救了我一命 quartz2D

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//
//  MyQuartzView.m
//  QuartzTest
//
//  Created by zenny_chen on 12-2-21.
//  Copyright (c) 2012年 GreenGames Studio. All rights reserved.
//
#import "MyQuartzView.h"
// Quartz2D以及Core Animation所需要的头文件
#import <QuartzCore/QuartzCore.h>
#import <CoreText/CoreText.h>
@implementation MyQuartzView
- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if(self) {
        // Initialization code
    }
    returnself;
}
// Only override drawRect: if you perform custom drawing.
// An empty implementation adversely affects performance during animation.
- (void)drawRect:(CGRect)rect
{
    // Drawing code
    
    // 创建Quartz上下文
    CGContextRef context = UIGraphicsGetCurrentContext();
    // 先填充一个alpha只为1的白色矩形
    CGContextSetRGBFillColor(context, 1.0f, 1.0f, 1.0f, 1.0f);
    CGContextFillRect(context, CGRectMake(0.0f, 0.0f, self.frame.size.width, self.frame.size.height));
    
    // 对于iOS坐标系，调整一下坐标系的表示，使得原点处于左下侧
    // 这样与我们平时在数学中用的坐标系可取得一致
    CGContextTranslateCTM(context, 0.0f, self.frame.size.height);
    CGContextScaleCTM(context, 1.0f, -1.0f);
    
    // 创建一个三角Path
    CGMutablePathRef path = CGPathCreateMutable();
    
    // 调用CGPathMoveToPoint来开启一个子Path
    CGPathMoveToPoint(path, &CGAffineTransformIdentity, 0.0f, self.frame.size.height);
    CGPathAddLineToPoint(path, &CGAffineTransformIdentity, 0.0f, 0.0f);
    CGPathAddLineToPoint(path, &CGAffineTransformIdentity, self.frame.size.width * 0.125f, 0.0f);
    CGPathAddLineToPoint(path, &CGAffineTransformIdentity, 0.0f, self.frame.size.height);
    CGPathCloseSubpath(path);
    
    // 设置Path的混合模式：
    // kCGBlendModeDestinationIn表示：如果alpha为0,那么采用目标像素
    CGContextSetBlendMode(context, kCGBlendModeDestinationIn);
    // 这里主要设置该path的alpha值为0
    CGContextSetRGBFillColor(context, 0.0f, 0.0f, 0.0f, 0.0f);
    
    // 添加Path并绘制该Path
    CGContextAddPath(context, path);
    CGContextFillPath(context);
    
    CGPathRelease(path);
}
@end

然后，我们再看看主控制器里面的代码：

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//
//  ViewController.m
//  QuartzTest
//
//  Created by zenny_chen on 12-2-21.
//  Copyright (c) 2012年 GreenGames Studio. All rights reserved.
//
#import "ViewController.h"
#import "MyQuartzView.h"
#import <QuartzCore/QuartzCore.h>
@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    
    // 设置主视图的背景色
    self.view.backgroundColor = [UIColor colorWithRed:0.2f green:0.2f blue:0.2f alpha:1.0f];
    
    // 创建一个红色背景的矩形图
    UIView *aView = [[UIView alloc] initWithFrame:CGRectMake(100.0f, 100.0f, 160.0f, 160.0f)];
    aView.backgroundColor = [UIColor redColor];
    [self.view addSubview:aView];
    [aView release];
    
    // 创建掩模视图，其尺寸与所要裁减的视图的尺寸一样
    MyQuartzView *myView = [[MyQuartzView alloc] initWithFrame:CGRectMake(0.0f, 0.0f, 160.0f, 160.0f)];
    
    // 这里要注意的是，必须将掩模视图的背景色的alpha值填充为0
    myView.backgroundColor = [UIColor clearColor];
    
    // 将掩模视图的layer作为被裁减视图的layer的mask
    aView.layer.mask = myView.layer;
    
    // 注意，这里的myView不能调release方法，
    // 因为aView.layer.mask = myView.layer这句并没有将myView给retain住。
}
- (void)viewDidUnload
{
    [super viewDidUnload];
    // Release any retained subviews of the main view.
}
- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    return(interfaceOrientation == UIInterfaceOrientationPortrait);
}
@end