179在屏幕中绘制一个三角形

效果如下：

ViewController.h

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController
@end

ViewController.m

#import "ViewController.h"
#import "KMTriangleView.h"

@interface ViewController ()
- (void)layoutUI;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self layoutUI];
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (void)layoutUI {
    KMTriangleView *triangleView = [[KMTriangleView alloc] initWithFrame:self.view.frame];
    self.view = triangleView;
}

@end

KMTriangleView.h

#import <UIKit/UIKit.h>

@interface KMTriangleView : UIView
@property CGPoint firstPoint;
@property CGPoint secondPoint;
@property CGPoint thirdPoint;
@property NSMutableArray *mArrPoint;

@end

KMTriangleView.m

#import "KMTriangleView.h"

@implementation KMTriangleView
#define kPointCount 3

- (id)initWithFrame:(CGRect)frame {
    if (self = [super initWithFrame:frame]) {
        self.backgroundColor = [UIColor whiteColor];
        UILabel *lblMsg = [[UILabel alloc] initWithFrame:CGRectMake(20, 20, self.frame.size.width, 40)];
        lblMsg.text = @"任意点击屏幕内的3点以确定一个三角形";
        [self addSubview:lblMsg];
        
        _mArrPoint = [[NSMutableArray alloc] initWithCapacity:kPointCount];
    }
    return self;
}

- (void)drawRect:(CGRect)rect {
    CGContextRef contextRef = UIGraphicsGetCurrentContext(); //获取绘制上下文对象实例
    CGContextSetRGBStrokeColor(contextRef, 0.5, 0.5, 0.5, 1.0); //设置笔画颜色
    CGContextSetLineWidth(contextRef, 2.0); //设置线条粗细大小
    CGPoint points[] = { _firstPoint, _secondPoint, _thirdPoint, _firstPoint };
    CGContextAddLines(contextRef, points, kPointCount+1); //设置线条的连接端点
    CGContextStrokePath(contextRef); //沿着要求的路径，开始绘制
}
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint touchPoint = [touch locationInView:self];
    [_mArrPoint addObject:[NSValue valueWithCGPoint:touchPoint]];
    
    if (_mArrPoint.count > 3) {
        [_mArrPoint removeObjectAtIndex:0];
    }
    if (_mArrPoint.count == 3) {
        _firstPoint = [_mArrPoint[0] CGPointValue];
        _secondPoint = [_mArrPoint[1] CGPointValue];
        _thirdPoint = [_mArrPoint[2] CGPointValue];
    }
    [self setNeedsDisplay]; //设置触发drawRect方法重新绘制内容；这句很关键，如不使用就不会触发drawRect方法
}

@end