue = [[NSOperationQueue alloc]init];
 
[NSURLConnection sendAsynchronousRequest:request queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
 
  if (connectionError == nil) {
      dispatch_async(dispatch_get_main_queue(), ^{
          UIImage * netImage = [UIImage imageWithData:data];
          
          self.image = netImage;
          
          [self saveImageToCache:netImage url:url];
      });
  } else {
      NSLog(@"%@", url);
      NSLog(@"加载图片出问题了%@", connectionError.localizedDescription);
  }
 
}];
 }
 

imageFromCacheWithURLString

 
(UIImage *)imageFromCacheWithURLString:(NSString *)urlString
 {
 if (urlString == nil || [urlString isEqualToString:@""]) {
 return nil;
 }
 NSString * md5String = [urlString md5];
 NSFileManager * fileManager = [NSFileManager defaultManager];
 NSError * error;
 __block UIImage * cacheImage = nil;
 
NSString * cacheImagePath = [self imageCachePath];
 NSArray * cacheImages = [fileManager contentsOfDirectoryAtPath:cacheImagePath error:&error];
 
if (error) {
 NSLog(@"获取缓存图片目录列表出错：%@", error.localizedDescription);
 return nil;
 }
 

[cacheImages enumerateObjectsUsingBlock:^(NSString * fileName, NSUInteger idx, BOOL *stop) {
    NSString * cacheImageItemPath = [cacheImagePath stringByAppendingPathComponent:fileName];
    if ([md5String isEqualToString:fileName]) {
        cacheImage = [UIImage imageWithContentsOfFile:cacheImageItemPath];
    }
}];

return cacheImage;

}
 

imageCachePath

- (NSString *)imageCachePath
 {
 NSString * documentsDirectory = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask, YES) objectAtIndex:0];
 NSString * cacheImagePath = [documentsDirectory stringByAppendingPathComponent:@"_imageCache"];

NSFileManager * fileManager = [NSFileManager defaultManager];
if ([fileManager fileExistsAtPath:cacheImagePath] == NO) {
    [fileManager createDirectoryAtPath:cacheImagePath withIntermediateDirectories:YES attributes:nil error:nil];
}

return cacheImagePath;

}

saveImageToCache

- (void)saveImageToCache:(UIImage *)image url:(NSString *)url
 {
 NSString * cacheImagePath = [self imageCachePath];
 NSString * md5FileName = [url md5];
 NSString * imageCachePath = [cacheImagePath stringByAppendingPathComponent:md5FileName];

[UIImagePNGRepresentation(image) writeToFile:imageCachePath atomically:YES];

}

需要注意一点的是MD5的使用。在判断缓存文件是否是我们需要的时候，我们使用url的md5值来做唯一性比较，这样会比较准确，但是会浪费一些时间开销。