iOS网络开发（6）网络图片加载开源框架SDWebImage

2025-01-04 技术教程

SDWebImage是一个第三方框架，用以实现网络图像的缓存，及处理等功能。

Github的托管下载地址： https://github.com/rs/SDWebImage

SDWebImage默认使用磁盘缓存，

在沙盒/Library/Cache中可以找到带WebImageCache字眼的目录，可以找到缓存的图片

SDWebImage以分类的形式，对UIKit中的控件扩展了网络图片加载接口，使用起来非常方便。

介绍SDWebImage中的几个分类：

UIImageView加载网络图片

UIButton加载网络图片

UIImage显示GIF图片

并介绍图片下载进度的监听，以及框架内实现流程的简要说明

UIImageView+WebCache分类

提供了大量的方法，从网络中加载图片，而且这些操作全部是异步的

-(void)sd_setImageWithURL:(NSURL*)url-(void)sd_setImageWithURL:(NSURL*)urlcompleted:(SDWebImageCompletionBlock)completedBlock

部分方法还提供了占位图片的操作

-(void)sd_setImageWithURL:(NSURL*)urlplaceholderImage:(UIImage*)placeholder-(void)sd_setImageWithURL:(NSURL*)urlplaceholderImage:(UIImage*)placeholdercompleted:(SDWebImageCompletionBlock)completedBlock

部分方法还提供了额外选项用于指定缓存的策略

-(void)sd_setImageWithURL:(NSURL*)urlplaceholderImage:(UIImage*)placeholderoptions:(SDWebImageOptions)options-(void)sd_setImageWithURL:(NSURL*)urlplaceholderImage:(UIImage*)placeholderoptions:(SDWebImageOptions)optionscompleted:(SDWebImageCompletionBlock)completedBlock

如：

UIButton+WebCache分类

与UIImageView+WebCache类似，不过UIButton上可以显示两个图片

背景图片的加载

-(void)sd_setBackgroundImageWithURL:(NSURL*)urlforState:(UIControlState)state-(void)sd_setBackgroundImageWithURL:(NSURL*)urlforState:(UIControlState)statecompleted:(SDWebImageCompletionBlock)completedBlock-(void)sd_setBackgroundImageWithURL:(NSURL*)urlforState:(UIControlState)stateplaceholderImage:(UIImage*)placeholder-(void)sd_setBackgroundImageWithURL:(NSURL*)urlforState:(UIControlState)stateplaceholderImage:(UIImage*)placeholdercompleted:(SDWebImageCompletionBlock)completedBlock-(void)sd_setBackgroundImageWithURL:(NSURL*)urlforState:(UIControlState)stateplaceholderImage:(UIImage*)placeholderoptions:(SDWebImageOptions)options-(void)sd_setBackgroundImageWithURL:(NSURL*)urlforState:(UIControlState)stateplaceholderImage:(UIImage*)placeholderoptions:(SDWebImageOptions)optionscompleted:(SDWebImageCompletionBlock)completedBlock

图片的加载

-(void)sd_setImageWithURL:(NSURL*)urlforState:(UIControlState)state-(void)sd_setImageWithURL:(NSURL*)urlforState:(UIControlState)statecompleted:(SDWebImageCompletionBlock)completedBlock-(void)sd_setImageWithURL:(NSURL*)urlforState:(UIControlState)stateplaceholderImage:(UIImage*)placeholder-(void)sd_setImageWithURL:(NSURL*)urlforState:(UIControlState)stateplaceholderImage:(UIImage*)placeholdercompleted:(SDWebImageCompletionBlock)completedBlock-(void)sd_setImageWithURL:(NSURL*)urlforState:(UIControlState)stateplaceholderImage:(UIImage*)placeholderoptions:(SDWebImageOptions)options-(void)sd_setImageWithURL:(NSURL*)urlforState:(UIControlState)stateplaceholderImage:(UIImage*)placeholderoptions:(SDWebImageOptions)optionscompleted:(SDWebImageCompletionBlock)completedBlock

UIImage+GIF分类，用于加载gif图片

这个分类提供了创建GIF图片对象的方法

+(UIImage*)sd_animatedGIFNamed:(NSString*)name+(UIImage*)sd_animatedGIFWithData:(NSData*)data

有兴趣的朋友，可以在框架中看看这两个方法的实现代码

图片下载进度的监听

很多App中，图片加载时会有一个加载的进度条，UIImageView同样提供了方法可以连续获得图片的下载进度：

-(void)sd_setImageWithURL:(NSURL*)urlplaceholderImage:(UIImage*)placeholderoptions:(SDWebImageOptions)optionsprogress:(SDWebImageDownloaderProgressBlock)progressBlockcompleted:(SDWebImageCompletionBlock)completedBlock

如：

SDWebImage的原理分析

SDWebImage支持异步的图片下载+缓存，加载图片的基本流程：

显示placeholderImage，SDWebImageManager根据URL开始处理图片

从内存缓存中查找图片是否已被下载，如果找到，则回调显示图片

如果没有，则在子线程中查找磁盘缓存，，如果找到，则回调显示图片

如果没有，则开始从URL中下载图片（使用一个SDWebImageDownloader下载器对象）

通过代理方式，得到图片下载数据后，进行图片解码后，回调显示图片

将图片保存到SDImageCache中，内存缓存和磁盘缓存同时保存，写入磁盘的操作也将在子线程中执行

SDImageCache在初始化时会注册一些消息通知

在内存警告或退到后台时清理内存缓存，

在程序退出时清理过期图片