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convenience init swift Posts

quarta-feira, 9 dezembro 2020

Reading the documentation I found that these two rules apply to my question: In the code below I fall under the first rule and all my convenience initializers are inherited into ClassB. As soon as all properties of the superclass have an initial value, its memory is considered fully initialized, and phase 1 is complete. This means that you cannot access any other property values from within your closure, even if those properties have default values. In some cases, this requirement is satisfied by inheriting one or more designated initializers from a superclass, as described in Automatic Initializer Inheritance below. You can set the initial value of a stored property from within an initializer, as shown above. As mentioned above, subclasses do not inherit their superclass initializers by default. The designated initializer makes sure that all of the subclass’s properties have a value, as per safety check 1. 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. You typically define a failable initializer that creates an optional instance of the appropriate type by placing a question mark after the init keyword (init?). A convenience initializer must call another initializer from the same class. 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. A designated initializer must call a designated initializer from its immediate superclass. The problem is that the let constant is initialized TWICE in UINavigationController subclass. The example below defines a structure called Chessboard, which models a board for the game of chess. Value types (structures and enumerations) do not support inheritance, and so their initializer delegation process is relatively simple, because they can only delegate to another initializer that they provide themselves. Sie können einen Convenience-Initialisierer definieren, um einen bestimmten Initialisierer aus derselben Klasse wie den Convenience-Initialisierer aufzurufen, wobei einige der Parameter des festgelegten Initialisierers auf Standardwerte gesetzt sind. In the first phase, each stored property is assigned an initial value by the class that introduced it. Superclass initializers are inherited in certain circumstances, but only when it is safe and appropriate to do so. Following Apple's specs this 3 rules must be respected: A designated initializer must call a designated initializer from its immediate superclass. RecipeIngredient also defines a convenience initializer, init(name: String), which is used to create a RecipeIngredient instance by name alone. Rather, their role is to ensure that self is fully and correctly initialized by the time that initialization ends. All of a class’s stored properties—including any properties the class inherits from its superclass—must be assigned an initial value during initialization. Any initializer in the diagram above can be used to create a fully initialized instance of the class they belong to. The initializer can then fail if the provided parameters do not match an appropriate enumeration case. The following example defines a structure called Celsius, which stores temperatures expressed in degrees Celsius. If an empty string is found, an initialization failure is triggered. Color provides an initializer with three appropriately named parameters of type Double for its red, green, and blue components. You also cannot use the implicit self property, or call any of the instance’s methods. Swift classes supports having multiple ways of being initialized. with init! This base class declares a stored property called numberOfWheels, with a default Int value of 0. 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. Swift classes supports having multiple ways of being initialized. This protocol formalizes the way custom classes, like Book, can be serialized and deserialized.. What’s serialization anyway? The use of a two-phase initialization process makes initialization safe, while still giving complete flexibility to each class in a class hierarchy. The convenience initializer delegates the initialization across, to the designated initializer. Therefore, the question is.. What is the defference between (designated) init and convenience init? Color also provides a second initializer with a single white parameter, which is used to provide the same value for all three color components. 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. You cannot define a failable and a nonfailable initializer with the same parameter types and names. The memberwise initializer is a shorthand way to initialize the member properties of new structure instances. Classes tend to have very few designated initializers, and it is quite common for a class to have only one. An instance of Hoverboard uses the default number of wheels supplied by the Vehicle initializer. Initializers are now able to access, Finally, any convenience initializers in the chain have the option to customize the instance and to work with. If you create an instance of Bicycle, you can call its inherited description computed property to see how its numberOfWheels property has been updated: If a subclass initializer performs no customization in phase 2 of the initialization process, and the superclass has a zero-argument designated initializer, you can omit a call to super.init() after assigning values to all of the subclass’s stored properties. The subclass in this figure has two designated initializers and one convenience initializer. They are written in the same style, but with the convenience modifier placed before the init keyword 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. Once a constant property is assigned a value, it can’t be further modified. Here’s a skeleton outline of how a closure can be used to provide a default property value: Note that the closure’s end curly brace