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Advanced Error Handling

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In this section, we will delve deeper into error handling in Swift. While basic error handling covers the essentials, advanced error handling techniques are crucial for building robust and resilient applications. We will cover custom error types, error propagation, and advanced techniques like rethrowing and converting errors.

Key Concepts

  1. Custom Error Types
  2. Error Propagation
  3. Rethrowing Functions
  4. Converting Errors

Custom Error Types

Swift allows you to define your own error types to provide more context and specificity when errors occur.

Defining Custom Error Types

You can define a custom error type by conforming to the Error protocol. Typically, you use an enum to represent different error cases.

enum NetworkError: Error {
    case badURL
    case requestFailed
    case unknown
}

Throwing Custom Errors

You can throw custom errors just like built-in errors.

func fetchData(from url: String) throws {
    guard url.starts(with: "http") else {
        throw NetworkError.badURL
    }
    // Simulate a request failure
    throw NetworkError.requestFailed
}

Handling Custom Errors

When handling custom errors, you can use a switch statement to handle different error cases.

do {
    try fetchData(from: "invalid-url")
} catch NetworkError.badURL {
    print("Invalid URL.")
} catch NetworkError.requestFailed {
    print("Request failed.")
} catch {
    print("An unknown error occurred.")
}

Error Propagation

Error propagation allows you to pass errors up the call stack to be handled at a higher level.

Propagating Errors

You can propagate errors by marking a function with the throws keyword.

func performNetworkRequest() throws {
    try fetchData(from: "http://example.com")
}

Handling Propagated Errors

You handle propagated errors in the same way as directly thrown errors.

do {
    try performNetworkRequest()
} catch {
    print("An error occurred: \(error)")
}

Rethrowing Functions

Rethrowing functions are functions that can throw errors, but only if one of their parameters throws an error.

Defining Rethrowing Functions

You use the rethrows keyword to define a rethrowing function.

func performOperation(_ operation: () throws -> Void) rethrows {
    try operation()
}

Using Rethrowing Functions

You can pass a throwing function to a rethrowing function.

func riskyOperation() throws {
    throw NetworkError.unknown
}

do {
    try performOperation(riskyOperation)
} catch {
    print("An error occurred: \(error)")
}

Converting Errors

Sometimes, you may need to convert one type of error to another. This is useful when you want to provide a consistent error interface.

Converting Errors Example

You can catch an error and throw a different error.

enum DataError: Error {
    case dataCorrupted
}

func loadData() throws {
    do {
        try fetchData(from: "http://example.com")
    } catch {
        throw DataError.dataCorrupted
    }
}

do {
    try loadData()
} catch DataError.dataCorrupted {
    print("Data is corrupted.")
} catch {
    print("An unknown error occurred.")
}

Practical Exercises

Exercise 1: Custom Error Types

Task: Define a custom error type for a file handling system and implement a function that throws these errors.

enum FileError: Error {
    case fileNotFound
    case unreadable
    case unknown
}

func readFile(at path: String) throws {
    guard path == "valid-path" else {
        throw FileError.fileNotFound
    }
    // Simulate an unreadable file
    throw FileError.unreadable
}

do {
    try readFile(at: "invalid-path")
} catch FileError.fileNotFound {
    print("File not found.")
} catch FileError.unreadable {
    print("File is unreadable.")
} catch {
    print("An unknown error occurred.")
}

Exercise 2: Error Propagation

Task: Implement a function that propagates errors from another function.

func processFile(at path: String) throws {
    try readFile(at: path)
}

do {
    try processFile(at: "valid-path")
} catch {
    print("An error occurred: \(error)")
}

Exercise 3: Rethrowing Functions

Task: Implement a rethrowing function that takes a closure and handles its errors.

func executeOperation(_ operation: () throws -> Void) rethrows {
    try operation()
}

do {
    try executeOperation {
        throw FileError.unknown
    }
} catch {
    print("An error occurred: \(error)")
}

Summary

In this section, we covered advanced error handling techniques in Swift, including custom error types, error propagation, rethrowing functions, and converting errors. These techniques are essential for building robust applications that can handle errors gracefully and provide meaningful feedback to users.

Next, we will explore memory management in Swift, which is crucial for optimizing the performance and efficiency of your applications.

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