涂鸦画板

#import "ViewController.h"

#import "DrawView.h"

@interface ViewController ()

@end

@implementation ViewController

-(void)loadView{

    self.view = [[DrawView alloc]init];

    self.view.backgroundColor = [UIColor whiteColor];

}

- (void)viewDidLoad {

    [super viewDidLoad];    
}

@end

////////////////////////////////////////////////////////////

#import "DrawView.h"

@interface DrawView()

@end

@implementation DrawView

- (instancetype)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        _lineArray = [NSMutableArray array];

        UIButton *btn = [UIButton buttonWithType:UIButtonTypeRoundedRect];

        btn.frame = CGRectMake(100, 420, 100, 40);

        btn.backgroundColor=[UIColor yellowColor];
        
        [btn setTitle:@"撤销" forState:UIControlStateNormal];

        [btn addTarget:self action:@selector(undoDraw) forControlEvents:UIControlEventTouchUpInside];

        [self addSubview:btn];
    }
    return self;
}

//撤销操作

-(void)undoDraw{
    [_lineArray removeLastObject];//删除最后一条线
    [self setNeedsDisplay];//让View重绘，自动重绘
}

//每次手指开始触摸,增加一个新的数组，数组记录划过的点,利用这些点绘画

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
     [_lineArray addObject:[NSMutableArray arrayWithCapacity:1]];
}
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{

    UITouch *touche = [touches anyObject];
    CGPoint point = [touche locationInView:self];

    //数组里需要id类型

    NSValue *value = [NSValue valueWithCGPoint:point];

    //把value装到_lineArray的最后一个元素（数组）里

    [[_lineArray lastObject] addObject:value];

    [self setNeedsDisplay];//重绘界面

}
 
-(void)drawRect:(CGRect)rect{
 
    CGContextRef context = UIGraphicsGetCurrentContext();//获得上下文

    CGContextSetStrokeColorWithColor(context, [UIColor redColor].CGColor);//画笔颜色

    for (int i=0; i<[_lineArray count]; i++) {

        NSMutableArray *pointArray = [[NSMutableArray alloc]initWithArray:[_lineArray objectAtIndex:i]];

        for (int j=0; j<[pointArray count]-1; j++) {

            //遍历所有的点

            NSValue *a = [pointArray objectAtIndex:j];

            CGPoint p1 = [a CGPointValue];

            NSValue *b = [pointArray objectAtIndex:j+1];

            CGPoint p2 = [b CGPointValue];


            CGContextMoveToPoint(context, p1.x, p1.y);

            CGContextAddLineToPoint(context, p2.x, p2.y);
        }        
    }

    //在当前的图形上下文中，根据画笔的颜色和画笔的路径，进行绘制

    CGContextStrokePath(context);
}

@end