Alternatively, you can override a superclass failable initializer with a subclass nonfailable initializer. Otherwise, the species property’s value is set, and initialization succeeds: You can use this failable initializer to try to initialize a new Animal instance and to check if initialization succeeded: If you pass an empty string value to the failable initializer’s species parameter, the initializer triggers an initialization failure: Checking for an empty string value (such as "" rather than "Giraffe") is not the same as checking for nil to indicate the absence of an optional String value. This ensures that the numberOfWheels inherited property is initialized by Vehicle before Bicycle has the opportunity to modify the property. Was it ever explained why the syntax is different? ", "This is not a defined temperature unit, so initialization failed. RecipeIngredient also defines a convenience initializer, init(name: String), which is used to create a RecipeIngredient instance by name alone. As a result, it automatically receives a default initializer, as described in Default Initializers. They are written in the same style, but with the convenience modifier placed before the init keyword Initializers are called to create a new instance of a particular type. Swift classes supports having multiple ways of being initialized. Note that if you override a failable superclass initializer with a nonfailable subclass initializer, the only way to delegate up to the superclass initializer is to force-unwrap the result of the failable superclass initializer. Designated init is default initatializer in swift.. 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. These kinds of closures or functions typically create a temporary value of the same type as the property, tailor that value to represent the desired initial state, and then return that temporary value to be used as the property’s default value. 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. The type of the array is inferred to be [ShoppingListItem]. If you want a custom subclass to present one or more of the same initializers as its superclass, you can provide a custom implementation of those initializers within the subclass. This initializer can be used to create a new Food instance with a specific name: The init(name: String) initializer from the Food class is provided as a designated initializer, because it ensures that all stored properties of a new Food instance are fully initialized. This convenience initializer assumes a quantity of 1 for any RecipeIngredient instance that is created without an explicit quantity. The figure below shows the overall initializer chain for all three classes: You can use all three of the inherited initializers to create a new ShoppingListItem instance: Here, a new array called breakfastList is created from an array literal containing three new ShoppingListItem instances. Argument labels must always be used in an initializer if they are defined, and omitting them is a compile-time error: If you do not want to use an argument label for an initializer parameter, write an underscore (_) instead of an explicit argument label for that parameter to override the default behavior. The class instance is not fully valid until the first phase ends. This means that classes have additional responsibilities for ensuring that all stored properties they inherit are assigned a suitable value during initialization. Swift’s approach prevents a situation in which a simple initializer from a superclass is inherited by a more specialized subclass and is used to create a new instance of the subclass that is not fully or correctly initialized. 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. If you use a closure to initialize a property, remember that the rest of the instance has not yet been initialized at the point that the closure is executed. Initialization parameters have the same capabilities and syntax as function and method parameters. One convenience initializer calls another convenience initializer, which in turn calls the single designated initializer. In either case, if you delegate to another initializer that causes initialization to fail, the entire initialization process fails immediately, and no further initialization code is executed. It’s simple: when you serialize an object, you convert it from a Swift-only data format to a stream (or “series”) of bytes. If the quantity is invalid, the entire initialization process fails immediately and no further initialization code is executed. Initializers prepare a class, struct or enum by setting stored properties before they are used. Because it provides a default value for all of the properties it introduces and does not define any initializers itself, ShoppingListItem automatically inherits all of the designated and convenience initializers from its superclass. To simplify the relationships between designated and convenience initializers, Swift applies the following three rules for delegation calls between initializers: 1. ). The Food class does not have a superclass, and so the init(name: String) initializer does not need to call super.init() to complete its initialization. A convenience initializer must call another initializer from the same class. The ultimate guide how to init your Swift data types, with the help of designated, convenience, failable intitializers and more. Before we can save and load data with NSKeyedArchiver, our Book class needs to adopt the NSCoding protocol. Because this convenience initializer overrides a designated initializer from its superclass, it must be marked with the override modifier (as described in Initializer Inheritance and Overriding). It introduces an Int property called quantity (in addition to the name property it inherits from Food) and defines two initializers for creating RecipeIngredient instances: The figure below shows the initializer chain for the RecipeIngredient class: The RecipeIngredient class has a single designated initializer, init(name: String, quantity: Int), which can be used to populate all of the properties of a new RecipeIngredient instance. 使用convenience关键字增加init方法. and vice versa. For more information, see Automatic Initializer Inheritance below. Alternatively, specify a default property value as part of the property’s declaration. 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. Instances of class types can also implement a deinitializer, which performs any custom cleanup just before an instance of that class is deallocated. The init(name: String) convenience initializer provided by RecipeIngredient takes the same parameters as the init(name: String) designated initializer from Food. The ShoppingListItem class models a recipe ingredient as it appears in a shopping list. You write a failable initializer by placing a question mark after the init keyword (init?). Any initializer in the diagram above can be used to create a fully initialized instance of the class they belong to. You can define a convenience initializer to call a designated initializer from the same class as the convenience initializer with some of the designated initializer’s parameters set to default values. You can use forced unwrapping in an initializer to call a failable initializer from the superclass as part of the implementation of a subclass’s nonfailable initializer. This process satisfies safety check 1 from Two-Phase Initialization above. In its simplest form, an initializer is like an instance method with no parameters, written using the init keyword: The example below defines a new structure called Fahrenheit to store temperatures expressed in the Fahrenheit scale. (Die Programmiersprache Swift -> Sprachführer -> Initialisierung) Damit es funktioniert: init (nm: String, ad: String) {numberPriorVisits = 0 super. 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. 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