Line Example - Updates to someone's code demonstrating how to extend a line beyond touch location and for me to learn some trigonometry.

GameScene.m

#import "GameScene.h"

@implementation GameScene {

    SKShapeNode *line; // Declaration of the SKShapeNode "line".

    // I thought storing a starting point was more convenient than calculating it in drawLine:.
    CGPoint startingPoint;
    CGFloat lineLength;

}

-(void)didMoveToView:(SKView *)view {

    /* Setup your scene here */

    self.backgroundColor = [SKColor blackColor];

    // Inititalize the SKShapeNode "line".

    line = [SKShapeNode node];

    [self addChild:line];

    [line setStrokeColor:[UIColor greenColor]];

    // The startingPoint will still be the origin of your path, but it will be (0, 0) in the
    // SKShapeNode's coordinate system as I thought that would make the math easier. Additionally
    // the SKShapeNode is positioned in the world space rather than the node, again to help make the
    // math easier.
    startingPoint = CGPointZero;
    line.position = CGPointMake(CGRectGetMidX(self.frame), CGRectGetMidY(self.frame) - (CGRectGetMidY(self.frame) * 0.5));

    // I'm being lazy here and just storing the value of the largest screen dimension to use that
    // as the line length to guarantee that the line goes to the edge.
    CGFloat width = self.frame.size.width;
    CGFloat height = self.frame.size.height;
    lineLength = (width > height) ? width : height;
}

-(void)drawLine:(CGPoint)endingPoint {

    // I'm not a trigonometry expert, so there is probably a better way to do this, but after
    // reading up to get a better understanding this seems to work.

    // First we consider that the line from startingPoint to endingPoint (the touch location)
    // is the hypotenuse of a right triangle...
    //     - with the adjacent leg length being the startingPoint.x to endingPoint.x
    //     - with the opposite leg length being the startingPoint.y to endingPoint.y
    // Since we made startingPoint CGPointZero, this is easy and we don't have to do any math to
    // calculate any offets or deltas.
    CGFloat adjacentLegLength = endingPoint.x;
    CGFloat oppositeLegLength = endingPoint.y;

    // Now that we have the lengths we can find the alpha angle with arctangent.
    CGFloat alphaAngle = atan2(oppositeLegLength, adjacentLegLength);

    // With all this we now have enough information to essentially scale the hypotenuse to extend
    // beyond our touch point. Again, not a trig expert so forgive inaccurate terminology here (^_^;)

    // Cosine is the relationship of the adjacent leg to the alpha angle, so we can use that to
    // figure out the actual x coordinate of the endpoint based on our hard-coded lineLength (our
    // new hypotenuse length).
    endingPoint.x = cos(alphaAngle) * lineLength;

    // Sine is the relationship of the opposite leg to the alpha angle, so we use that to find y.
    endingPoint.y = sin(alphaAngle) * lineLength;

    // I guess if you're doing some type of reflection, you probably want to end up doing some math
    // to find x or y at the bounds of the frame instead of what I've done here. Then you can use
    // more trig to continue the line at the appropriate angle. Hopefully this gets you in the right
    // direction?


    CGMutablePathRef pathToDraw = CGPathCreateMutable();

    // Now, this below is about 1/4 of the way up the screen.

    CGPathMoveToPoint(pathToDraw, NULL, startingPoint.x, startingPoint.y); // Starting Point.

    CGPathAddLineToPoint(pathToDraw, NULL, endingPoint.x, endingPoint.y); // Where to draw the line TO.

    CGFloat dashedPattern[2];

    dashedPattern[0] = 20.0; // The length of them.

    dashedPattern[1] = 20.0; // The distance between them.

    CGFloat dottedPattern[2];

    dottedPattern[0] = 5.0; // The length of them.

    dottedPattern[1] = 20.0; // The distance between them.

    CGPathRef dashed = CGPathCreateCopyByDashingPath(pathToDraw, NULL, 0, dashedPattern, 2); // Reference, Transform, Phase, Lengths, Count.

    CGPathRef dotted = CGPathCreateCopyByDashingPath(pathToDraw, NULL, 0, dottedPattern, 2); // Reference, Transform, Phase, Lengths, Count.

    line.path = dashed; // Turn the SKShapeNode "line" into a dotted or dashed line, and not just empty.

    CGPathRelease(dashed);

}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {

    for (UITouch *touch in touches) {

        CGPoint location = [touch locationInNode:line];

        [self drawLine:location]; // Call the drawLine function to draw the line to the finger's location.

    }

}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {

    for (UITouch *touch in touches) {

        CGPoint location = [touch locationInNode:line];

        [self drawLine:location]; // Keep calling the drawLine function, so it'll stay updated with where your finger is at.
        
    }
    
}

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    
    line.path = nil; // Remove the SKShapeNode "line" from the screen when you release your touch.
    
}

-(void)update:(NSTimeInterval)currentTime {
    
    /* Update function ... you know what it does ;) */

}

@end