Canvas for drawing in iOS Swift

Introduction

A canvas lets you draw shapes, lines, or pictures on the screen. It helps you create custom drawings or designs in your app.

When you want users to draw or doodle on the screen.

To create custom graphics like charts or shapes.

For making simple games that need drawing.

To add signatures or handwriting input.

When you want to show dynamic drawings that change.

Syntax

iOS Swift

class MyView: UIView {
  override func draw(_ rect: CGRect) {
    super.draw(rect)
    let context = UIGraphicsGetCurrentContext()!
    // Drawing code here
  }
}

The draw(_:) method is where you put your drawing code.

Use UIGraphicsGetCurrentContext() to get the canvas to draw on.

Examples

This fills the whole canvas with red color.

iOS Swift

override func draw(_ rect: CGRect) {
  super.draw(rect)
  let context = UIGraphicsGetCurrentContext()!
  context.setFillColor(UIColor.red.cgColor)
  context.fill(rect)
}

This draws a blue line from point (10,10) to (100,100).

iOS Swift

override func draw(_ rect: CGRect) {
  super.draw(rect)
  let context = UIGraphicsGetCurrentContext()!
  context.setStrokeColor(UIColor.blue.cgColor)
  context.setLineWidth(5)
  context.move(to: CGPoint(x: 10, y: 10))
  context.addLine(to: CGPoint(x: 100, y: 100))
  context.strokePath()
}

Sample App

This app shows a yellow background with a red diagonal line from top-left to bottom-right.

iOS Swift

import UIKit

class DrawingView: UIView {
  override func draw(_ rect: CGRect) {
    super.draw(rect)
    guard let context = UIGraphicsGetCurrentContext() else { return }
    context.setFillColor(UIColor.yellow.cgColor)
    context.fill(rect)

    context.setStrokeColor(UIColor.red.cgColor)
    context.setLineWidth(4)
    context.move(to: CGPoint(x: 20, y: 20))
    context.addLine(to: CGPoint(x: rect.width - 20, y: rect.height - 20))
    context.strokePath()
  }
}

class ViewController: UIViewController {
  override func viewDidLoad() {
    super.viewDidLoad()
    let drawingView = DrawingView(frame: view.bounds)
    drawingView.backgroundColor = .white
    view.addSubview(drawingView)
  }
}

Important Notes

Always call super.draw(rect) if you override draw in a subclass that already draws something.

Drawing happens only inside the draw(_:) method.

Use setNeedsDisplay() to refresh the canvas and redraw.

Summary

Canvas drawing uses UIView's draw(_:) method.

Get the drawing context with UIGraphicsGetCurrentContext().

Use context methods to draw shapes, lines, and colors.

Practice

1. What is the primary method to override when drawing custom shapes on a UIView in Swift?

easy

A. draw(_ rect: CGRect)

B. viewDidLoad()

C. layoutSubviews()

D. updateConstraints()

Solution

1. Step 1: Understand UIView drawing lifecycle

   Custom drawing on a UIView is done by overriding the draw(_:) method.

2. Step 2: Identify the correct method signature

   The method draw(_ rect: CGRect) provides the area to draw and is called automatically when the view needs to render.

3. Final Answer:

   draw(_ rect: CGRect) -> Option A

4. Quick Check:

   Custom drawing uses draw(_:) [OK]

Hint: Remember: draw(_:) is the canvas entry point [OK]

Common Mistakes:

• Confusing draw(_:) with viewDidLoad()
• Trying to draw in layoutSubviews()
• Using updateConstraints() for drawing

2. Which Swift code correctly obtains the current graphics context inside draw(_:) for drawing?

easy

A. let context = UIView.getContext()

B. let context = CGContext.current()

C. let context = GraphicsContext.shared

D. let context = UIGraphicsGetCurrentContext()

Solution

1. Step 1: Recall the UIKit method for graphics context

   UIGraphicsGetCurrentContext() is the correct UIKit function to get the current drawing context.

2. Step 2: Verify method names and usage

   Other options are not valid UIKit methods or properties for getting the context.

