调用流程

1.查看内存是否有对应的图片
2.查看硬盘是否有对应的图片
3.去网络下载对应的图片

decode

/**
 Create a decoded CGImage by the provided CGImage and orientation. This follows The Create Rule and you are response to call release after usage.
 It will detect whether image contains alpha channel, then create a new bitmap context with the same size of image, and draw it. This can ensure that the image do not need extra decoding after been set to the imageView.
 
 @param cgImage The CGImage
 @param orientation The EXIF image orientation.
 @return A new created decoded image
 */
+ (CGImageRef _Nullable)CGImageCreateDecoded:(_Nonnull CGImageRef)cgImage orientation:(CGImagePropertyOrientation)orientation CF_RETURNS_RETAINED;

+ (CGImageRef)CGImageCreateDecoded:(CGImageRef)cgImage orientation:(CGImagePropertyOrientation)orientation {
    if (!cgImage) {
        return NULL;
    }
    size_t width = CGImageGetWidth(cgImage);
    size_t height = CGImageGetHeight(cgImage);
    if (width == 0 || height == 0) return NULL;
    size_t newWidth;
    size_t newHeight;
    switch (orientation) {
        case kCGImagePropertyOrientationLeft:
        case kCGImagePropertyOrientationLeftMirrored:
        case kCGImagePropertyOrientationRight:
        case kCGImagePropertyOrientationRightMirrored: {
            // These orientation should swap width & height
            newWidth = height;
            newHeight = width;
        }
            break;
        default: {
            newWidth = width;
            newHeight = height;
        }
            break;
    }
    
    BOOL hasAlpha = [self CGImageContainsAlpha:cgImage];
    // iOS prefer BGRA8888 (premultiplied) or BGRX8888 bitmapInfo for screen rendering, which is same as `UIGraphicsBeginImageContext()` or `- [CALayer drawInContext:]`
    // Though you can use any supported bitmapInfo (see: https://developer.apple.com/library/content/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_context/dq_context.html#//apple_ref/doc/uid/TP30001066-CH203-BCIBHHBB ) and let Core Graphics reorder it when you call `CGContextDrawImage`
    // But since our build-in coders use this bitmapInfo, this can have a little performance benefit
    CGBitmapInfo bitmapInfo = kCGBitmapByteOrder32Host;
    bitmapInfo |= hasAlpha ? kCGImageAlphaPremultipliedFirst : kCGImageAlphaNoneSkipFirst;
    CGContextRef context = CGBitmapContextCreate(NULL, newWidth, newHeight, 8, 0, [self colorSpaceGetDeviceRGB], bitmapInfo);
    if (!context) {
        return NULL;
    }
    
    // Apply transform
    CGAffineTransform transform = SDCGContextTransformFromOrientation(orientation, CGSizeMake(newWidth, newHeight));
    CGContextConcatCTM(context, transform);
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), cgImage); // The rect is bounding box of CGImage, don't swap width & height
    CGImageRef newImageRef = CGBitmapContextCreateImage(context);
    CGContextRelease(context);
    
    return newImageRef;
}