iOS画图，截图及清空

#import "ViewController.h"
#import "SecondVC.h"


#define WIDTH [UIScreen mainScreen].bounds.size.width
#define HEIGHT [UIScreen mainScreen].bounds.size.height

@interface ViewController ()
{
    UIImageView *_canvasView;//画布
    CGPoint _startPoint;//记录开始坐标
    
}
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    _canvasView = [[UIImageView alloc] initWithFrame:self.view.bounds];
    [self.view addSubview:_canvasView];
    
    UIButton *clearBtn = [UIButton buttonWithType:UIButtonTypeCustom];
    clearBtn.frame = CGRectMake(50, 20, 100, 30);
    [clearBtn addTarget:self action:@selector(clearBtnClick) forControlEvents:UIControlEventTouchUpInside];
    [clearBtn setTitle:@"清空" forState:UIControlStateNormal];
    clearBtn.backgroundColor = [UIColor orangeColor];
    [self.view addSubview:clearBtn];
    
    UIButton *cutPicBtn = [UIButton buttonWithType:UIButtonTypeCustom];
    cutPicBtn.frame = CGRectMake(250, 20, 100, 30);
    [cutPicBtn addTarget:self action:@selector(cutPicBtnClick) forControlEvents:UIControlEventTouchUpInside];
    [cutPicBtn setTitle:@"截图" forState:UIControlStateNormal];
    cutPicBtn.backgroundColor = [UIColor orangeColor];
    [self.view addSubview:cutPicBtn];
    
}


- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    _startPoint = [[touches anyObject] locationInView:_canvasView];

}

- (void)touchesMoved:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    CGPoint movePoint = [[touches anyObject] locationInView:_canvasView];

//    移动的点有很多，
    @autoreleasepool {
        UIGraphicsBeginImageContext(CGSizeMake(WIDTH, HEIGHT));//开始一个图片的绘图
        
        [_canvasView drawRect:_canvasView.frame];//绘制原来已经存在的线条
        
        CGContextRef context = UIGraphicsGetCurrentContext();//绘图缓存区，当前的是一张图片
        CGContextSetLineWidth(context, 5);
        CGContextSetRGBStrokeColor(context, 1, 0, 0, 1);
        CGContextSetLineCap(context, kCGLineCapRound);//边帽
        CGContextSetLineJoin(context, kCGLineJoinRound);//缝合
        
        CGContextMoveToPoint(context, _startPoint.x, _startPoint.y);
        CGContextAddLineToPoint(context, movePoint.x, movePoint.y);
        CGContextStrokePath(context);
        _canvasView.image = UIGraphicsGetImageFromCurrentImageContext();//获取当前图片绘制区域内的图片
        
        UIGraphicsEndImageContext();//关闭图片绘制
    }
    
    _startPoint = movePoint;
}


#pragma mark - 清除按钮
- (void)clearBtnClick
{
    
    _canvasView.image = nil;//这并不是一个好办法。
    
}

#pragma mark - 剪切按钮
- (void)cutPicBtnClick
{
    UIGraphicsBeginImageContext(CGSizeMake(WIDTH, HEIGHT));//开始一个图片的绘图
//    在屏幕加载之后截图
    [self.view drawViewHierarchyInRect:self.view.frame afterScreenUpdates:YES];
    
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();//获取当前图片绘制区域内的图片
    
    UIGraphicsEndImageContext();
    
    SecondVC *vc = [[SecondVC alloc] init];
    vc.image = image;
    [self presentViewController:vc animated:YES completion:nil];
}