Add the test case testSDGraphicsImageRenderer, update the documentation

SDWebImage/Core/SDGraphicsImageRenderer.h

@@ -10,10 +10,11 @@
 
 /**
  These following class are provided to use `UIGraphicsImageRenderer` with polyfill, which allows write cross-platform(AppKit/UIKit) code and avoid runtime version check.
- Compared to `UIGraphicsBeginImageContext`, `UIGraphicsImageRenderer` use dynamic bitmap info from your draw code to generate CGContext, not always use ARGB8888, which is more performant on RAM usage.
+ Compared to `UIGraphicsBeginImageContext`, `UIGraphicsImageRenderer` use dynamic bitmap from your draw code to generate CGContext, not always use ARGB8888, which is more performant on RAM usage.
+ Which means, if you draw CGImage/CIImage which contains grayscale only, the underlaying bitmap context use grayscale, it's managed by system and not a fixed type. (actually, the `kCGContextTypeAutomatic`)
  For usage, See more in Apple's documentation: https://developer.apple.com/documentation/uikit/uigraphicsimagerenderer
  For UIKit on iOS/tvOS 10+, these method just use the same `UIGraphicsImageRenderer` API.
- For others (macOS/watchOS or iOS/tvOS 10-), these method use the `SDImageGraphics.h` to implements the same behavior.
+ For others (macOS/watchOS or iOS/tvOS 10-), these method use the `SDImageGraphics.h` to implements the same behavior (but without dynamic bitmap support)
 */
 
 typedef void (^SDGraphicsImageDrawingActions)(CGContextRef _Nonnull context);

Tests/Tests/SDUtilsTests.m

@@ -12,6 +12,7 @@
 #import "SDDisplayLink.h"
 #import "SDInternalMacros.h"
 #import "SDFileAttributeHelper.h"
+#import "UIColor+HexString.h"
 
 @interface SDUtilsTests : SDTestCase
 
@@ -107,6 +108,32 @@
     expect(hasAttr).beFalsy();
 }
 
+- (void)testSDGraphicsImageRenderer {
+    // Main Screen
+    SDGraphicsImageRendererFormat *format = SDGraphicsImageRendererFormat.preferredFormat;
+#if SD_UIKIT
+    CGFloat screenScale = [UIScreen mainScreen].scale;
+#elif SD_MAC
+    CGFloat screenScale = [NSScreen mainScreen].backingScaleFactor;
+#endif
+    expect(format.scale).equal(screenScale);
+    expect(format.opaque).beFalsy();
+#if SD_UIKIT
+    expect(format.preferredRange).equal(SDGraphicsImageRendererFormatRangeAutomatic);
+#elif SD_MAC
+    expect(format.preferredRange).equal(SDGraphicsImageRendererFormatRangeStandard);
+#endif
+    CGSize size = CGSizeMake(100, 100);
+    SDGraphicsImageRenderer *renderer = [[SDGraphicsImageRenderer alloc] initWithSize:size format:format];
+    UIColor *color = UIColor.redColor;
+    UIImage *image = [renderer imageWithActions:^(CGContextRef  _Nonnull context) {
+        [color setFill];
+        CGContextFillRect(context, CGRectMake(0, 0, size.width, size.height));
+    }];
+    expect(image.scale).equal(format.scale);
+    expect([[image sd_colorAtPoint:CGPointMake(50, 50)].sd_hexString isEqualToString:color.sd_hexString]).beTruthy();
+}
+
 - (void)testSDScaledImageForKey {
     // Test nil
     expect(SDScaledImageForKey(nil, nil)).beNil();