UIImageCreateUsingBlock.m

//
//  UIImageCreateUsingBlock.m
//
//  Copyright (c) 2013 Zachary Waldowski.
//  Licensed under MIT - Provided as-is.
//

#import <UIKit/UIKit.h>
#import <CoreGraphics/CoreGraphics.h>

static inline size_t aligned_size(size_t size, size_t alignment) {
	size_t r = size + --alignment + 2;
	return (r + 2 + alignment) & ~alignment;
}

UIImage *UIImageCreateUsingBlock(CGSize size, BOOL opaque, void(^drawingBlock)(void)) {
	BOOL isMain = [NSThread isMainThread];
	CGContextRef context = NULL;
	CGFloat scale;
	
	if (isMain) {
		UIGraphicsBeginImageContextWithOptions(size, opaque, 0.0);
		context = UIGraphicsGetCurrentContext();
	} else {
		CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
		scale = [UIScreen mainScreen].scale;
		CGImageAlphaInfo alphaInfo = 0;
		if (opaque) {
			alphaInfo |= kCGImageAlphaNoneSkipFirst;
		} else {
			alphaInfo |= kCGImageAlphaPremultipliedFirst;
		}
		
		// RGB - 32 bpp - 8 bpc - available on both OS X and iOS
		const size_t bitsPerPixel = 32;
		const size_t bitsPerComponent = 8;
		const size_t widthPixels = size.width * scale;
		
		// Quartz 2D Programming Guide
		// "When you create a bitmap graphics context, you’ll get the best
		// performance if you make sure the data and bytesPerRow are 16-byte aligned."
		size_t bytesPerRow = widthPixels * bitsPerPixel;
		size_t alignedBytesPerRow = aligned_size(bytesPerRow, 16);
		
		context = CGBitmapContextCreate(NULL, widthPixels, size.height * scale, bitsPerComponent, alignedBytesPerRow, colorSpace, alphaInfo);
		CGColorSpaceRelease(colorSpace);
		CGContextScaleCTM(context, scale, -1 * scale);
		CGContextTranslateCTM(context, 0, -1 * size.height);
		CGContextClipToRect(context, (CGRect){ CGPointZero, size });
		UIGraphicsPushContext(context);
	}
	
	if (drawingBlock) drawingBlock();
	
	UIImage *retImage = nil;
	
	if (isMain) {
		retImage = UIGraphicsGetImageFromCurrentImageContext();
		UIGraphicsEndImageContext();
	} else {
		UIGraphicsPopContext();
		CGImageRef cgImage = CGBitmapContextCreateImage(context);
		retImage = [UIImage imageWithCGImage:cgImage scale:scale orientation:UIImageOrientationUp];
		CGImageRelease(cgImage);
		CGContextRelease(context);
	}
	
	return retImage;
}