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Swift 2
Error Handling in Practice
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cockscomb
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No content
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Recent work
• Rewrite Hatena Blog for iOS with Swift
• Hatena Summer Internship 2015 “iOS App Course”
• Sample code: hatena/swift-Sample-GitHubSearch
• Textbook: Available soon
• Secret projects‥
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I moved our codes
into Swift 2 the past week
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Swift 2
Error Handling in Practice
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Which is better?
func parseInt(string: String) -> Int?
func parseInt(string: String) throws -> Int
func parseInt(string: String) throws -> Int?
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Which is better?
func parseInt(string: String) -> Int?
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Which is better?
func parseJSON(data: NSData) -> JSONObject?
func parseJSON(data: NSData) throws -> JSONObject
func parseJSON(data: NSData) throws -> JSONObject?
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Which is better?
func parseJSON(data: NSData) throws -> JSONObject
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func parseJSON(data: NSData) throws -> JSONObject
do {
let json = try parseJSON(data)
// Something
} catch {
// Handle error
}
if let json = try? parseJSON(data) {
// Something
}
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enum JSONDecodeError: ErrorType {
case MissingRequiredKey(String)
case UnexpectedType(key: String,
expected: Any.Type, actual: Any.Type)
}
struct JSONObject {
let raw: [String : AnyObject]
func getValue(key: String) throws -> T {
guard let value = raw[key] else {
throw JSONDecodeError.MissingRequiredKey(key)
}
guard let typedValue = value as? T else {
throw JSONDecodeError.UnexpectedType(
key: key,
expected: T.self,
actual: value.dynamicType)
}
return typedValue
}
}
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do {
let json = try parseJSON(data)
let userName: String = try json.getValue("user_name")
// Something
} catch {
// Handle error
}
if let json = try? parseJSON(data),
let userName: String = try? json.getValue("user_name") {
// Something
}
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Which is better?
enum JSONDecodeError: ErrorType {
case MissingRequiredKey(String)
case UnexpectedType(key: String,
expected: Any.Type,
actual: Any.Type)
}
enum JSONDecodeError: ErrorType {
case MissingRequiredKey
case UnexpectedType
}
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Which is better?
enum JSONDecodeError: ErrorType {
case MissingRequiredKey(String)
case UnexpectedType(key: String,
expected: Any.Type,
actual: Any.Type)
}
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Do more better
enum JSONDecodeError: ErrorType,
CustomDebugStringConvertible {
case MissingRequiredKey(String)
case UnexpectedType(key: String,
expected: Any.Type, actual: Any.Type)
var debugDescription: String {
switch self {
case .MissingRequiredKey(let key):
return "Required key '\(key)' missing"
case let .UnexpectedType(key: key,
expected: expected, actual: actual):
return "Unexpected type '\(actual)' was
supplied for '\(key): \(expected)'"
}
}
}
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Do more better
struct JSONObject {
let raw: [NSString : AnyObject]
/**
Get typed value for the key
- parameters:
- key: JSON key
- returns: Typed value
*/
func getValue(key: String) throws -> T {
guard let value = raw[key] else {
throw JSONDecodeError.MissingRequiredKey(key)
}
...
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Do more better
struct JSONObject {
let raw: [NSString : AnyObject]
/**
Get typed value for the key
- parameters:
- key: JSON key
- returns: Typed value
- throws: JSONDecodeError
*/
func getValue(key: String) throws -> T {
guard let value = raw[key] else {
throw JSONDecodeError.MissingRequiredKey(key)
}
...
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as NSError
enum MyError: ErrorType {
case SuperError
}
let error = MyError.SuperError as NSError
print(error.localizedDescription)
// The operation couldn’t be completed.
(Module.MyError error 0.)
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enum MyError: ErrorType {
case SuperError
static func description(code: Int) -> String? {
switch code {
case 0: return "Super error occurred"
default: return nil
}
}
}
NSError.setUserInfoValueProviderForDomain("Module.MyError")
{ (error, userInfoKey) -> AnyObject? in
switch userInfoKey {
case NSLocalizedDescriptionKey:
return MyError.description(error.code)
default: break
}
return nil
}
let error = MyError.SuperError as NSError
print(error.localizedDescription) // Super error occurred
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Error handling in practice
• Functions can return Optional type or throw error
• ErrorType should provide informations in detail
• Documentation comment with possible error kind
• Treating ErrorType as NSError have some problems