我正在尝试将此objc代码转换为swift:CGContextRefcontextRef=CGBitmapContextCreate(NULL,proposedRect.size.width,proposedRect.size.height,CGImageGetBitsPerComponent(imageRef),0,colorSpaceRef,(CGBitmapInfo)kCGImageAlphaPremultipliedFirst);NSGraphicsContext*context=[NSGraphicsContextgraphicsContextWithGraphicsPort