Drawing Polygons and Detecting Selection withtouchesBegan in UIKit

In this tutorial, we will explore how to draw a list of polygons using UIBezierPath and detect which polygon was selected by handling the touchesBegan event.

Introduction to UIBezierPath

UIBezierPath is a powerful class in UIKit that allows us to create complex shapes with multiple paths. It’s commonly used for drawing custom views, like polygons, circles, or even more complex shapes.

To create a polygon using UIBezierPath, we need to define an array of coordinates that form the shape of our polygon. Here’s an example of creating a simple rectangle:

UIBezierPath *path = [UIBezierPath bezierPath];
[path moveToPoint:CGPointMake(10, 10)];
[path addLineToPoint:CGPointMake(100, 10)];
[path addLineToPoint:CGPointMake(100, 100)];
[path addLineToPoint:CGPointMake(10, 100)];
[path closePath];

In this example, we create a new UIBezierPath instance and define the coordinates for the rectangle. The moveToPoint: method sets the starting point of the path, while addLineToPoint: adds a line to the path at the specified coordinates.

Creating Polygons

To create more complex polygons, you can use the same approach as above, but with multiple addLineToPoint: calls:

UIBezierPath *path = [UIBezierPath bezierPath];
[path moveToPoint:CGPointMake(10, 10)];
[path addLineToPoint:CGPointMake(100, 10)];
[path addLineToPoint:CGPointMake(150, 50)];
[path addLineToPoint:CGPointMake(200, 100)];
[path closePath];

In this example, we create a new polygon with four points.

Drawing Polygons on the Screen

To draw our polygons on the screen, we can use CAShapeLayer and set its path property to our UIBezierPath instance:

CAShapeLayer *layer = [CAShapeLayer layer];
layer.path = path;
layer.strokeColor = [UIColor blueColor].CGColor;
layer.lineWidth = 2;
[self.view.layer addSublayer:layer];

This will render the polygon on the screen.

Detecting Polygon Selection with touchesBegan

To detect which polygon was selected, we need to handle the touchesBegan event. When a user touches our view, touchesBegan: is called, and we can check if the touch point lies within any of our polygons:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    CGPoint p = [[touches anyObject] locationInView:self.view];

    for (CAShapeLayer *layer in self.view.layer.sublayers) {
        UIBezierPath *path = layer.path;
        if ([path containsPoint:p]) {
            NSLog(@"Touched on polygon!");
            // Handle the selection logic here
        }
    }
}

In this example, we loop through all sublayers of our view’s layer and check if the touch point lies within any of their paths. If it does, we log a message indicating that the user touched on a polygon.

Handling Polygon Selection

To handle the selection logic, you can use various approaches, such as:

• Showing a popup or alert with information about the selected polygon
• Changing the color or appearance of the selected polygon
• Enabling or disabling certain actions based on the selection

For example:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    CGPoint p = [[touches anyObject] locationInView:self.view];

    for (CAShapeLayer *layer in self.view.layer.sublayers) {
        UIBezierPath *path = layer.path;
        if ([path containsPoint:p]) {
            NSLog(@"Touched on polygon!");
            // Show a popup with information about the selected polygon
            UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Selected Polygon" message:@"The user touched on this polygon!" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil, nil];
            [alertView show];
        }
    }
}

In this example, we create an AVAlertView to display a popup with information about the selected polygon.

Conclusion

Drawing polygons and detecting selection with touchesBegan is a powerful feature in UIKit. By using UIBezierPath and handling the touchesBegan event, you can create custom views that respond to user interactions. With this tutorial, you should now have a solid understanding of how to create and detect polygon selection using UIBezierPath and CAShapeLayer.

Last modified on 2023-12-30