Simple drawing view based on UIPanGestureRecognizer.

DrawView.h

#import <UIKit/UIKit.h>

@interface DrawView : UIView

@property (nonatomic) CGMutablePathRef drawingPath;
@property (nonatomic) CGPoint lastPoint;

@end

DrawView.m

#import "DrawView.h"

CGRect DrawViewCGRectFromPointsMinimumSize(CGPoint p1, CGPoint p2, CGSize size) {
    return CGRectMake(MIN(p1.x, p2.x), MIN(p1.y, p2.y), MAX(fabs(p1.x - p2.x), size.width), MAX(fabs(p1.y - p2.y), size.height));
}
void GetArrayPoints_CGPathApplierFunc(void *info, const CGPathElement *element) {
    NSMutableArray *array = (__bridge NSMutableArray *)info;
    if (element->type == kCGPathElementMoveToPoint || element->type == kCGPathElementAddLineToPoint) {
        CGPoint point = element->points[0];
        [array addObject:[NSValue valueWithCGPoint:point]];
    }
}


#define DrawLineWidth 4.0

@implementation DrawView

#pragma mark -
#pragma mark - Life cycle
- (instancetype)initWithCoder:(NSCoder *)aDecoder {
    self = [super initWithCoder:aDecoder];
    if (self) {
        UIPanGestureRecognizer *panner = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(panGestureDidPan:)];
        [self addGestureRecognizer:panner];
    }
    return self;
}
- (void)dealloc {
	if (_drawingPath != NULL) {
        CGPathRelease(_drawingPath);
        _drawingPath = NULL;
    }
}

#pragma mark -
#pragma mark - Draw
- (void)drawRect:(CGRect)rect {
    if (_drawingPath != NULL) {
        CGContextRef ctx = UIGraphicsGetCurrentContext();
        CGContextSaveGState(ctx);
        
        CGContextSetLineWidth(ctx, DrawLineWidth);
        CGContextSetLineCap(ctx, kCGLineCapRound);
        CGContextSetLineJoin(ctx, kCGLineJoinRound);
        CGContextSetMiterLimit(ctx, 2.0);
        
        CGContextSetStrokeColorWithColor(ctx, [[UIColor orangeColor] CGColor]);
        
        CGContextAddPath(ctx, _drawingPath);
        CGContextDrawPath(ctx, kCGPathStroke);
        
        CGContextRestoreGState(ctx);
    }
}

#pragma mark -
#pragma mark - Gesture
- (void)panGestureDidPan:(UIPanGestureRecognizer *)panner {
    // Start
    if (panner.state == UIGestureRecognizerStateBegan) {
        // Reset the path
        if (_drawingPath != NULL) {
            CGPathRelease(_drawingPath);
            _drawingPath = NULL;
        }
        _lastPoint = CGPointZero;
        
        // Create a new path
        _drawingPath = CGPathCreateMutable();
        
        // Start the new path
        CGPoint point = [panner locationInView:self];
        CGPathMoveToPoint(_drawingPath, nil, point.x, point.y);
    
    // Change
    } else if (panner.state == UIGestureRecognizerStateChanged) {
        if (_drawingPath != NULL) {
            // Add to the path
            CGPoint point = [panner locationInView:self];
            CGPathAddLineToPoint(_drawingPath, nil, point.x, point.y);
            
            // Draw the line
            if (CGPointEqualToPoint(_lastPoint, CGPointZero)) {
                // If there a last point then we'll redraw everything
                [self setNeedsDisplay];
            } else {
                // Only redraw what has changed.
                CGRect render = DrawViewCGRectFromPointsMinimumSize(_lastPoint, point, CGSizeMake(DrawLineWidth, DrawLineWidth));
                [self setNeedsDisplayInRect:UIEdgeInsetsInsetRect(render, UIEdgeInsetsMake(-DrawLineWidth, -DrawLineWidth, -DrawLineWidth, -DrawLineWidth))];
            }
            _lastPoint = point;
        }
        
    // Ended
    } else if (panner.state == UIGestureRecognizerStateEnded) {
        
        NSMutableArray *array = [NSMutableArray array];
        CGPathApply(_drawingPath, (__bridge void *)(array), &GetArrayPoints_CGPathApplierFunc);
        
        NSLog(@"%@",array);
        
        // Reset the path
        if (_drawingPath != NULL) {
            CGPathRelease(_drawingPath);
            _drawingPath = NULL;
        }
        _lastPoint = CGPointZero;
    }
}

@end