普通版本完成的锯齿很严重 但是Ios系统中仅CGContextClearRect 并不存在cyclo等方法。 网上查了一些资料。 发现还是利用到了CG 中的Mask来实现效果图：
这种效果可以自定义画笔的粗细等相关设置，不会像普通模式中出现比较严重的锯齿。
具体分析如下：
1.获取需要隐藏View的layer的CGImageRef用于后面配合掩码
2.开辟符合1的CGImageRef的CFMutableDataRef空间 给下面的掩码BitmapContext使用
3.通过CFMutableDataRef获得CGDataProviderRef （maskCreate用到）
3.BitmapContextCreate
4.BitmapContext填充黑色 并且设置画笔相关(白色，宽度，线帽(cap))
5.CGImageMaskCreate 将CGDataProviderRef 传入 则得到CGImageRef指向的是刚才创建BitmapContext。改变BitmapContext 则此CGImageRef内容也会改变
6.CGImageCreateWithMask - 通过Mask的CGImageRef创建CGImageRef 此CGImageRef随 Maskde CGImageRef变化（所以 BitmapContext改变 Mask改变 mask掩码后的CGImageRef改变）
7.释放
8.手指触摸时，根据上一个CGPoint 和此次的CGPoint 给bitmapContext新增path 并且绘制。然后调用【self setNeedsDisplay】，在重绘中(-drawRect) 重新获取通过mask后的CGImageRef 获取最新的UIImage
9.将最新的UIImage drawRect到界面
代码：
//
// PLLScrathView.h
// PLLScratchViewDemo
//
// Created by liu poolo on 14-7-31.
// Copyright (c) 2014年 liu poolo. All rights reserved.
//
#import <UIKit/UIKit.h>
#import <QuartzCore/QuartzCore.h>
@interface PLLScratchView : UIView
@property (nonatomic,assign) float sizeBrush;
-(void)setHideView:(UIView*) hideView;
@end
//
// PLLScrathView.m
// PLLScratchViewDemo
//
// Created by liu poolo on 14-7-31.
// Copyright (c) 2014年 liu poolo. All rights reserved.
//
#import "PLLScratchView.h"
@interface PLLScratchView(){
CGContextRef _contextMask;//maskContext 用户touch 改变的context
CGImageRef _scratchCGImg;//CGimageRef 封装_contextMask 图片信息 _contextMask改变 跟着改变 直到 调用生成UIImage
CGPoint currPonit;
CGPoint prePoint;
}
@end
@implementation PLLScratchView
@synthesize sizeBrush;
- (id)initWithFrame:(CGRect)frame
{
self = [super initWithFrame:frame];
if (self) {
[self setOpaque:NO];
//设置透明 如果不透明就没法看到下一层了
self.sizeBrush=10.0f;
}
return self;
}
- (void)drawRect:(CGRect)rect
{
[super drawRect:rect];
UIImage* imageToDraw=[UIImage imageWithCGImage:_scratchCGImg];
[imageToDraw drawInRect:self.frame];
}
//setSizeBrush before setHideView
-(void)setHideView:(UIView*) hideView{
CGColorSpaceRef colorSpace=CGColorSpaceCreateDeviceGray();
CGFloat scale = [UIScreen mainScreen].scale;
//获得当前传入View的CGImage
UIGraphicsBeginImageContextWithOptions(hideView.bounds.size, NO, 0);
hideView.layer.contentsScale=scale;
[hideView.layer renderInContext:UIGraphicsGetCurrentContext()];
CGImageRef hideCGImg=UIGraphicsGetImageFromCurrentImageContext().CGImage;
UIGraphicsEndImageContext();
//绘制Bitmap掩码
size_t width=CGImageGetWidth(hideCGImg);
size_t height=CGImageGetHeight(hideCGImg);
CFMutableDataRef pixels;
pixels=CFDataCreateMutable(NULL, width*height);
//创建一个可变的dataRef 用于bitmap存储记录
_contextMask = CGBitmapContextCreate(CFDataGetMutableBytePtr(pixels), width, height , 8, width, colorSpace, kCGImageAlphaNone);
//数据提供者
CGDataProviderRef dataProvider=CGDataProviderCreateWithCFData(pixels);
//填充黑色背景 mask中黑色范围为显示内容 白色为不显示
CGContextSetFillColorWithColor(_contextMask, [UIColor blackColor].CGColor);
CGContextFillRect(_contextMask, self.frame);
CGContextSetStrokeColorWithColor(_contextMask, [UIColor whiteColor].CGColor);
CGContextSetLineWidth(_contextMask, self.sizeBrush);
CGContextSetLineCap(_contextMask, kCGLineCapRound);
CGImageRef mask=CGImageMaskCreate(width, height, 8, 8, width, dataProvider, nil, NO);
_scratchCGImg=CGImageCreateWithMask(hideCGImg, mask);
CGImageRelease(mask);
CGColorSpaceRelease(colorSpace);
}
-(void)scratchViewFrom:(CGPoint)startPoint toEnd:(CGPoint)endPoint{
float scale=[UIScreen mainScreen].scale;
//CG的Y与UI的是反的 UI的y0在左上角 CG在左下
CGContextMoveToPoint(_contextMask, startPoint.x*scale, (self.frame.size.height-startPoint.y)*scale);
CGContextAddLineToPoint(_contextMask, endPoint.x*scale,(self.frame.size.height-endPoint.y)*scale);
CGContextStrokePath(_contextMask);
[self setNeedsDisplay];
}
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
[super touchesBegan:touches withEvent:event];
}
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{
[super touchesMoved:touches withEvent:event];
UITouch *touch=[touches anyObject];
currPonit=[touch locationInView:self];
prePoint=[touch previousLocationInView:self];
[self scratchViewFrom:prePoint toEnd:currPonit];
}
-(void)toucheseEnd:(NSSet *)touches withEvent:(UIEvent *)event{
[super touchesEnded:touches withEvent:event];
UITouch *touch=[touches anyObject];
currPonit=[touch locationInView:self];
prePoint=[touch previousLocationInView:self];
[self scratchViewFrom:prePoint toEnd:currPonit];
}
-(void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event{
[super touchesCancelled:touches withEvent:event];
}
@end