主要思路：同过UIPanGestureRecognizer移动手势创建UIBezierPath线条，把线条添加到数组里面，然后重新实现- (void)drawRect:(CGRect)rect，把所有的线条画出来。

主要代码

// 画线的宽度
@property(nonatomic,assign)CGFloat lineWidth;

// 线条颜色
@property(nonatomic,retain)UIColor* pathColor;

// 加载背景图片
@property(nonatomic,strong)UIImage* image;

// 清屏
- (void)clear;

// 撤销
- (void)undo;

// 橡皮擦
- (void)eraser;

// 保存
- (void)save;

// 重绘UI
- (void)drawRect:(CGRect)rect {
    
    for (DrawBezierPath* path in self.paths) {
        
        if ([path isKindOfClass:[UIImage class]]) {
            // 画图片
            UIImage* image = (UIImage*)path;
            [image drawInRect:rect];
        }else{
            // 设置画笔颜色
            [path.pathColor set];
            // 绘制
            [path stroke];
        }
    }
}

// 懒加载属性
- (NSMutableArray*)paths{
    
    if (_paths == nil) {
        _paths = [NSMutableArray array];
    }
    return _paths;
}

// 重写image属性
- (void)setImage:(UIImage *)image{
    
    _image = image;
    
    // 将图片加入到线条数组中
    [self.paths addObject:image];
    
    [self setNeedsDisplay];
}

#pragma mark - Init

// 初始化
- (void)setUp{
    
    // 添加平移手势
    UIPanGestureRecognizer* panGes = [[UIPanGestureRecognizer alloc]initWithTarget:self action:@selector(panGes:)];
    [self addGestureRecognizer:panGes];
    
    // 默认值
    self.lineWidth = 1;
    self.pathColor = [UIColor blackColor];
}

#pragma mark - Event

// 平移事件
- (void)panGes:(UIPanGestureRecognizer*)ges{
    
    // 获取当前点
    CGPoint curPoint = [ges locationInView:self];
    
    if (ges.state == UIGestureRecognizerStateBegan) { // 开始移动
        
        // 创建贝塞尔曲线
        _bezierPath = [[DrawBezierPath alloc] init];
        
        // 设置线条宽度
        _bezierPath.lineWidth = _lineWidth;
        
        // 线条默认颜色
        _bezierPath.pathColor = _pathColor;
        
        // 设置起始点
        [_bezierPath moveToPoint:curPoint];
        
        [self.paths addObject:_bezierPath];
    }
    
    // 连线
    [_bezierPath addLineToPoint:curPoint];
    
    // 重绘
    [self setNeedsDisplay];
}

#pragma mark - Method

// 清屏
- (void)clear{
    
    [self.paths removeAllObjects];
    
    [self setNeedsDisplay];
}

// 撤销
- (void)undo{
    
    [self.paths removeLastObject];
    
    [self setNeedsDisplay];
}

// 橡皮擦
- (void)eraser{
    
    self.pathColor = [UIColor whiteColor];
    
    [self setNeedsDisplay];
}

// 保存
- (void)save{
    
    // ---- 截图操作
    // 开启上下文
    UIGraphicsBeginImageContextWithOptions(self.frame.size, NO, 0);
    
    // 获取当前上下文
    CGContextRef context = UIGraphicsGetCurrentContext();
    
    // 渲染图层到上下文
    [self.layer renderInContext:context];
    
    // 从上下文中获取图片
    UIImage* image = UIGraphicsGetImageFromCurrentImageContext();
    
    // 关闭上下文
    UIGraphicsEndImageContext();
    
    // ---- 保存图片
    UIImageWriteToSavedPhotosAlbum(image, self, @selector(image:didFinishSavingWithError:contextInfo:), nil);
    
}