Reading sdwebimage source code Notes

1. SDWebImage Maximum concurrent number ：6.
2. Default download timeout ：15s.
3. Default cache time ： a week .

 

1、SDWebImage Download process 【 These include SDWebImage Cache policy （ Second level cache policy ）】

imageView sd_setImageWithUrl:placeholder:options:context:progress{}completed{}

The outermost layer is a SDWebImageManager Single case , Managing two other singletons , One is downloaded SDWebImageDownloader, It manages multiple download operations operation. The other is cache processing SDImageCache. The specific process ：

1、 Every time I download pictures , First, according to imageview Of operationDictionary Is there the same key, If there is, it will key Corresponding Operation Cancel and delete .

2、 Add a new task to operation In the dictionary , After loading the picture, it will be based on operation In and out, judge whether it shows , Show and remove if any operation. This is a SD Ways to avoid reuse .

3、 Then it will asynchronously go to the main thread to set the image as placeholder, Set up placehoder Pictures are not cached .

4、 Judge url There is , So called SDWebImageManager Of loadWithUrl: The function passes through SDImageCache From the cache according to CacheKey That is, for downloading url turn MD5 As key To see if the picture is already in the cache .

5、 If the picture is cached in memory , Then call back to Manager, display picture .

6、 If the picture is not in memory , Then it will be generated to find out whether the image exists from the hard disk , If it exists, it will be loaded into memory , If the memory is small, clear the memory first and then save the cache , Callback display picture after reading .

7、 If the picture cannot be read from the hard disk , It means you need to download pictures . then , adopt SDWebImageLoader To download pictures .

9、 Picture download by NSUrlConnection To do it , Monitor the download progress of pictures

10、 stay didFinishLoading Callback method , Process pictures , And turn to the main thread to show the picture

11、 Then save the picture in SDImageCache in , Memory cache and hard disk storage .

 

2、SDWebImage Image cache cleaning strategy

1、 Get the last modification date of the picture in the disk , Read the attributes of the file, such as the last modification date 、 Size and file path .

2、 Classify the pictures according to the last modification date , Store those files that have exceeded the maximum storage time in the delete array , Other files are saved in another dictionary . Calculate the size after deleting the file to be deleted

3、 Delete the corresponding file according to the file path

4、 Determine whether the remaining file size still exceeds the maximum disk capacity of the user

5、 If it exceeds, the remaining files will be modified and sorted , Delete the file with the earliest modification time , Until the remaining file size is less than maxCacheSize.

 

3、 Cache cleanup timing

1、 When the system issues a low memory notification , All image caches in memory will be deleted

2、 When the program enters the background , The disk file data will be cleaned

3、 When receiving the program shutdown notice , Clean up the file data on the disk