版本记录
| 版本号 | 时间 |
|---|---|
| V1.0 | 2017.07.13 |
前言
我们做APP，文字和图片是绝对不可缺少的元素，特别是图片一般存储在图床里面，一般公司可以委托第三方保存，NB的公司也可以自己存储图片，ios有很多图片加载的第三方框架，其中最优秀的莫过于SDWebImage，它几乎可以满足你所有的需求，用了好几年这个框架，今天想总结一下。感兴趣的可以看其他几篇。
1. SDWebImage探究（一）
SDWebImage入口
我们前面讲的是SDWebImage框架的基础部分，下面我们就说一下SDWebImage框架的原理等其他部分。我们比较常用的就是用#import "UIImageView+WebCache.h"这个类，我们先看一下入口。
/**
* Set the imageView `image` with an `url`.
*
* The download is asynchronous and cached.
*
* @param url The url for the image.
*/
- (void)sd_setImageWithURL:(nullable NSURL *)url;
上面这个方法，会调用下面其他的几个方法，分别如下，其中最下面的这个方法是参数最齐全的。
/**
* Set the imageView `image` with an `url` and a placeholder.
*
* The download is asynchronous and cached.
*
* @param url The url for the image.
* @param placeholder The image to be set initially, until the image request finishes.
* @see sd_setImageWithURL:placeholderImage:options:
*/
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder;
/**
* Set the imageView `image` with an `url`, placeholder and custom options.
*
* The download is asynchronous and cached.
*
* @param url The url for the image.
* @param placeholder The image to be set initially, until the image request finishes.
* @param options The options to use when downloading the image. @see SDWebImageOptions for the possible values.
*/
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options;
/**
* Set the imageView `image` with an `url`.
*
* The download is asynchronous and cached.
*
* @param url The url for the image.
* @param completedBlock A block called when operation has been completed. This block has no return value
* and takes the requested UIImage as first parameter. In case of error the image parameter
* is nil and the second parameter may contain an NSError. The third parameter is a Boolean
* indicating if the image was retrieved from the local cache or from the network.
* The fourth parameter is the original image url.
*/
- (void)sd_setImageWithURL:(nullable NSURL *)url
completed:(nullable SDExternalCompletionBlock)completedBlock;
/**
* Set the imageView `image` with an `url`, placeholder.
*
* The download is asynchronous and cached.
*
* @param url The url for the image.
* @param placeholder The image to be set initially, until the image request finishes.
* @param completedBlock A block called when operation has been completed. This block has no return value
* and takes the requested UIImage as first parameter. In case of error the image parameter
* is nil and the second parameter may contain an NSError. The third parameter is a Boolean
* indicating if the image was retrieved from the local cache or from the network.
* The fourth parameter is the original image url.
*/
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
completed:(nullable SDExternalCompletionBlock)completedBlock;
/**
* Set the imageView `image` with an `url`, placeholder and custom options.
*
* The download is asynchronous and cached.
*
* @param url The url for the image.
* @param placeholder The image to be set initially, until the image request finishes.
* @param options The options to use when downloading the image. @see SDWebImageOptions for the possible values.
* @param completedBlock A block called when operation has been completed. This block has no return value
* and takes the requested UIImage as first parameter. In case of error the image parameter
* is nil and the second parameter may contain an NSError. The third parameter is a Boolean
* indicating if the image was retrieved from the local cache or from the network.
* The fourth parameter is the original image url.
*/
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
completed:(nullable SDExternalCompletionBlock)completedBlock;
/**
* Set the imageView `image` with an `url`, placeholder and custom options.
*
* The download is asynchronous and cached.
*
* @param url The url for the image.
* @param placeholder The image to be set initially, until the image request finishes.
* @param options The options to use when downloading the image. @see SDWebImageOptions for the possible values.
* @param progressBlock A block called while image is downloading
* @note the progress block is executed on a background queue
* @param completedBlock A block called when operation has been completed. This block has no return value
* and takes the requested UIImage as first parameter. In case of error the image parameter
* is nil and the second parameter may contain an NSError. The third parameter is a Boolean
* indicating if the image was retrieved from the local cache or from the network.
* The fourth parameter is the original image url.