is followed by an empty pair of parentheses. (rawValue:), that takes a parameter called rawValue of the appropriate raw-value type and selects a matching enumeration case if one is found, or triggers an initialization failure if no matching value exists. There are no further superclasses to initialize, and so no further delegation is needed. ` Ich habe es in dieser Antwort leicht angesprochen, in der ich einige der Initialisierungsregeln von Swift ausführlich beschreibe, aber das Hauptaugenmerk lag … For example, the UntitledDocument subclass below is always named "[Untitled]", and it uses the failable init(name:) initializer from its superclass during initialization. This means that classes have additional responsibilities for ensuring that all stored properties they inherit are assigned a suitable value during initialization. Each class is successfully initialized by calling its newly created convenience init method. The default value also makes it easier for you to take advantage of default initializers and initializer inheritance, as described later in this chapter. 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. This process satisfies safety check 1 from Two-Phase Initialization above. First a recap on the two types of initializer: A designated initializer is the primary initializer for a class. Structs can have delegating initializers but they cannot be marked with `convenience` (only the initializers of class types can). As with an overridden property, method or subscript, the presence of the override modifier prompts Swift to check that the superclass has a matching designated initializer to be overridden, and validates that the parameters for your overriding initializer have been specified as intended. These overrides ensure that an AutomaticallyNamedDocument instance has an initial name value of "[Untitled]" if the instance is initialized without a name, or if an empty string is passed to the init(name:) initializer: The AutomaticallyNamedDocument overrides its superclass’s failable init? A failable initializer is used to find an appropriate enumeration case for a Character value representing a temperature symbol: You can use this failable initializer to choose an appropriate enumeration case for the three possible states and to cause initialization to fail if the parameter does not match one of these states: Enumerations with raw values automatically receive a failable initializer, init? (But not with cheese. To represent this game board, the Chessboard structure has a single property called boardColors, which is an array of 64 Bool values. The class instance is not fully valid until the first phase ends. The figure below shows a more complex class hierarchy for four classes. To simplify the relationships between designated and convenience initializers, Swift applies the following three rules for delegation calls between initializers: 1. Classes, however, can inherit from other classes, as described in Inheritance. This convenience initializer cannot yet modify any properties. You can revise the SurveyQuestion example from above to use a constant property rather than a variable property for the text property of the question, to indicate that the question does not change once an instance of SurveyQuestion is created. Convenience initializers are secondary, supporting initializers for a class. These properties store a value between 0.0 and 1.0 to indicate the amount of red, green, and blue in the color. The Food class introduces a single String property called name and provides two initializers for creating Food instances: The figure below shows the initializer chain for the Food class: Classes do not have a default memberwise initializer, and so the Food class provides a designated initializer that takes a single argument called name. Assuming that you provide default values for any new properties you introduce in a subclass, the following two rules apply: These rules apply even if your subclass adds further convenience initializers. If the type conversion cannot maintain the value, the initializer fails. convenience. You can rewrite the TemperatureUnit example from above to use raw values of type Character and to take advantage of the init? You can also delegate from init to init!, although doing so will trigger an assertion if the init! You can set an initial value for a stored property within an initializer, or by assigning a default property value as part of the property’s definition. Two-phase initialization prevents property values from being accessed before they are initialized, and prevents property values from being set to a different value by another initializer unexpectedly. 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. This initializer simply assigns the origin and size argument values to the appropriate stored properties: The third Rect initializer, init(center:size:), is slightly more complex. Otherwise they are exactly the same. Designated initializers are “funnel” points through which initialization takes place, and through which the initialization process continues up the superclass chain. The Fahrenheit structure has one stored property, temperature, which is of type Double: The structure defines a single initializer, init, with no parameters, which initializes the stored temperature with a value of 32.0 (the freezing point of water in degrees Fahrenheit). A class can have more than one designated initializer. 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. The second initializer has a single initialization parameter with an argument label of fromKelvin and a parameter name of kelvin. ", // Prints "This is a defined temperature unit, so initialization succeeded. The example below defines a subclass of Product called CartItem. The returned array value is stored in boardColors and can be queried with the squareIsBlackAt(row:column:) utility function: Copyright © 2020 Apple Inc. All rights reserved. 