CODEWITHSUNDEEP
iosdrag-and-dropuigesturerecognizer
Jonathan Rachlin
Jonathan Rachlin

Reputation: 391

How to drag a line in ios using UIPanGestureRecognizer

I am writing a program where the user will drag a line from one UILabel to another. I created a UIView subclass called DragView, and overrode the drawRect method. I made the UIView a subview of the RootController view. The DragView is definitely visible, the drawRect method is definitely being called, but no line is visible.

These are (what I think are) the relevant pieces of code.

//DragView.m

- (void)drawRect:(CGRect)rect
{
    if (context == nil)
    {
        context = UIGraphicsGetCurrentContext();
    }

    if (_drawLineFlag)
    {

        CGContextSetLineWidth(context,2.0); 
        CGColorSpaceRef colorspace = CGColorSpaceCreateDeviceRGB();
        CGFloat components[] = {0.0,0.0,1.0,1.0};
        CGColorRef color = CGColorCreate(colorspace, components);

        CGContextSetStrokeColorWithColor(context,color);

        CGContextMoveToPoint(context,_startX, _startY);

        CGContextAddLineToPoint(context,_currentX,_currentY);

        CGContextStrokePath(context);   
    } 
}

DrawProgramAppDelegate

- (void) initializeUI
{
.....

    dragView = [[DragView alloc] initWithFrame:CGRectMake(0.0f,0.0f,1024.0f,768.0f)] ;
    [view addSubview: dragView];
    UIPanGestureRecognizer *panGestureRecognizer = [[UIPanGestureRecognizer alloc] initWithTarget:self  action:@selector(handlePan:)];
    [dragView addGestureRecognizer:panGestureRecognizer];
.....

}

The event handler is:

- (void) handlePan:(UIPanGestureRecognizer *) recognizer             
{
    CGPoint point = [recognizer translationInView:dragView];

    [dragView setCurrentX:point.x];
    [dragView setCurrentY:point.y];
    [dragView setDrawLineFlag:YES];

    [view bringSubviewToFront:drawView];
    [dragView drawRect:CGRectMake (0.0f,0.0f,768.0f, 1024.0f)];
}

Many thanks for your help.

Jon

Upvotes: 0

Views: 1635

Answers (1)

jbat100
jbat100

Reputation: 16827

It seems to me you are trying to draw a path (you should take a look at this post). Here it seems you just draw a line from the last point to the current point

CGContextMoveToPoint(context,_startX, _startY);
CGContextAddLineToPoint(context,_currentX,_currentY);

which given the frequency of the call would only draw a tiny line.

You should not call drawRect:

[dragView drawRect:CGRectMake (0.0f,0.0f,768.0f, 1024.0f)];

You should tell the system the view needs displaying

[dragView setNeedsDisplay];

Also you should check the gesture recognizer state, defined as 

typedef enum {
   UIGestureRecognizerStatePossible,
   UIGestureRecognizerStateBegan,
   UIGestureRecognizerStateChanged,
   UIGestureRecognizerStateEnded,
   UIGestureRecognizerStateCancelled,
   UIGestureRecognizerStateFailed,
   UIGestureRecognizerStateRecognized = UIGestureRecognizerStateEnded
} UIGestureRecognizerState;

And possibly only draw if the state is UIGestureRecognizerStateChanged or UIGestureRecognizerStateEnded

Upvotes: 1

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