iOS：CALayer核心动画层上绘图

    //创建子层(使用默认的锚点)
    CALayer *subLayer = [[CALayer alloc]init];
    
    subLayer.bounds = CGRectMake(0,0,100, 100);
    
    subLayer.position = self.view.center;
    
    subLayer.backgroundColor = [[UIColor redColor]CGColor];
    
    //设置圆角半径,设置为50,此时的子层为圆形
    subLayer.cornerRadius = 50;
    
    [self.view.layer addSublayer:subLayer];

    //设置层的代理
    subLayer.delegate = self;

    //只有发送setNeedsDisplay这个消息时才会调用代理方法
    [subLayer setNeedsDisplay];

#pragma mark -CALayer的代理方法
-(void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx
{
    //画矩形
    CGContextAddRect(ctx, CGRectMake(0, 0, 50, 50));
    
    //设置颜色
    CGFloat components[4] = {0.0,1.0,0.0,1.0};
    CGContextSetStrokeColor(ctx, components);

    //这种方式也可以设置颜色
    //CGContextSetStrokeColorWithColor(ctx,[[UIColor greenColor]CGColor]);    

    //旋转坐标系
    CGContextScaleCTM(ctx, 1, -1);
    CGContextTranslateCTM(ctx, 0, -100);
    
    //画笑脸图像
    UIImage *image = [UIImage imageNamed:@"1.png"];
    CGContextDrawImage(ctx, CGRectMake(0, 0, 100, 100), [image CGImage]);
    
    //画路径
    CGContextDrawPath(ctx, kCGPathStroke);
}

#import "ViewController.h"
#import "MyLayer.h"

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    //创建子层
    MyLayer *myLayer = [[MyLayer alloc]init];
    myLayer.bounds = CGRectMake(0, 0, 100, 100);
    
    myLayer.position = CGPointMake(100, 100);
    
    //设置子层背景色为灰色
    myLayer.backgroundColor = [[UIColor grayColor]CGColor];
    
    [self.view.layer addSublayer:myLayer];
    
    [myLayer setNeedsDisplay];
}
@end

#import "MyLayer.h"
#import <UIKit/UIKit.h>

@implementation MyLayer
//重写这个方法
-(void)drawInContext:(CGContextRef)ctx
{
    
    //绘制矩形
    CGContextAddRect(ctx, CGRectMake(0, 0, 50, 50));
    
    
    //第一种设置颜色方式：
    //设置颜色空间(选择配色方案:RGB,红、绿、蓝)
    CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
    CGContextSetStrokeColorSpace(ctx, colorspace);
    
    //数组中四个内容：前三个分别为红绿蓝颜色值,后一个为透明度
    CGFloat components[4] = {0.0,1.0,0.0,1.0};
    CGContextSetStrokeColor(ctx, components);
    
    
    //这是另一种比较简单的设置颜色的方式
    //CGContextSetStrokeColorWithColor(ctx, [[UIColor greenColor]CGColor]);
    
    
    //绘制描边路径
    CGContextDrawPath(ctx, kCGPathStroke);
    
    //释放create出的属性,防止内存泄露
    CGColorSpaceRelease(colorspace);
}
@end