绘图的几种方式

一 使用CALayerDelegate来绘图

@interface ViewController ()<CALayerDelegate>
@end

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    
    CALayer *layer = [CALayer layer];
    layer.frame = CGRectMake(87.5, 80, 200, 100);
    layer.delegate = self;
    [self.view.layer addSublayer:layer];
    //不调用下面的语句，将不会执行CALayerDelegate中的方法
    [layer setNeedsDisplay];
}

- (void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx
{
    CGSize size = layer.bounds.size;
    CGMutablePathRef path = CGPathCreateMutable();
    CGContextMoveToPoint(ctx, 0, size.height);
    CGContextAddLineToPoint(ctx, size.width, size.height);
    CGContextAddLineToPoint(ctx, size.width / 2, 0);
    CGContextClosePath(ctx);
    
    UIColor *color = [UIColor colorWithPatternImage:[UIImage imageNamed:@"star"]];
    CGContextSetFillColorWithColor(ctx, color.CGColor);
    CGContextAddPath(ctx, path);//记得将path添加进来
    CGPathRelease(path);
    CGContextFillPath(ctx);
}

二 使用UIView的drawRect:方法来绘图

@interface TestView : UIView

@end

- (void)drawRect:(CGRect)rect 
{    
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    
    UIColor *color = [UIColor colorWithPatternImage:[UIImage imageNamed:@"star"]];
    CGContextSetFillColorWithColor(ctx, color.CGColor);
    CGContextFillEllipseInRect(ctx, rect);
}

调用

TestView *testView = [[TestView alloc] initWithFrame:CGRectMake(87.5, 220, 200, 200)];
testView.backgroundColor = [UIColor whiteColor];
[self.view addSubview:testView];

三 使用UIGraphicsBeginImageContext相关方法来绘图

- (void)drawAnyWhere
{
    //下面两句顺序不能颠倒
    //UIGraphicsBeginImageContext(CGSizeMake(200, 100));
    //不能使用上一个方法，因为Retina屏的scale为2或3，而上一个方法的scale是1，会造成图片模糊，0.0表示根据ping'm
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(200, 100), NO, 0.0);
    CGContextRef ctx = UIGraphicsGetCurrentContext();
    
    CGMutablePathRef path = CGPathCreateMutable();
    CGPathAddEllipseInRect(path, NULL, CGRectMake(0, 0, 200, 100));
    CGContextAddPath(ctx, path);
    UIColor *color = [UIColor colorWithPatternImage:[UIImage imageNamed:@"star"]];
    CGContextSetFillColorWithColor(ctx, color.CGColor);

    CGContextFillPath(ctx);
    CGPathRelease(path);

    //获取绘制的图像
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(87.5, 470, 200, 100)];
    imageView.image = image;

    [self.view addSubview:imageView];
}

效果

效果图