Custom ViewModifiers in iOS Swift - Mini App: Build & Ship

Build: StyledTextScreen

This screen shows a text label styled using a custom ViewModifier to add padding, background color, and rounded corners.

Target UI

-------------------------
|                       |
|   Hello, SwiftUI!      |
|                       |
-------------------------

Create a custom ViewModifier named 'RoundedBackground' that adds padding, a blue background color, and rounded corners.

Apply this modifier to a Text view displaying 'Hello, SwiftUI!'

Use a VStack to center the text vertically and horizontally on the screen.

Starter Code

iOS Swift

import SwiftUI

struct StyledTextScreen: View {
    var body: some View {
        VStack {
            // TODO: Add Text with custom modifier here
        }
        .frame(maxWidth: .infinity, maxHeight: .infinity)
        .background(Color.white)
    }
}

// TODO: Define custom ViewModifier here

struct StyledTextScreen_Previews: PreviewProvider {
    static var previews: some View {
        StyledTextScreen()
    }
}

Task 1

Task 2

Task 3

Solution

iOS Swift

import SwiftUI

struct RoundedBackground: ViewModifier {
    func body(content: Content) -> some View {
        content
            .padding(16)
            .background(Color.blue)
            .cornerRadius(10)
            .foregroundColor(.white)
    }
}

extension View {
    func roundedBackground() -> some View {
        self.modifier(RoundedBackground())
    }
}

struct StyledTextScreen: View {
    var body: some View {
        VStack {
            Text("Hello, SwiftUI!")
                .roundedBackground()
        }
        .frame(maxWidth: .infinity, maxHeight: .infinity)
        .background(Color.white)
        .ignoresSafeArea()
    }
}

struct StyledTextScreen_Previews: PreviewProvider {
    static var previews: some View {
        StyledTextScreen()
    }
}

We created a custom ViewModifier called RoundedBackground. It adds padding around the content, sets a blue background color, rounds the corners with a radius of 10, and changes the text color to white for contrast.

We also added a convenient extension on View to apply this modifier easily with .roundedBackground().

Inside the StyledTextScreen, we put a Text view with the string "Hello, SwiftUI!" and applied our custom modifier. The VStack centers the text vertically and horizontally by expanding to fill the screen.

This approach keeps styling reusable and clean, just like putting on a favorite jacket to look nice.

Final Result

Completed Screen

-------------------------
|                       |
|   Hello, SwiftUI!      |
|   (blue rounded box)   |
|                       |
-------------------------

The text 'Hello, SwiftUI!' is shown in white on a blue rounded rectangle.

The text is centered vertically and horizontally on the screen.

No user interaction is required; this is a static styled label.

Stretch Goal

Add a toggle button that switches the background color of the text between blue and green using the custom ViewModifier.

💡 Hint

Use a @State variable to track the color and pass it as a parameter to the ViewModifier.

Practice

1. What is the main purpose of creating a custom ViewModifier in SwiftUI?

easy

A. To reuse a set of view styles and behaviors easily

B. To create a new type of view from scratch

C. To handle user input events directly

D. To manage app navigation between screens

Solution

1. Step 1: Understand what a ViewModifier does

   A ViewModifier bundles styling and behavior changes for views.

2. Step 2: Identify the main use

   It helps reuse these styles easily across many views.

3. Final Answer:

   To reuse a set of view styles and behaviors easily -> Option A

4. Quick Check:

   Custom ViewModifiers = reuse styles [OK]

Hint: Think: reuse styles and behaviors with one modifier [OK]

Common Mistakes:

• Confusing ViewModifier with creating new views
• Thinking it handles navigation or input directly
• Assuming it manages app state

2. Which of the following is the correct way to apply a custom ViewModifier named Highlight to a Text view?

easy

A. Text("Hello").modifier(Highlight())

B. Text("Hello").applyModifier(Highlight)

C. Text("Hello").useModifier(Highlight())

D. Text("Hello").addModifier(Highlight)

Solution

1. Step 1: Recall the correct method name

   The method to apply a ViewModifier is .modifier().

2. Step 2: Check syntax for applying Highlight

   It should be .modifier(Highlight()) with parentheses to create the modifier instance.

