Error protocol conformance in Swift - Step-by-Step Execution

Concept Flow - Error protocol conformance

Define Error Type

↓

Conform to Error Protocol

↓

Throw Error

↓

Catch Error

↓

Handle Error

This flow shows how to define a custom error type by conforming to the Error protocol, then throwing and catching that error.

Execution Sample

Swift

enum MyError: Error {
    case badInput
}

func check(input: Int) throws {
    if input < 0 { throw MyError.badInput }
}

Defines a custom error type and a function that throws this error if input is negative.

Execution Table

Step	Action	Condition	Result	Output
1	Call check(input: 5)	5 < 0?	False	No error thrown
2	Function returns normally	-	-	-
3	Call check(input: -3)	-3 < 0?	True	Throw MyError.badInput
4	Error caught in catch block	-	-	Handle error

💡 Execution stops normally or after error is caught and handled.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
input	-	5	-3	-3
errorThrown	nil	nil	MyError.badInput	MyError.badInput

Key Moments - 2 Insights

Why do we need to conform to the Error protocol?

Only types that conform to the Error protocol can be thrown as errors in Swift, as shown in step 3 where MyError.badInput is thrown.

What happens if the condition to throw is false?

The function returns normally without throwing, as seen in step 2 when input is 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'errorThrown' after step 1?

A nil

B MyError.badInput

C 5

D Undefined

💡 Hint

Check variable_tracker row for 'errorThrown' after step 1.

At which step does the error get thrown?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at execution_table row where 'Throw MyError.badInput' appears.

If input was 0, what would happen according to the execution flow?

A Error thrown

B Function returns normally

C Crash

D Infinite loop

💡 Hint

Refer to the condition 'input < 0' in execution_table steps.

Concept Snapshot

Define an enum or struct conforming to Error protocol.
Use 'throw' keyword to throw errors.
Use 'try' to call throwing functions.
Catch errors with 'do-catch' blocks.
Only Error-conforming types can be thrown.

Full Transcript

This visual execution shows how to create a custom error type by conforming to the Error protocol in Swift. The sample code defines an enum MyError with a case badInput. The function check(input:) throws this error if the input is less than zero. The execution table traces calling check with positive and negative inputs, showing when the error is thrown and caught. The variable tracker follows the input value and whether an error was thrown. Key moments clarify why conforming to Error is required and what happens when the condition to throw is false. The quiz tests understanding of error throwing steps and variable states. The snapshot summarizes the syntax and rules for error protocol conformance in Swift.

Practice

1. What is the main purpose of the Error protocol in Swift?

easy

A. To create network connections

B. To handle user interface events

C. To manage memory allocation automatically

D. To define custom error types that can be thrown and caught

Solution

1. Step 1: Understand the role of the Error protocol

   The Error protocol is designed to let developers define their own error types in Swift.

2. Step 2: Identify the purpose of custom error types

   Custom error types allow throwing and catching specific errors in code, improving error handling.

3. Final Answer:

   To define custom error types that can be thrown and caught -> Option D

4. Quick Check:

   Error protocol = custom error types [OK]

Hint: Error protocol is for creating your own error types [OK]

Common Mistakes:

• Confusing Error protocol with UI or memory management
• Thinking Error protocol manages network connections
• Assuming Error protocol is for general programming tasks

2. Which of the following is the correct way to declare an enum that conforms to the Error protocol?

easy

A. enum NetworkError: Error { case disconnected, timeout }

B. enum NetworkError { case disconnected, timeout }

C. enum NetworkError: String { case disconnected, timeout }

D. enum NetworkError: Int { case disconnected, timeout }

Solution

1. Step 1: Check enum conformance syntax

   To conform to the Error protocol, the enum must explicitly declare : Error after its name.

2. Step 2: Verify the enum cases

   The cases can be simple without raw values; raw values are optional and unrelated to Error conformance.