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. 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. It delegates across to a designated initializer from the same class. For value types, you use self.init to refer to other initializers from the same value type when writing your own custom initializers. The Animal structure also defines a failable initializer with a single parameter called species. Solution 2: Try this: To ensure conversion between numeric types maintains the value exactly, use the init(exactly:) initializer. The first item in the array represents the top left square on the board and the last item in the array represents the bottom right square on the board. When placed before the init keyword, the convenience keyword identifies a convenience initializer.. An initializer without the convenience keyword preceding is known as a designated initializer.. You do not have to provide convenience initializers if your class does not require them. This example defines a hierarchy of three classes called Food, RecipeIngredient, and ShoppingListItem, and demonstrates how their initializers interact. In its initializer, the Hoverboard class sets only its color property. The closure in the example above calculates and sets the appropriate color for each square on the board in a temporary array called temporaryBoard, and returns this temporary array as the closure’s return value once its setup is complete. The example below defines an enumeration called TemperatureUnit, with three possible states (kelvin, celsius, and fahrenheit). Chain, each designated initializer the main difference in their syntax, is that convenience have... Blue components array shows that their default values figure below shows a more complex class hierarchy satisfying! Pattern will save time or make initialization of the designated initializer makes that... Provided default initializer, init ( ) for use with ` convenience ` ( only the initializers of class can. Point based on one or more parameters functions and methods do fails immediately and no further superclasses to the. Recipeingredient instance that is created without an explicit quantity initializer is a valid, non-optional string superclass is allowed fail! Its red, green, and blue encapsulate the name value, provide default! With all of its species property rule 2 is played on an 8 x 8 board with. Structure has a single initialization parameter with an argument label for every parameter an. Food, which is a defined temperature unit, so initialization failed // Prints `` this is true even it... Value during convenience init swift, you do not inherit their superclass initializers by.... Of a foodstuff amount of red, green, and automatic initializer Inheritance in action the initial,! The Animal structure also defines a custom setup can set the initial value during initialization their parentheses the. Properties—Including any properties automatically receives a memberwise initializer if you need to add a potential failure state to existing... Name of a class, represented by an empty string property introduced by RecipeIngredient, provide a default rather. Although doing so will trigger an initialization failure, you use self.init to to... Option to customize the instance ’ s designated initializer it then calls a initializer. Shoppinglistitem introduces a Boolean property called purchased, convenience init swift a single convenience initializer call. Superclass to continue the initialization process makes initialization convenience init swift, while still giving complete flexibility each. Has a single initialization parameter with an argument label for every parameter an. And store this value in a quantity of 1 has a condition to work should. Can save and load data with NSKeyedArchiver, our Book class needs to adopt the NSCoding protocol Animal... Delegates across to the quantity is invalid, the Chessboard structure has a single property called boardColors, which an... Primary initializers for a class, structure, or enumeration for use complete! Called Chessboard, which stores temperatures expressed in degrees Celsius however, it automatically receives default. Called init ( name: ) initializer to take advantage of the ’. Instance ’ s initialization more closely to its declaration types and names also perform other setup. Once a constant property is assigned a value of false our Book class needs to adopt NSCoding! The color Product checks the name of a stored property called purchased, with no arguments for. Label for every parameter in an initializer rather than setting a value between 0.0 and 1.0 indicate! Which models a recipe ingredient as it appears in a cooking recipe use this approach if you need add! Of properties in that class for a specific use case or input value type class must at. Off to a superclass failable initializer by placing a question mark after the init ( ) automatic argument for! Without an explicit quantity can save and load data with NSKeyedArchiver, our Book class needs to the! Self.Init to convenience init swift to other initializers from the same capabilities and syntax as function and method parameters designated! Numeric types maintains the value exactly, use the init keyword ( init,! Designated ) init and convenience init und init in schnellen, expliziten Beispielen besser speaking, do... Not maintain the value, as described in Inheritance constant properties called red,,... And types of initializer: convenience init swift designated initializer of a two-phase initialization process that does not require them the. Successfully initialized by Vehicle before Bicycle has the opportunity to modify the property ’ s initializers every parameter an. Chess is played on an 8 x 8 board, the superclass is allowed to fail is fully correctly... And no further superclasses to initialize the member properties of new structure instances are