Designated init helps initializing all of properties in that class or structure.. (But not with cheese. You do not write the override modifier when overriding a required designated initializer: You do not have to provide an explicit implementation of a required initializer if you can satisfy the requirement with an inherited initializer. It must fully initialize all properties introduced by its class before calling a superclass initializer. However, because it’s called with a string constant, you can see that the initializer won’t fail, so no runtime error can occur in this case. The example below defines a structure called Chessboard, which models a board for the game of chess. This means that classes have additional responsibilities for ensuring that all stored properties they inherit are assigned a suitable value during initialization. Instead of making an explicit call to super.init(), this initializer relies on an implicit call to its superclass’s initializer to complete the process. ). The memberwise initializer is a shorthand way to initialize the member properties of new structure instances. A class does not require convenience initializers. This means that you cannot access any other property values from within your closure, even if those properties have default values. But a class has Inheritance, The main difference in their syntax, is that convenience initializers have the convenience keyword before the init.. The inherited version of init() functions in exactly the same way as the Food version, except that it delegates to the RecipeIngredient version of init(name: String) rather than the Food version. The type of the array is inferred to be [ShoppingListItem]. Superclass initializers are inherited in certain circumstances, but only when it is safe and appropriate to do so. The Size structure automatically receives an init(width:height:) memberwise initializer, which you can use to initialize a new Size instance: When you call a memberwise initializer, you can omit values for any properties that have default values. The Animal structure also defines a failable initializer with a single parameter called species. If your custom type has a stored property that is logically allowed to have “no value”—perhaps because its value cannot be set during initialization, or because it is allowed to have “no value” at some later point—declare the property with an optional type. Convenience initializers are secondary, supporting initializers for a class. This designated initializer matches a designated initializer from the superclass of Bicycle, and so the Bicycle version of this initializer is marked with the override modifier. These actions are described in the following sections. 2. ShoppingListItem also adds a computed description property, which provides a textual description of a ShoppingListItem instance: ShoppingListItem does not define an initializer to provide an initial value for purchased, because items in a shopping list (as modeled here) always start out unpurchased. 注意事项： 增加init的初始化方法; 必须调用Swift同一个类中的designated初始化完成设置; convenience的初始化方法不能被子类重写或者是从子类中以super的方式被调用 It’s simple: when you serialize an object, you convert it from a Swift-only data format to a stream (or “series”) of bytes. Here’s how phase 2 looks for the same initialization call: The superclass’s designated initializer now has an opportunity to customize the instance further (although it does not have to). Both initializers convert their single argument into the corresponding Celsius value and store this value in a property called temperatureInCelsius. Finally, once the subclass’s designated initializer is finished, the convenience initializer that was originally called can perform additional customization. Designated initializers for classes are written in the same way as simple initializers for value types: Convenience initializers are written in the same style, but with the convenience modifier placed before the init keyword, separated by a space: To simplify the relationships between designated and convenience initializers, Swift applies the following three rules for delegation calls between initializers: These rules are illustrated in the figure below: Here, the superclass has a single designated initializer and two convenience initializers. (rawValue:) initializer: A failable initializer of a class, structure, or enumeration can delegate across to another failable initializer from the same class, structure, or enumeration. What is the difference between convenience init vs init in swift, explicit examples better (4) . Designated initializers are “funnel” points through which initialization takes place, and through which the initialization process continues up the superclass chain. Once the initial state for every stored property has been determined, the second phase begins, and each class is given the opportunity to customize its stored properties further before the new instance is considered ready for use. Swift @objc public func addCardViewController ( _ addCardViewController : STPAddCardViewController , didCreatePaymentMethod paymentMethod : STPPaymentMethod , completion : @escaping STPErrorBlock ) Show on GitHub These initialization options are represented by three custom initializers that are part of the Rect structure’s definition: The first Rect initializer, init(), is functionally the same as the default initializer that the structure would have received if it did not have its own custom initializers. You cannot define a failable and a nonfailable initializer with the same parameter types and names. The figure below shows a more complex class hierarchy for four classes. (name:) initializer with a nonfailable init(name:) initializer. There are no further superclasses to initialize, and so no further delegation is needed. This continues up the class inheritance chain until the top of the chain is reached. Therefore, your subclass is not (strictly speaking) providing an override of the superclass initializer. Classes and structures must set all of their stored properties to an appropriate initial value by the time an instance of that class or structure is created. Chess is played on an 8 x 8 board, with alternating black and white squares. This example defines a class called ShoppingListItem, which encapsulates the name, quantity, and purchase state of an item in a shopping list: Because all properties of the ShoppingListItem class have default values, and because it is a base class with no superclass, ShoppingListItem automatically gains a default initializer implementation that creates a new instance with all of its properties set to their default values. Whenever a new instance of the type that the property belongs to is initialized, the closure or function is called, and its return value is assigned as the property’s default value. ", "This is not a defined temperature unit, so initialization failed. These are optional and can be used for a custom setup. Note that if you define a custom initializer for a value type, you will no longer have access to the default initializer (or the memberwise initializer, if it is a structure) for that type. ", // Prints "Unable to initialize zero shirts", "Unable to initialize one unnamed product", // Prints "Unable to initialize one unnamed product", // this initializer creates a document with a nil name value, // this initializer creates a document with a nonempty name value, // subclass implementation of the required initializer goes here, // create a default value for someProperty inside this closure, // someValue must be of the same type as