3. Final Answer:

   enum NetworkError: Error { case disconnected, timeout } -> Option A

4. Quick Check:

   Enum conforming to Error = enum Name: Error { cases } [OK]

Hint: Add ': Error' after enum name to conform [OK]

Common Mistakes:

• Omitting ': Error' conformance
• Using raw value types without conforming to Error
• Confusing enum raw types with protocol conformance

3. What will be the output of the following Swift code?

enum FileError: Error {
    case notFound
    case unreadable
}

func readFile(_ name: String) throws -> String {
    if name == "missing.txt" {
        throw FileError.notFound
    }
    return "File content"
}

 do {
    let content = try readFile("missing.txt")
    print(content)
} catch FileError.notFound {
    print("File not found error caught")
} catch {
    print("Other error")
}

medium

A. File not found error caught

B. File content

C. Other error

D. No output

Solution

1. Step 1: Analyze the function behavior

   The function throws FileError.notFound when the filename is "missing.txt".

2. Step 2: Check the do-catch block

   The catch block specifically catches FileError.notFound and prints "File not found error caught".

3. Final Answer:

   File not found error caught -> Option A

4. Quick Check:

   Throw and catch specific error = prints catch message [OK]

Hint: Throws matching catch block runs, prints its message [OK]

Common Mistakes:

• Expecting function to return normally
• Ignoring specific catch and expecting generic catch
• Confusing error thrown with error caught

4. Identify the error in the following Swift code snippet:

enum LoginError: Error {
    case invalidUsername
    case invalidPassword
}

func login(user: String, password: String) throws {
    if user.isEmpty {
        throw LoginError.invalidUsername
    } else if password.isEmpty {
        throw LoginError.invalidPassword
    }
}

try login(user: "", password: "1234")

medium

A. Function login should not throw errors

B. Missing do-catch block to handle the thrown error

C. Enum does not conform to Error protocol

D. Invalid enum case names

Solution

1. Step 1: Check enum conformance

   The enum correctly conforms to the Error protocol with valid cases.

2. Step 2: Analyze function call

   The function is called with try but outside a do-catch block or without try? or try!, causing a compile-time error.

3. Final Answer:

   Missing do-catch block to handle the thrown error -> Option B

4. Quick Check:

   Throwing function call needs do-catch or try? [OK]

Hint: Use do-catch when calling throwing functions [OK]

Common Mistakes:

• Calling throwing function without do-catch
• Assuming enum conformance is missing
• Misunderstanding error handling syntax

5. You want to create a custom error enum DatabaseError that conforms to Error and includes cases for connectionFailed, queryFailed, and timeout. Additionally, you want to add a computed property message that returns a user-friendly description for each case. Which of the following is the correct implementation?

hard

A. struct DatabaseError: Error {
var message: String
}

B. enum DatabaseError {
case connectionFailed
case queryFailed
case timeout

var message: String {
return "Error occurred"
}
}

C. enum DatabaseError: Error {
case connectionFailed
case queryFailed
case timeout

var message: String {
switch self {
case .connectionFailed: return "Connection failed"
case .queryFailed: return "Query failed"
case .timeout: return "Operation timed out"
}
}
}

D. enum DatabaseError: Error {
case connectionFailed(String)
case queryFailed(String)
case timeout(String)

var message: String {
return "Error"
}
}

Solution

1. Step 1: Confirm enum conforms to Error and defines cases

   enum DatabaseError: Error { case connectionFailed case queryFailed case timeout var message: String { switch self { case .connectionFailed: return "Connection failed" case .queryFailed: return "Query failed" case .timeout: return "Operation timed out" } } } correctly declares an enum conforming to Error with the required cases.

2. Step 2: Check computed property implementation

   The computed property message uses a switch on self to return descriptive strings for each case.

3. Final Answer:

   enum with computed property and switch -> Option C

4. Quick Check:

   Enum + Error + computed property with switch = enum DatabaseError: Error { case connectionFailed case queryFailed case timeout var message: String { switch self { case .connectionFailed: return "Connection failed" case .queryFailed: return "Query failed" case .timeout: return "Operation timed out" } } } [OK]

Hint: Use enum with switch in computed property for messages [OK]

Common Mistakes:

• Not conforming enum to Error
• Using struct instead of enum for error cases
• Ignoring switch on self in computed property