Canvas for drawing in iOS Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Canvas in iOS Swift development?

A Canvas is a view or area where you can draw shapes, lines, or images using code. It acts like a blank paper for your app to create drawings.

beginner

Which class in Swift is commonly used to create a custom drawing area?

You use a <code>UIView</code> subclass and override its <code>draw(_ rect: CGRect)</code> method to perform custom drawing on the Canvas.

intermediate

What is the role of UIBezierPath in drawing on Canvas?

UIBezierPath helps you create paths like lines, curves, and shapes that you can stroke or fill on the Canvas.

intermediate

How do you trigger a Canvas to redraw its content?

Call setNeedsDisplay() on the Canvas view to tell iOS to call draw(_ rect: CGRect) again and update the drawing.

intermediate

Why is it important to keep drawing code inside draw(_ rect: CGRect)?

Because iOS calls this method whenever the view needs to update. Drawing outside this method may not appear or cause performance issues.

Which method do you override to draw on a UIView Canvas?

A viewDidLoad()

B init()

C layoutSubviews()

D draw(_ rect: CGRect)

The draw(_ rect: CGRect) method is where you put your drawing code for a UIView.

What class helps you create shapes and lines for drawing?

A UILabel

B UIBezierPath

C UIButton

D UIImageView

UIBezierPath is used to create paths like lines and shapes for drawing.

How do you tell the Canvas to refresh its drawing?

A setNeedsDisplay()

B reloadData()

C refresh()

D update()

setNeedsDisplay() marks the view as needing to be redrawn, triggering draw(_ rect: CGRect).

What happens if you draw outside the draw(_ rect:) method?

A Drawing may not appear or cause issues

B The app crashes

C Drawing always appears immediately

D The system ignores the drawing

Drawing outside draw(_ rect:) may not show or cause performance problems.

Which UIKit class do you subclass to create a custom Canvas?

A UIScrollView

B UITableView

C UIView

D UIViewController

You subclass UIView to create a custom drawing Canvas.

Explain how to create a simple drawing Canvas in iOS Swift.

Think about the steps to show custom lines or shapes on screen.

Describe why the draw(_ rect: CGRect) method is important for Canvas drawing.

Consider how iOS manages view updates.

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