*/
- (void)sd_setImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock;
/**
* Set the imageView `image` with an `url` and optionally a placeholder image.
*
* The download is asynchronous and cached.
*
* @param url The url for the image.
* @param placeholder The image to be set initially, until the image request finishes.
* @param options The options to use when downloading the image. @see SDWebImageOptions for the possible values.
* @param progressBlock A block called while image is downloading
* @note the progress block is executed on a background queue
* @param completedBlock A block called when operation has been completed. This block has no return value
* and takes the requested UIImage as first parameter. In case of error the image parameter
* is nil and the second parameter may contain an NSError. The third parameter is a Boolean
* indicating if the image was retrieved from the local cache or from the network.
* The fourth parameter is the original image url.
*/
- (void)sd_setImageWithPreviousCachedImageWithURL:(nullable NSURL *)url
placeholderImage:(nullable UIImage *)placeholder
options:(SDWebImageOptions)options
progress:(nullable SDWebImageDownloaderProgressBlock)progressBlock
completed:(nullable SDExternalCompletionBlock)completedBlock;
入口就这些，下面继续向下研究。
SDWebImage原理
下面我们就看一下SDWebImage的原理。
- 入口
setImageWithURL:placeholderImage:options:会先把 placeholderImage 显示，然后 SDWebImageManager 根据 URL 开始处理图片。 - 进入
SDWebImageManagerdownloadWithURL:delegate:options:userInfo:，交给SDImageCache从缓存查找图片是否已经下载queryDiskCacheForKey:delegate:userInfo:。 - 先从内存图片缓存查找是否有图片，如果内存中已经有图片缓存，
SDImageCacheDelegate回调imageCache:didFindImage:forKey:userInfo:到SDWebImageManager。 -
SDWebImageManagerDelegate回调webImageManager:didFinishWithImage:到UIImageView+WebCache等前端展示图片。 - 如果内存缓存中没有，生成
NSInvocationOperation
添加到队列开始从硬盘查找图片是否已经缓存。 - 根据 URLKey 在硬盘缓存目录下尝试读取图片文件。这一步是在
NSOperation进行的操作，所以回主线程进行结果回调notifyDelegate:。 - 如果上一操作从硬盘读取到了图片，将图片添加到内存缓存中
（如果空闲内存过小，会先清空内存缓存）。SDImageCacheDelegate回调imageCache:didFindImage:forKey:userInfo:。进而回调展示图片。 - 如果从硬盘缓存目录读取不到图片，
说明所有缓存都不存在该图片，需要下载图片，
回调imageCache:didNotFindImageForKey:userInfo:。 - 共享或重新生成一个下载器
SDWebImageDownloader开始下载图片。 - 图片下载由
NSURLSession(NSURLConnection已经被剔除了)来做，实现相关 delegate 来判断图片下载中、下载完成和下载失败。 -
session:didReceiveData:中利用ImageIO做了按图片下载进度加载效果。 -
sessionDidFinishLoading:数据下载完成后交给SDWebImageDecoder做图片解码处理。 - 图片解码处理在一个
NSOperationQueue完成，不会拖慢主线程 UI。如果有需要对下载的图片进行二次处理，最好也在这里完成，效率会好很多。 - 在主线程
notifyDelegateOnMainThreadWithInfo:宣告解码完成，imageDecoder:didFinishDecodingImage:userInfo:回调给SDWebImageDownloader。 -
imageDownloader:didFinishWithImage:回调给SDWebImageManager告知图片下载完成。 - 通知所有的
downloadDelegates下载完成，回调给需要的地方展示图片。 - 将图片保存到
SDImageCache中，内存缓存和硬盘缓存同时保存。 - 写文件到硬盘也在以单独
NSInvocationOperation完成，
避免拖慢主线程。 -
SDImageCache在初始化的时候会注册一些消息通知，在内存警告或退到后台的时候清理内存图片缓存，应用结束的时候清理过期图片。 -
SDWI也提供了UIButton+WebCache和MKAnnotationView+WebCache，方便使用。 -
SDWebImagePrefetcher可以预先下载图片，方便后续使用。
总的流程图如下所示。
原理流程图
这一篇就先说这么多了，后续跟进~~~
后记
未完，待续~~~
思念