3. Final Answer:

   let context = UIGraphicsGetCurrentContext() -> Option D

4. Quick Check:

   Use UIGraphicsGetCurrentContext() to get context [OK]

Hint: Use UIGraphicsGetCurrentContext() inside draw(_:) [OK]

Common Mistakes:

• Using non-existent methods like CGContext.current()
• Trying to get context outside draw(_:) method
• Confusing UIView methods with graphics context

3. What will be the visible result of this draw(_:) code snippet?

override func draw(_ rect: CGRect) {
  guard let ctx = UIGraphicsGetCurrentContext() else { return }
  ctx.setFillColor(UIColor.red.cgColor)
  ctx.fill(CGRect(x: 10, y: 10, width: 100, height: 50))
}

medium

A. A blue rectangle at position (10,10) sized 100x50

B. A red rectangle at position (10,10) sized 100x50

C. No visible shape because fill color is not set

D. A red circle at position (10,10) sized 100x50

Solution

1. Step 1: Analyze fill color and shape

   The code sets fill color to red and fills a rectangle at (10,10) with width 100 and height 50.

2. Step 2: Confirm shape and color match

   The shape is a rectangle, not a circle, and color is red, not blue.

3. Final Answer:

   A red rectangle at position (10,10) sized 100x50 -> Option B

4. Quick Check:

   Fill color red + fill rect = red rectangle [OK]

Hint: Fill color sets shape color; fill draws the shape [OK]

Common Mistakes:

• Assuming default color is blue
• Confusing rectangle with circle
• Forgetting to unwrap context safely

4. Identify the error in this draw(_:) method that prevents drawing a blue circle:

override func draw(_ rect: CGRect) {
  let ctx = UIGraphicsGetCurrentContext()
  ctx.setFillColor(UIColor.blue.cgColor)
  ctx.fillEllipse(in: CGRect(x: 20, y: 20, width: 80, height: 80))
}

medium

A. UIColor.blue.cgColor is invalid

B. Incorrect method fillEllipse(in:) used

C. Missing guard or forced unwrap for context

D. CGRect coordinates are out of bounds

Solution

1. Step 1: Check context unwrapping

   The context is optional and not safely unwrapped, causing a compile error because ctx is optional.

2. Step 2: Confirm method and color correctness

   fillEllipse(in:) and UIColor.blue.cgColor are correct and CGRect is valid.

3. Final Answer:

   Missing guard or forced unwrap for context -> Option C

4. Quick Check:

   Always unwrap context safely before drawing [OK]

Hint: Always unwrap UIGraphicsGetCurrentContext() safely [OK]

Common Mistakes:

• Ignoring optional context leading to no drawing
• Thinking fillEllipse(in:) is wrong method
• Assuming color cgColor is invalid

5. You want to draw a green rectangle with a black border of 5 points thickness inside draw(_:). Which code snippet correctly achieves this?

hard

A. ctx.setFillColor(UIColor.green.cgColor)
ctx.fill(rect)
ctx.setStrokeColor(UIColor.black.cgColor)
ctx.setLineWidth(5)
ctx.stroke(rect)

B. ctx.setStrokeColor(UIColor.green.cgColor)
ctx.stroke(rect)
ctx.setFillColor(UIColor.black.cgColor)
ctx.fill(rect)
ctx.setLineWidth(5)

C. ctx.setFillColor(UIColor.black.cgColor)
ctx.fill(rect)
ctx.setStrokeColor(UIColor.green.cgColor)
ctx.setLineWidth(5)
ctx.stroke(rect)

D. ctx.setLineWidth(5)
ctx.setFillColor(UIColor.green.cgColor)
ctx.stroke(rect)
ctx.setStrokeColor(UIColor.black.cgColor)
ctx.fill(rect)

Solution

1. Step 1: Fill rectangle with green color

   Set fill color to green and fill the rect to paint the inside.

2. Step 2: Draw black border with stroke

   Set stroke color to black, line width to 5, then stroke the rect to draw border.

3. Final Answer:

   Fill green then stroke black with line width 5 -> Option A

4. Quick Check:

   Fill first, then stroke for border [OK]

Hint: Fill shape first, then stroke border with line width [OK]

Common Mistakes:

• Swapping fill and stroke colors
• Setting line width after stroke call
• Calling stroke before fill