Modify the definition of SDImageFormat to extern to avoid creating multiple global variables

[tujinqiu]

I found that several formats about SDImageFormat are static variables defined in a header file. When the program is compiled and linked, any .m file that imports or indirectly imports this header will create a static variable inside the file. This results in an increase in useless binary size. In fact, only need to declare these variables as extern and assign values ​​inside a certain .m file, so that there will only be one copy of these variables in the entire binary.

[dreampiggy]

Looks good for me.

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By the way, I want to talk about this SDImageFormat as well

This shit design (using a enum) is from the history SDWebImage 2.x, and changed into typedef NSUInteger in 4.x, but not been refactored during the 5.x release.

The most stupid is that this is actually not Extensible, though even it can be. The code using NSUInteger in whatever version is bad, because the int value based can be conflict, it's not decentralized, unlike our coder API design. (See https://github.com/SDWebImage/SDWebImage/wiki/Coder-Plugin-List, this ugly int based typedef is centralized)

In SD 6.0, I'll totally remove this bad design, use class instead.

// In SDWebImage Core repo
@interface SDImageFormat : NSObject
@property (readonly, nonnull) NSString *UTI;
@property (readonly, nonnull) NSString *preferredFilenameExtension;
@property (readonly, nonnull) NSString *preferredMIMEType;
@end

// Each coder repo, like JPEGXL
@interface SDImageFormat (JPEGXL)
@property (class, readonly) SDImageFormat *JPEGXL;
@end 

// WebP repo
@interface SDImageFormat (WebP)
@property (class, readonly) SDImageFormat *WebP;
@end 

// APNG
@interface SDImageFormat (APNG)
@property (class, readonly) SDImageFormat *APNG;
@end 

[dreampiggy]

Compiler can do optimization based on enum cases number, these changes is no needed.

[tujinqiu]

The compiler reports an error when declare these variables as extern.

[dreampiggy]

So, is this cause API break (assume user use code like this in switch case?)

If so, why not totally break and use class declarations instead

[dreampiggy]

I remembered...In 5.c

This static declaration is written by me to allows "switch case" works

This code is indeed, so seems this is not possible in 5.x

[dreampiggy]

So, at least, we need to fix the 10+ individual coder repo as well (they use static declaration)

Can we wait for the total break changes to use class based SDImageForamt instead ?

[tujinqiu]

So, at least, we need to fix the 10+ individual coder repo as well (they use static declaration)

Can we wait for the total break changes to use class based SDImageForamt instead ?

This is just a small optimization, looking forward to version 6.0