SDWebImage-图片自动缓存、异步加载实用库（转贴）

SDWebImage是一个可以自动管理图片加载的类库。

因为大家都知道，图片加载非常耗流量，所以在移动平台上对于图片的处理就要异常小心了。因此就必须用到本地缓存了。

而我之前写的一个小型App，因为每张图片的名字都是GUID生成的，不会被修改了。所以会每次都检查一下本地是否有这个文件名的文件存在了。这样最大限度的减小了网络流量，不需要每次都加载一次。

不过呢，SDWebImage的功能不仅仅仅限于此，功能更为强大。最基本的有一个UIImageView的category，用法很简单

[imageView setImageWithURL:[NSURLURLWithString:@"http://www.ioslib.com/ioslib.png"]];

另外呢，还有一个SDWebImageManager，使用它可以进行一些异步加载的工作，关于这部分内容，可以参见官方的文档：

https://github.com/rs/SDWebImage#readme

SDWebImage是托管在Github上的：http://github.com/rs/SDWebImage

SDWebImage——简化网络图片处理

用SDWebImage调用网站上的图片，跟本地调用内置在应用包里的图片一样简单。操作也很简单，举例说明

Using UIImageView+WebCache category with UITableView

Just #import the UIImageView+WebCache.h header, and call the setImageWithURL:placeholderImage:method from the tableView:cellForRowAtIndexPath: UITableViewDataSource method. Everything will behandled for you, from async downloads to caching management.

#import "UIImageView+WebCache.h"

...

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *MyIdentifier = @"MyIdentifier";

UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:MyIdentifier];

if (cell == nil)
{
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
reuseIdentifier:MyIdentifier] autorelease];
}

// Here we use the new provided setImageWithURL: method to load the web image
[cell.imageView setImageWithURL:[NSURL URLWithString:@"http://www.domain.com/path/to/image.jpg"]
placeholderImage:[UIImage imageNamed:@"placeholder.png"]];

cell.textLabel.text = @"My Text";
return cell;
}

Using SDWebImageManager

The SDWebImageManager is the class behind the UIImageView+WebCache category. It ties theasynchronous downloader with the image cache store. You can use this classe directly to benefitsfrom web image downloading with caching in another context than a UIView
(ie: with Cocos).

Here is a simple example of how to use SDWebImageManager:

SDWebImageManager *manager = [SDWebImageManager sharedManager];

UIImage *cachedImage = [manager imageWithURL:url];

if (cachedImage)
{
// Use the cached image immediatly
}
else
{
// Start an async download
[manager downloadWithURL:url delegate:self];
}

Your class will have to implement the SDWebImageManagerDelegate protocol, and to implement thewebImageManager:didFinishWithImage: method from this protocol:

- (void)webImageManager:(SDWebImageManager *)imageManager didFinishWithImage:(UIImage *)image
{
// Do something with the downloaded image
}

Using Asynchronous Image Downloader Independently

It is possible to use the async image downloader independently. You just have to create an instanceof SDWebImageDownloader using its convenience constructor downloaderWithURL:delegate:.

downloader = [SDWebImageDownloader downloaderWithURL:url delegate:self];

The download will start immediately and the imageDownloader:didFinishWithImage: method from theSDWebImageDownloaderDelegate protocol will be called as soon as the download of image is completed.

Using Asynchronous Image Caching Independently

It is also possible to use the NSOperation based image cache store independently. SDImageCachemaintains a memory cache and an optional disk cache. Disk cache write operations are performedasynchronous so it doesn't add unnecessary latency to the UI.

The SDImageCache class provides a singleton instance for convenience but you can create your owninstance if you want to create separated cache namespaces.

To lookup the cache, you use the imageForKey: method. If the method returns nil, it means the cachedoesn't currently own the image. You are thus responsible of generating and caching it. The cachekey is an application unique identifier for the image to cache.
It is generally the absolute URL ofthe image.

UIImage *myCachedImage = [[SDImageCache sharedImageCache] imageFromKey:myCacheKey];

By default SDImageCache will lookup the disk cache if an image can't be found in the memory cache.You can prevent this from happening by calling the alternative method imageFromKey:fromDisk: with anegative second argument.

To store an image into the cache, you use the storeImage:forKey: method:

[[SDImageCache sharedImageCache] storeImage:myImage forKey:myCacheKey];

By default, the image will be stored in memory cache as well as on disk cache (asynchronously). Ifyou want only the memory cache, use the alternative method storeImage:forKey:toDisk: with a negativethird argument.

转自 http://www.isdada.com/sdwebimage.html

偶然所得，不敢独享，与大家分享之～