3. Final Answer:

   Text("Hello").modifier(Highlight()) -> Option A

4. Quick Check:

   Use .modifier() with instance [OK]

Hint: Remember: .modifier() takes an instance with () [OK]

Common Mistakes:

• Using wrong method names like applyModifier or addModifier
• Forgetting parentheses after modifier name
• Passing type instead of instance

3. Given this custom ViewModifier:

struct RedTitle: ViewModifier {
  func body(content: Content) -> some View {
    content
      .font(.title)
      .foregroundColor(.red)
  }
}

What will this code display?

Text("Welcome").modifier(RedTitle())

medium

A. Text "Welcome" with no changes

B. Text "Welcome" in default color with title font

C. Text "Welcome" in red color with default font

D. Text "Welcome" in red color with title font

Solution

1. Step 1: Analyze the RedTitle modifier

   It sets the font to title and text color to red.

2. Step 2: Apply modifier to Text("Welcome")

   The text will appear with title font size and red color.

3. Final Answer:

   Text "Welcome" in red color with title font -> Option D

4. Quick Check:

   RedTitle = red + title font [OK]

Hint: Look at modifier body: font and color changes [OK]

Common Mistakes:

• Ignoring the font change
• Assuming default color remains
• Confusing modifier effects

4. What is wrong with this custom ViewModifier code?

struct BlueBackground: ViewModifier {
  func body(content: Content) -> some View {
    content.background(Color.blue)
  }

  func body(content: Content) -> some View {
    content.padding()
  }
}

medium

A. Background modifier is not allowed inside ViewModifier

B. Two body functions cause a duplicate method error

C. Missing return keyword in body functions

D. Padding modifier must be applied before background

Solution

1. Step 1: Check method definitions in struct

   There are two body(content:) methods defined, which is not allowed.

2. Step 2: Understand Swift rules for method overloading

   Swift does not allow two methods with the same signature in one struct.

3. Final Answer:

   Two body functions cause a duplicate method error -> Option B

4. Quick Check:

   Duplicate body() methods = error [OK]

Hint: Only one body(content:) allowed per ViewModifier [OK]

Common Mistakes:

• Trying to overload body method
• Forgetting return keyword (not needed here)
• Misunderstanding modifier order rules

5. You want to create a custom ViewModifier that adds a rounded border only if a Boolean highlight is true. Which of these implementations correctly applies the border conditionally?

hard

A. struct ConditionalBorder: ViewModifier {
var highlight: Bool
func body(content: Content) -> some View {
if highlight {
content.border(Color.green, width: 2)
} else {
content
}
}
}

B. struct ConditionalBorder: ViewModifier {
var highlight: Bool
func body(content: Content) -> some View {
content.border(highlight ? Color.green : Color.clear, width: 2)
}
}

C. struct ConditionalBorder: ViewModifier {
var highlight: Bool
func body(content: Content) -> some View {
content
.border(Color.green, width: highlight ? 2 : 0)
}
}

D. struct ConditionalBorder: ViewModifier {
var highlight: Bool
func body(content: Content) -> some View {
content
.border(Color.green, width: highlight)
}
}

Solution

1. Step 1: Eliminate compile errors

   struct ConditionalBorder: ViewModifier { var highlight: Bool func body(content: Content) -> some View { if highlight { content.border(Color.green, width: 2) } else { content } } } returns different view types in if-else (compiler error). struct ConditionalBorder: ViewModifier { var highlight: Bool func body(content: Content) -> some View { content .border(Color.green, width: highlight) } } passes Bool to width (CGFloat expected, type error).

2. Step 2: Compare remaining options

   struct ConditionalBorder: ViewModifier { var highlight: Bool func body(content: Content) -> some View { content.border(highlight ? Color.green : Color.clear, width: 2) } } uses Color.clear with fixed width 2: invisible but expands frame (layout shift). struct ConditionalBorder: ViewModifier { var highlight: Bool func body(content: Content) -> some View { content .border(Color.green, width: highlight ? 2 : 0) } } uses conditional width 2/0: border only when true, no layout shift.

3. Final Answer:

   content.border(Color.green, width: highlight ? 2 : 0) -> Option C

4. Quick Check:

   Conditional width = correct conditional border [OK]

Hint: Ternary on width for conditional border without layout shift [OK]

Common Mistakes:

• Returning different view types in if-else inside body
• Passing Bool where CGFloat is expected
• Using Color.clear with fixed width (layout shift)