分析的版本

pod 'SDWebImage', '~> 5.0.6'

SDWebImage默认清除磁盘缓存的时长是7天。

/**
* The maximum length of time to keep an image in the disk cache, in seconds.
* Setting this to a negative value means no expiring.
* Setting this to zero means that all cached files would be removed when do expiration check.
* Defaults to 1 week.
*/
@property (assign, nonatomic) NSTimeInterval maxDiskAge;

那么SDWebImage是如何进行操作的？
进入SDImageCache的头文件会发现有这么一段代码

- (void)deleteOldFilesWithCompletionBlock:(nullable SDWebImageNoParamsBlock)completionBlock {
dispatch_async(self.ioQueue, ^{
[self.diskCache removeExpiredData];
if (completionBlock) {
dispatch_async(dispatch_get_main_queue(), ^{
completionBlock();
});
}
});
}

调用的地方有两个（APP将要中断，进入后台之后）

- (void)applicationWillTerminate:(NSNotification *)notification

- (void)applicationDidEnterBackground:(NSNotification *)notification