UIView that can be used as a map marker on GoogleMaps. marker.icon = [[[MapClusterPinView alloc] initWithCount:12] image];

MapClusterPinView.h

//
//  MapClusterPinView.h
//
//  Created by Rob Booth on 10/9/15.
//  Copyright © 2015 Rob Booth. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface MapClusterPinView : UIView

- (instancetype _Nullable)initWithCount:(NSInteger)count;
- (UIImage * _Nonnull)image;

@end

MapClusterPinView.m

//
//  MapClusterPinView.m
//
//  Created by Rob Booth on 10/9/15.
//  Copyright © 2015 Rob Booth. All rights reserved.
//

#import "MapClusterPinView.h"

@interface MapClusterPinView ()

@property (assign) CGSize size;
@property (strong, nonatomic) UIColor *circleColor;
@property (strong, nonatomic) NSString *count;

@end

@implementation MapClusterPinView

- (instancetype)initWithCount:(NSInteger)count
{
	self = [super init];
	if (self)
	{
		self.size = CGSizeMake(30, 30);
		if (count < 10)
		{
			self.size = CGSizeMake(15,15);
		}
		else if (count < 101)
		{
			self.size = CGSizeMake(20,20);
		}
		self.circleColor = [self randomColor];
		self.count = [NSString stringWithFormat:@"%ld%@", (count < 100) ? count : 100, (count > 100) ? @"+" : @""];
	}
	return self;
}

- (UIColor *)randomColor
{
	// colors from http://designmodo.github.io/Flat-UI/
	NSArray *colors = @[
		[UIColor colorWithRed:0.10196078431372549 green:0.7372549019607844  blue:0.611764705882353    alpha:1.0],
		[UIColor colorWithRed:0.10196078431372549 green:0.7372549019607844  blue:0.611764705882353    alpha:1.0],
		[UIColor colorWithRed:0.08627450980392157 green:0.6274509803921569  blue:0.5215686274509804   alpha:1.0],
		[UIColor colorWithRed:0.1803921568627451  green:0.8                 blue:0.44313725490196076  alpha:1.0],
		[UIColor colorWithRed:0.15294117647058825 green:0.6823529411764706  blue:0.3764705882352941   alpha:1.0],
		[UIColor colorWithRed:0.20392156862745098 green:0.596078431372549   blue:0.8588235294117647   alpha:1.0],
		[UIColor colorWithRed:0.1607843137254902  green:0.5019607843137255  blue:0.7254901960784313   alpha:1.0],
		[UIColor colorWithRed:0.6078431372549019  green:0.34901960784313724 blue:0.7137254901960784   alpha:1.0],
		[UIColor colorWithRed:0.5568627450980392  green:0.26666666666666666 blue:0.6784313725490196   alpha:1.0],
		[UIColor colorWithRed:0.20392156862745098 green:0.28627450980392155 blue:0.3686274509803922   alpha:1.0],
		[UIColor colorWithRed:0.17254901960784313 green:0.24313725490196078 blue:0.3137254901960784   alpha:1.0],
		[UIColor colorWithRed:0.9450980392156862  green:0.7686274509803922  blue:0.058823529411764705 alpha:1.0],
		[UIColor colorWithRed:0.9529411764705882  green:0.611764705882353   blue:0.07058823529411765  alpha:1.0],
		[UIColor colorWithRed:0.9019607843137255  green:0.49411764705882355 blue:0.13333333333333333  alpha:1.0],
		[UIColor colorWithRed:0.8274509803921568  green:0.32941176470588235 blue:0                    alpha:1.0],
		[UIColor colorWithRed:0.9058823529411765  green:0.2980392156862745  blue:0.23529411764705882  alpha:1.0],
		[UIColor colorWithRed:0.7529411764705882  green:0.2235294117647059  blue:0.16862745098039217  alpha:1.0],
		[UIColor colorWithRed:0.9254901960784314  green:0.9411764705882353  blue:0.9450980392156862   alpha:1.0],
		[UIColor colorWithRed:0.7411764705882353  green:0.7647058823529411  blue:0.7803921568627451   alpha:1.0],
		[UIColor colorWithRed:0.5843137254901961  green:0.6470588235294118  blue:0.6509803921568628   alpha:1.0],
		[UIColor colorWithRed:0.4980392156862745  green:0.5490196078431373  blue:0.5529411764705883   alpha:1.0]
	];
	
	return colors[arc4random() % colors.count];
}

- (void)drawRect:(CGRect)rect
{
	//// General Declarations
	CGContextRef context = UIGraphicsGetCurrentContext();
	
	//// Circle Drawing
	UIBezierPath* circlePath = [UIBezierPath bezierPathWithOvalInRect: CGRectMake(0, 0, self.size.width, self.size.height)];
	[self.circleColor setFill];
	[circlePath fill];
	[UIColor.whiteColor setStroke];
	circlePath.lineWidth = 1;
	[circlePath stroke];
	
	
	//// Text Drawing
	CGRect textRect = CGRectMake(0, self.size.height/2 - 5, self.size.width, 10);
	NSMutableParagraphStyle* textStyle = NSMutableParagraphStyle.defaultParagraphStyle.mutableCopy;
	textStyle.alignment = NSTextAlignmentCenter;
	
	NSDictionary* textFontAttributes = @{NSFontAttributeName: [UIFont fontWithName: @"Avenir-Heavy" size: 10], NSForegroundColorAttributeName: UIColor.whiteColor, NSParagraphStyleAttributeName: textStyle};
	
	CGFloat textTextHeight = [self.count boundingRectWithSize: CGSizeMake(textRect.size.width, INFINITY)  options: NSStringDrawingUsesLineFragmentOrigin attributes: textFontAttributes context: nil].size.height;
	CGContextSaveGState(context);
	CGContextClipToRect(context, textRect);
	[self.count drawInRect: CGRectMake(CGRectGetMinX(textRect), CGRectGetMinY(textRect) + (CGRectGetHeight(textRect) - textTextHeight) / 2, CGRectGetWidth(textRect), textTextHeight) withAttributes: textFontAttributes];
	CGContextRestoreGState(context);
}

- (UIImage *)image
{
	if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {
		UIGraphicsBeginImageContextWithOptions(self.size, NO, [[UIScreen mainScreen] scale]);
	} else {
		UIGraphicsBeginImageContext(self.size);
	}
	[self drawRect:CGRectMake(0, 0, self.size.width, self.size.height)];
	
	UIImage * img = UIGraphicsGetImageFromCurrentImageContext();
	
	UIGraphicsEndImageContext();
	
	return img;
}

@end