Ranjit Ranjit - 23 days ago 4
iOS Question

Undo/Redo for drawing in iOS

I am working on a drawing app, I want to do Undo/Redo, for this I am saving CGPath on touches ended to NSMutableArray, But I am not understanding how I should render the CGPaths on click of Undo button


As I am using BezierPaths, So, I first decided to go with a simple approach of just stroking this path, without CGPath,

EDIT2: As my Undo is happening in segments(i,e in parts and not whole path is deleted), I decided to create an array of array, So I have made changes accordingly and now I will be drawing in CGlayer, with taking CGPath

So here "parentUndoArray" is array of arrays.

So I have done this way

I have a class called DrawingPath which will do the drawing


@interface DrawingPath : NSObject

@property (strong, nonatomic) NSString *pathWidth;
@property (strong,nonatomic) UIColor *pathColor;
@property (strong,nonatomic) UIBezierPath *path;

- (void)draw;



#import "DrawingPath.h"

@implementation DrawingPath

@synthesize pathWidth = _pathWidth;
@synthesize pathColor = _pathColor;
@synthesize path = _path;

- (id)init {

if (!(self = [super init] ))
return nil;

_path = [[UIBezierPath alloc] init];

[_path setLineWidth:2.0f];

return self;

- (void)draw

[self.pathColor setStroke];
[self.path stroke];


So now in my DrawingView, I do it this way

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
ctr = 0;
bufIdx = 0;
UITouch *touch = [touches anyObject];
pts[0] = [touch locationInView:self];
isFirstTouchPoint = YES;

[m_undoArray removeAllObjects];//On every touches began clear undoArray


-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
UITouch *touch = [touches anyObject];

CGPoint p = [touch locationInView:self];
pts[ctr] = p;

if (ctr == 4)
pts[3] = midPoint(pts[2], pts[4]);

for ( int i = 0; i < 4; i++)
pointsBuffer[bufIdx + i] = pts[i];

bufIdx += 4;

dispatch_async(drawingQueue, ^{

self.currentPath = [[DrawingPath alloc] init];
[self.currentPath setPathColor:self.lineColor];

if (bufIdx == 0) return;

LineSegment ls[4];
for ( int i = 0; i < bufIdx; i += 4)
if (isFirstTouchPoint) // ................. (3)

ls[0] = (LineSegment){pointsBuffer[0], pointsBuffer[0]};
[self.currentPath.path moveToPoint:ls[0].firstPoint];

// [offsetPath addLineToPoint:ls[0].firstPoint];
isFirstTouchPoint = NO;

ls[0] = lastSegmentOfPrev;


float frac1 = self.lineWidth/clamp(len_sq(pointsBuffer[i], pointsBuffer[i+1]), LOWER, UPPER); // ................. (4)
float frac2 = self.lineWidth/clamp(len_sq(pointsBuffer[i+1], pointsBuffer[i+2]), LOWER, UPPER);
float frac3 = self.lineWidth/clamp(len_sq(pointsBuffer[i+2], pointsBuffer[i+3]), LOWER, UPPER);

ls[1] = [self lineSegmentPerpendicularTo:(LineSegment){pointsBuffer[i], pointsBuffer[i+1]} ofRelativeLength:frac1]; // ................. (5)
ls[2] = [self lineSegmentPerpendicularTo:(LineSegment){pointsBuffer[i+1], pointsBuffer[i+2]} ofRelativeLength:frac2];
ls[3] = [self lineSegmentPerpendicularTo:(LineSegment){pointsBuffer[i+2], pointsBuffer[i+3]} ofRelativeLength:frac3];

[self.currentPath.path moveToPoint:ls[0].firstPoint]; // ................. (6)
[self.currentPath.path addCurveToPoint:ls[3].firstPoint controlPoint1:ls[1].firstPoint controlPoint2:ls[2].firstPoint];
[self.currentPath.path addLineToPoint:ls[3].secondPoint];
[self.currentPath.path addCurveToPoint:ls[0].secondPoint controlPoint1:ls[2].secondPoint controlPoint2:ls[1].secondPoint];
[self.currentPath.path closePath];

lastSegmentOfPrev = ls[3]; // ................. (7)

[m_undoArray addObject:self.currentPath];

CGPathRef cgPath = self.currentPath.path.CGPath;
mutablePath = CGPathCreateMutableCopy(cgPath);

dispatch_async(dispatch_get_main_queue(), ^{
bufIdx = 0;
[self setNeedsDisplay];


pts[0] = pts[3];
pts[1] = pts[4];
ctr = 1;

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
[parentUndoArray addObject:m_undoArray];


My drawRect method is below

EDIT: Now my DrawRect has two cases

- (void)drawRect:(CGRect)rect
switch (m_drawStep)
case DRAW:

CGContextRef context = UIGraphicsGetCurrentContext();//Get a reference to current context(The context to draw)

CGContextRef layerContext = CGLayerGetContext(self.currentDrawingLayer);
CGContextAddPath(layerContext, mutablePath);
CGContextSetStrokeColorWithColor(layerContext, self.lineColor.CGColor);
CGContextSetFillColorWithColor(layerContext, self.lineColor.CGColor);
CGContextDrawPath(layerContext, kCGPathFillStroke);
// CGPathRelease(mutablePath);

CGContextDrawLayerInRect(context,rectSize, self.newDrawingLayer);
CGContextDrawLayerInRect(context, self.bounds, self.permanentDrawingLayer);
CGContextDrawLayerInRect(context, self.bounds, self.currentDrawingLayer );


case UNDO:
for(int i = 0; i<[m_parentUndoArray count];i++)
NSMutableArray *undoArray = [m_parentUndoArray objectAtIndex:i];

for(int i =0; i<[undoArray count];i++)
DrawingPath *drawPath = [undoArray objectAtIndex:i];
[drawPath draw];


[super drawRect:rect];

EDIT2:Now the problem what I am facing now is that, even if I draw small paths or large paths, the data in the array of array is same. But infact, small path should contain lesser drawingPath object and large path should contain more drawingPath object in undoArray, which at last is added to array of array called "ParentUndoArray

Here are the the screen shots,

1.first screen Shot of drawn line at a stretch without lifting the finger

enter image description here

2, After doing undo operation once, only segment of that line is removed

enter image description here


I have found a solution for this, we need to create a array of array of DrawingPaths:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
   // Do the above code, then
   [m_undoArray addObject:self.currentPath];

- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
   [m_parentUndoArray addObject:[NSArray arrayWithArray:m_undoArray]];

and then stroke the path in DrawRect.