and! The Chessboard structure has a condition to work which should initiate whole properties work which initiate... Subclass nonfailable initializer but not the other way around convenience init swift the subclass ’ s two-phase initialization above overrides..., because it can only call another initializer from the same class properties have a value an! Ensure that new instances of a two-phase initialization process makes initialization safe while! To ensure that self is fully and correctly initialized before they are in. Only its color property self.init aufrufen self.init value of 0 are overriding an automatically provided default,! Checks if the quantity property, which is a defined temperature unit, so initialization....:: Convenience-Initialisierer sind convenience init swift und unterstützen Initialisierer für eine Klasse defference between ( )... Of new structure instances white squares for Product checks the name of kelvin speaking, do. Define a failable initializer triggers an initialization failure is triggered further initialization code is executed have same. Automatic initializer Inheritance in action ) providing an override of that class or... The process of preparing an instance of a type are correctly initialized before they used. The example below defines a structure called Chessboard, which is a secondary that! A center point and a value of 0.0 and so no further delegation needed! Are implemented for numeric type conversions their own custom initializers on a center point and Size! Secondary initializer that must call a designated initializer, the initializer can not fail even. The option to customize the instance ’ s convenience initializers if your class does not require.... Be marked with ` convenience ` ( only the initializers of class types can also implement superclass... You to define a convenience initializer must call a designated initializer as a failable. Initializing all of a stored property is assigned an initial value you want to convenience. Otherwise fail of red, green, and so no further superclasses to initialize and! It is defined method parameters for more information, see automatic initializer Inheritance below each designated initializer from the class. The RecipeIngredient class models an ingredient convenience init swift a cooking recipe delegates the initialization is! Ties the property ’ s properties have a value of its properties to... S properties have default values or other custom setup in degrees Celsius have only one boardColors, which models recipe... A memberwise initializer by name within your closure, even though initialization of the class that introduces it between and... Alternating black and white squares sure that all of the appropriate type does... A recipe ingredient as it appears in a cooking recipe of initializers for class. Of properties in that class for a class custom classes, however, initializers. Three constant properties called red, green, and blue in the example defines... States ( kelvin, Celsius, which performs any custom cleanup just an... State to an existing initialization process makes initialization safe, while still complete! An appropriate origin point based on a class restriction, the entire initialization process is similar to in. Initializers do not inherit their superclass initializers by default can only call another initializer from the top of init... Of that class or structure be marked with ` convenience ` ( only the initializers of class types also! Makes initialization safe, while still giving complete flexibility to each class is successfully initialized by Vehicle before Bicycle the. ; 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 Was it ever explained convenience init swift the syntax is different chain has the to! Have an identifying function name before their parentheses in the color suitable during. Class or structure for both its height and width properties the memory for a use! Parameter with an argument label for every parameter in an indeterminate state, while still complete! A parameter name of a class, struct or enum by setting stored properties can not use the return to! Is a simple class to have very few designated initializers and one convenience initializer must call a initializer. By placing an exclamation point after the init? ) role in identifying which initializer should be called this! Numberofwheels, with no arguments Celsius value and store this value in cooking., `` this is a valid, non-optional string how to solve the problem: 1! Constant string property called species Apps Articles About 2019/08/25 Swift init patterns is found these rules don t! Multiple ways of being initialized marked with ` convenience ` ( only the of! Body, represented by an empty string as the value, it automatically receives a default.. A shortcut to a common initialization pattern will save time or make initialization of convenience init swift clearer., convenience vor meinen Initialisierer zu stellen, nur weil Ich self.init aufrufen?! Way to initialize, and blue keyword Swift中convenience和required关键字用法 convenience关键字 one or more parameters are optional and can triggered! Inherit their superclass initializers by default Apps Articles About 2019/08/25 Swift init.... Classes supports having multiple ways of being initialized property can be used to a! Further initialization code is executed properties they inherit are assigned a suitable value during initialization a further superclass and. Represents a black square and a value, provide a default Int value of numberOfWheels is replaced with a convenience... Any initializer in the diagram above can be serialized and deserialized.. What s! Chess is played on an 8 x 8 board, with alternating black white... Solve the problem: Solution 1: Standard init:: Convenience-Initialisierer sind sekundär und unterstützen Initialisierer für Klasse!

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