Canvas for drawing in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a UIView subclass that can draw a red circle.

iOS Swift

class DrawingView: UIView {
  override func draw(_ rect: CGRect) {
    guard let context = UIGraphicsGetCurrentContext() else { return }
    context.setFillColor(UIColor.[1].cgColor)
    context.fillEllipse(in: rect)
  }
}

Drag options to blanks, or click blank then click option'

AColor

BRed

CredColor

Dred

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Red' instead of lowercase 'red'.

Using 'redColor' which is not a UIColor property.

The correct property is 'red' with lowercase 'r' in UIColor to get the red color.

2fill in blank

medium

Complete the code to add the DrawingView as a subview and set its frame.

iOS Swift

let drawingView = DrawingView()
drawingView.frame = CGRect(x: 0, y: 0, width: 200, height: 200)
self.view.[1](drawingView)

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AaddSubview

BaddView

CinsertSubview

DappendSubview

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'addView' which does not exist.

Using 'appendSubview' which is not a valid method.

To add a view as a child, use 'addSubview' method on the parent view.

3fill in blank

hard

Fix the error in the draw method to set the stroke color to blue.

iOS Swift

override func draw(_ rect: CGRect) {
  guard let context = UIGraphicsGetCurrentContext() else { return }
  context.setStrokeColor(UIColor.[1].cgColor)
  context.stroke(rect)
}

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AblueColor

Bblue

CBLUE

DBlue

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Blue' or 'BLUE'.

Using 'blueColor' which is not a UIColor property.

UIColor properties are lowercase, so 'blue' is correct to get blue color.

4fill in blank

hard

Fill both blanks to draw a green rectangle with a 5 point line width.

iOS Swift

override func draw(_ rect: CGRect) {
  guard let context = UIGraphicsGetCurrentContext() else { return }
  context.setStrokeColor(UIColor.[1].cgColor)
  context.setLineWidth([2])
  context.stroke(rect)
}

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Agreen

Bblue

C5

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong color names like 'blue'.

Using line width values that are too small or strings.

Use 'green' for the stroke color and '5' for the line width to draw a thick green rectangle.

5fill in blank

hard

Fill all three blanks to draw a filled yellow ellipse with a black border.

iOS Swift

override func draw(_ rect: CGRect) {
  guard let context = UIGraphicsGetCurrentContext() else { return }
  context.setFillColor(UIColor.[1].cgColor)
  context.fillEllipse(in: rect)
  context.setStrokeColor(UIColor.[2].cgColor)
  context.setLineWidth([3])
  context.strokeEllipse(in: rect)
}

Drag options to blanks, or click blank then click option'

Ayellow

Bblack

C2

Dwhite

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing fill and stroke colors.

Using too thick or zero line width.

Fill color is yellow, stroke color is black, and line width is 2 points for a visible border.

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