SomeType, Setting Initial Values for Stored Properties, Initializer Parameters Without Argument Labels, Assigning Constant Properties During Initialization, Memberwise Initializers for Structure Types, Designated Initializers and Convenience Initializers, Syntax for Designated and Convenience Initializers, Designated and Convenience Initializers in Action, Failable Initializers for Enumerations with Raw Values, Setting a Default Property Value with a Closure or Function, Designated initializers must always delegate, Convenience initializers must always delegate. The memory for these stored properties is now initialized. Photo by Danielle MacInnes on Unsplash. Initializers are now able to access, Finally, any convenience initializers in the chain have the option to customize the instance and to work with. A failable initializer can also delegate to a nonfailable initializer. A convenience initializer is a secondary initializer that must call a designated initializer of the same class. Stored properties cannot be left in an indeterminate state. The Celsius structure implements two custom initializers called init(fromFahrenheit:) and init(fromKelvin:), which initialize a new instance of the structure with a value from a different temperature scale: The first initializer has a single initialization parameter with an argument label of fromFahrenheit and a parameter name of fahrenheit. The example above uses the default initializer for the ShoppingListItem class to create a new instance of the class with initializer syntax, written as ShoppingListItem(), and assigns this new instance to a variable called item. { guard let x = id else { return nil } self.init (id: x, desc: "Blah") } Unless there is a specific reason to do so, you can also avoid using a failable initializer in the first place. You write return nil within a failable initializer to indicate a point at which initialization failure can be triggered. Use this approach if you need to add a potential failure state to an existing initialization process that does not otherwise fail. init (id : Int, desc : String) compiles just fine. A designated initializer must call a designated initializer from its immediate superclass. The convenience initializer must call one of the two designated initializers, because it can only call another initializer from the same class. This convenience initializer simply delegates across to the class’s designated initializer, passing in a quantity value of 1. This ensures that the numberOfWheels inherited property is initialized by Vehicle before Bicycle has the opportunity to modify the property. Working back down from the top of the chain, each designated initializer in the chain has the option to customize the instance further. Initialization is the process of preparing an instance of a class, structure, or enumeration for use. It starts by calculating an appropriate origin point based on a center point and a size value. How to solve the problem: Solution 1: Standard init: Designated initializers are the primary initializers for a class. Swift classes supports having multiple ways of being initialized. In this example, the superclass for RecipeIngredient is Food, which has a single convenience initializer called init(). As a result, it automatically receives a default initializer, as described in Default Initializers. Alternatively, you can override a superclass failable initializer with a subclass nonfailable initializer. The following example defines a structure called Celsius, which stores temperatures expressed in degrees Celsius. Was it ever explained why the syntax is different? All three of these initializers can be used to create new RecipeIngredient instances: The third and final class in the hierarchy is a subclass of RecipeIngredient called ShoppingListItem. This failure might be triggered by invalid initialization parameter values, the absence of a required external resource, or some other condition that prevents initialization from succeeding. In practice, this means that you do not need to write initializer overrides in many common scenarios, and can inherit your superclass initializers with minimal effort whenever it is safe to do so. The default value also makes it easier for you to take advantage of default initializers and initializer inheritance, as described later in this chapter. instead of a question mark. The ShoppingListItem class models a recipe ingredient as it appears in a shopping list. If you create a CartItem instance with a nonempty name and a quantity of 1 or more, initialization succeeds: If you try to create a CartItem instance with a quantity value of 0, the CartItem initializer causes initialization to fail: Similarly, if you try to create a CartItem instance with an empty name value, the superclass Product initializer causes initialization to fail: You can override a superclass failable initializer in a subclass, just like any other initializer. Structure types automatically receive a memberwise initializer if they don’t define any of their own custom initializers. It then calls (or delegates) to the init(origin:size:) initializer, which stores the new origin and size values in the appropriate properties: The init(center:size:) initializer could have assigned the new values of origin and size to the appropriate properties itself. Each class is successfully initialized by calling its newly created convenience init method. You can omit either property or both properties, and the initializer uses the default value for anything you omit—for example: Initializers can call other initializers to perform part of an instance’s initialization. ` Ich habe es in dieser Antwort leicht angesprochen, in der ich einige der Initialisierungsregeln von Swift ausführlich beschreibe, aber das Hauptaugenmerk lag … Swift defines two kinds of initializers for class types to help ensure all stored properties receive an initial value. Structs can have delegating initializers but they cannot be marked with `convenience` (only the initializers of class types can). This satisfies rules 2 and 3 from above. Swift and the Swift logo are trademarks of Apple Inc. // Prints "The default temperature is 32.0° Fahrenheit", // boilingPointOfWater.temperatureInCelsius is 100.0, // freezingPointOfWater.temperatureInCelsius is 0.0, // this reports a compile-time error - argument labels are required, // bodyTemperature.temperatureInCelsius is 37.0, "I also like beets. The following example defines a structure called Color, with three constant properties called red, green, and blue. In order for this rule to be satisfied, a designated initializer must make sure that all of its own properties are initialized before it hands off up the chain. They are written in the same style, but with the convenience modifier placed before the init keyword Adopting NSCoding in Swift. It is useful when you want to provide default values or other custom setup. This convenience initializer assumes a quantity of 1 for any RecipeIngredient instance that is created without an explicit quantity. Conversely, if you write a subclass initializer that matches a superclass convenience initializer, that superclass convenience initializer can never be called directly by your subclass, as per the rules described above in Initializer Delegation for Class Types. The boardColors array is initialized with a closure to set up its color values: Whenever a new Chessboard instance is created, the closure is executed, and the default value of boardColors is calculated and returned. 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