Added macros to expand SPI symbol to Selector, which can make it easy to distinguish and maintain in the future

SDWebImage/Core/SDAnimatedImageView.m

@@ -470,10 +470,10 @@
 // NSImageView use a subview. We need this subview's layer for actual rendering.
 // Why using this design may because of properties like `imageAlignment` and `imageScaling`, which it's not available for UIImageView.contentMode (it's impossible to align left and keep aspect ratio at the same time)
 - (NSView *)imageView {
-    NSImageView *imageView = imageView = objc_getAssociatedObject(self, NSSelectorFromString(@"_imageView"));
+    NSImageView *imageView = imageView = objc_getAssociatedObject(self, SD_SEL_SPI(imageView));
     if (!imageView) {
         // macOS 10.14
-        imageView = objc_getAssociatedObject(self, NSSelectorFromString(@"_imageSubview"));
+        imageView = objc_getAssociatedObject(self, SD_SEL_SPI(imageSubview));
     }
     return imageView;
 }

SDWebImage/Core/UIImage+Metadata.m

@@ -8,6 +8,7 @@
 
 #import "UIImage+Metadata.h"
 #import "NSImage+Compatibility.h"
+#import "SDInternalMacros.h"
 #import "objc/runtime.h"
 
 @implementation UIImage (Metadata)
@@ -42,14 +43,14 @@
             return YES;
         }
         // SVG
-        SEL SVGSelector = NSSelectorFromString(@"_CGSVGDocument");
+        SEL SVGSelector = SD_SEL_SPI(CGSVGDocument);
         if ([self respondsToSelector:SVGSelector] && [self performSelector:SVGSelector]) {
             return YES;
         }
     }
     if (@available(iOS 11.0, tvOS 11.0, watchOS 4.0, *)) {
         // PDF
-        SEL PDFSelector = NSSelectorFromString(@"_CGPDFPage");
+        SEL PDFSelector = SD_SEL_SPI(CGPDFPage);
         if ([self respondsToSelector:PDFSelector] && [self performSelector:PDFSelector]) {
             return YES;
         }

SDWebImage/Private/SDInternalMacros.h

@@ -21,6 +21,18 @@
 #define SD_OPTIONS_CONTAINS(options, value) (((options) & (value)) == (value))
 #endif
 
+#ifndef SD_CSTRING
+#define SD_CSTRING(str) #str
+#endif
+
+#ifndef SD_NSSTRING
+#define SD_NSSTRING(str) @(SD_CSTRING(str))
+#endif
+
+#ifndef SD_SEL_SPI
+#define SD_SEL_SPI(name) NSSelectorFromString([NSString stringWithFormat:@"_%@", SD_NSSTRING(name)])
+#endif
+
 #ifndef weakify
 #define weakify(...) \
 sd_keywordify \