糗事百科的閱讀方式跟微博很想，它是按時間線排布的。所以過幾分鐘刷新出來的內(nèi)容會是全新的。那這種情況其實做緩存的意義不是很大。這邊只是個實例而已，演示如何把圖片緩存到本地，在需要時優(yōu)先加載本地圖片，如果不存在再從網(wǎng)絡(luò)獲取，獲取成功后緩存起來。

所以整個流程是：

所有的操作均在UIImageView+Util這個UIImageView擴展類中實現(xiàn)，具體代碼如下：

<pre>

• (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(@"加載圖片出問題了%@", connectionError.localizedDescription);
    }
  

  }];
  }
  </pre>

imageFromCacheWithURLString
<pre>

• (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(@"獲取緩存圖片目錄列表出錯：%@", 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;

}
</pre>

imageCachePath

<pre>- (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;

}</pre>

saveImageToCache

<pre>- (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];

}</pre>

需要注意一點的是MD5的使用。在判斷緩存文件是否是我們需要的時候，我們使用url的md5值來做唯一性比較，這樣會比較準(zhǔn)確，但是會浪費一些時間開銷。