Example.m

[UIImage tttImageWithSize:CGSizeMake(44, 44) 
    			  drawing:^(CGContextRef ctx, CGSize size) {

    UIColor *blue = [UIColor blueColor];
    CGContextSetFillColorWithColor(ctx, blue.CGColor);
	
    CGFloat half = size.height / 2;
    rect = (CGRect){{0, half}, {size.width, half}};
    CGContextFillRect(ctx, rect);
	
}];

UIImage+TTTDrawing.h

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

typedef void (^TTTDrawingBlock)(CGContextRef ctx, CGSize size);

@interface UIImage (TTTDrawing)

+ (UIImage *)tttImageWithSize:(CGSize)size drawing:(TTTDrawingBlock)drawing;

@end

UIImage+TTTDrawing.m

#import "UIImage+TTTDrawing.h"

@implementation UIImage (TTTDrawing)

+ (UIImage *)tttImageWithSize:(CGSize)size drawing:(TTTDrawingBlock)drawing
{
    UIGraphicsBeginImageContextWithOptions(size, NO, [UIScreen mainScreen].scale);
    CGContextRef context = UIGraphicsGetCurrentContext();

    drawing(context, size);

    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return image;
}

@end