模拟UIAlertView的模态背景遮罩

MJModalOverlayer.h

//
//  MJModalOverlayer.h
//  Overlayer
//
//  Created by manjun.han on 13-3-13.
//  Copyright (c) 2013年 com.hmj. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface MJModalOverlayer : UIView

@end

MJModalOverlayer.m

//
//  MJOverlayer.m
//  Overlayer
//
//  Created by manjun.han on 13-3-13.
//  Copyright (c) 2013年 com.hmj. All rights reserved.
//

#import "MJModalOverlayer.h"

@implementation MJModalOverlayer

- (id)init
{
    self = [super init] ;
    if (self) {
        self.backgroundColor = [UIColor clearColor] ;
    }
    return self ;
}
- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame] ;
    
    if(self){
        self.backgroundColor = [UIColor clearColor] ;
    }
    return self ;
}

- (void)drawRect:(CGRect)rect
{
    
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    
    // 渐变颜色
    UIColor *gradientOuter = [UIColor colorWithRed: 0 green: 0 blue: 0 alpha: 0.4];
    
    UIColor *gradientInner = [UIColor colorWithRed: 0 green: 0 blue: 0 alpha: 0.1];
	
    NSArray *radialGradientColors = @[(id)gradientInner.CGColor, (id)gradientOuter.CGColor];
    
    // 渐变locations
    CGFloat radialGradientLocations[] = {0, 0.5, 1};
    
    CGGradientRef radialGradient = CGGradientCreateWithColors(colorSpace, (__bridge CFArrayRef)radialGradientColors, radialGradientLocations);
	
    // 剪切路径
    UIBezierPath *rectanglePath = [UIBezierPath bezierPathWithRect: CGRectMake(rect.origin.x, rect.origin.y, rect.size.width, rect.size.height)];
    
    CGContextSaveGState(ctx);
    
    [rectanglePath addClip];
    
    CGContextDrawRadialGradient(ctx, radialGradient, CGPointMake(rect.origin.x + rect.size.width / 2, rect.origin.y + rect.size.height / 2), rect.size.width / 4,
             					     CGPointMake(rect.origin.x + rect.size.width / 2, rect.origin.y + rect.size.height / 2), rect.size.width,
							 kCGGradientDrawsBeforeStartLocation | kCGGradientDrawsAfterEndLocation);
    CGContextRestoreGState(ctx);
	
    // 释放资源
    CGGradientRelease(radialGradient);
    
    CGColorSpaceRelease(colorSpace);
}

@end