Swift makes it easy to create arrays in your code using an array literal: simply surround a comma-separated list of values with square brackets. Without any other information, Swift creates an array that includes the specified values, automatically inferring the array’s Element type. Python的创始人为吉多范罗苏姆，当时他在阿姆斯特丹的荷兰数学和计算机科学研究学会工作。 1989年的圣诞节期间，吉多范罗苏姆为了在打发时间，决心开发一个新的脚本解释程式，作为ABC语言的一种继承，替代使用Unix shell和C语言进行系统管理，担负同 Amoeba操作系统 （ 英语 ： Amoeba (operating. Bridging Between Set and NSSet. You can bridge between Set and NSSet using the as operator. For bridging to be possible, the Element type of a set must be a class, an @objc protocol (a protocol imported from Objective-C or marked with the @objc attribute), or a type that bridges to a Foundation type. Bridging from Set to NSSet always takes O(1) time and space. A color literal allows you to select colors using a color picker directly from your code. You can either select a color or type in the HEX code. You can also set the opacity. Note: Color literals are great for static colors. For colors that adapt to dark mode, I suggest using colors in the Assets Catalog.

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Object literals are very useful and they often make your code easier to read and understand. String literals are a bit different, though. It is true that a string literal is the easiest solution to create a String object. It is straightforward and everyone understands what is going on. But there is a price you pay every time you use a string literal. Did you know that?

Common Examples

Let me show you a few common examples of string literals as they are used in Swift and Cocoa development.

I bet these examples look familiar. In each example, we use one or more string literals to carry out a task. What is wrong with these examples?

While there is nothing inherently wrong with the use of string literals, there is room for improvement. String literals very often lead to stringly typed code, a play on strongly typed.

What Is Stringly Typed Code

When strings, and very often string literals, are used instead of a more appropriate type, we speak of stringly typed code. Some forms of stringly typing are obvious while others are more subtle. This is an obvious example of stringly typed code.

When working with coordinates, there is a better solution than using strings for the location's latitude and longitude. This is a better implementation that adds safety and clarity.

We can take it one step further by asking the Core Location framework for help.

Another obvious example is the use of a String object instead of a URL object. There is a reason why Apple pushes APIs that accept a URL object instead of a path, that is, a String object.

Subtle Forms of Stringly Typed Code

There are many subtle forms of stringly typed code that are often overlooked. These forms are easy to avoid with a tiny amount of refactoring. Let me show you how I get rid of the string literals in the examples I showed you earlier.

Images

Whenever I use an API that accepts a String object, I am on my guard. Why? It is an opportunity for a string literal to make its way into my codebase. This is a very common example.

This is how the majority of developers create UIImage instances. The UIImage class explicitly asks for a String instance. What is wrong with this and how can we improve this example?

Let me ask you a few questions first. What happens if the asset's name changes? And what happens if you misspell the asset's name? Because init(named:) returns an optional UIImage instance, the compiler won't warn you if there isn't an asset with the name you passed to init(named:). How can we solve this problem?

There are several solutions. I usually create an extension for UIImage that looks something like this.

Does this look overly complex to you? I can understand that it may look too complex of a solution for the problem. Bear with me, though. This solution makes clever use of enumerations to create namespaces. It is a pattern I use very, very often and it is one of the patterns I swear by.

The resulting API looks and feels elegant and intuitive. Don't you agree? This is what it looks like at the call site.

Notice that I use an exclamation mark in the UIImage extension to forced unwrap the result of init(named:). As I write elsewhere, I rarely use Swift's exclamation mark. One exception is the loading of assets and resources from the application's bundle. Such an operation should never fail. If it fails, then I have other things to worry about and the application crashing due to a runtime error is the least of my worries.

You may be wondering if instantiating a UIImage instance in an extension is a wise thing to do. Don't worry, though. Static properties like this are instantiated lazily. That is obviously a good thing since we want to load an asset the moment it is needed. This improves performance and reduces memory consumption.

If you are worried about memory consumption, then you can turn the static constant property into a static computed property.

Storyboards

I use a similar solution for loading storyboards and instantiating the view controllers they contain. I create an extension for UIStoryboard and define a static constant property.

It is important to realize that the storyboard is kept in memory for the lifetime of the application once it is instantiated. If that isn't what you want, then you can turn the static constant property into a static computed property.

A new instance of the storyboard is created every time the main computed property is accessed. You can use a similar strategy for the view controllers of a storyboard. Take a look at the following example and notice the use of the exclamation mark.

An added benefit is that the pattern makes the instantiation of the OnboardingViewController class more concise at the call site.

This is much better than the alternative, especially if you need to instantiate the view controller in multiple locations.

Localization

You can use a similar strategy for localization. It is unfortunate that Apple doesn't add support for this in Xcode. I would be nice to take advantage of autocompletion when localizing an application.

Segues

The last example that is worth mentioning is the use of segues. The approach I am currently using is simple yet effective. I define a private enum, Segue, in the view controller. For every segue of the view controller, I define a static constant property. This means I no longer need to deal with string literals or constant stored properties.

What Do We Gain

The Xcode theme I use highlights string literals in bright red. It shows me an overview of the string literals I am using and, when I see too much red, it is time for some refactoring.

But you may be wondering why this is necessary. I hope I have convinced you that the use of string literals can break your application without the compiler warning you about it. That is the inherent risk of string literals and stringly typed code.

The solutions I listed come with a few key benefits. You can take advantage of Xcode's autocompletion and, more important, the compiler helps you find common bugs that are the result of typos. Refactoring also becomes easier and less prone to errors. You don't need to rely on search and replace to update the project.

Automation

Even if you use the solutions I listed, you still need to be careful. For example, you need to make sure that the identifier of a segue matches the one you use in your codebase and that is where automation can prove very useful.

There are several libraries and frameworks that automatically generate the extensions I showed you. The code generation usually takes place at compile time. This means that you eliminate typos, benefit from code completion, and, when you make a change, it is automatically propagated.

I don't use any of these solutions, but I have to admit that it is tempting if you care about safety and consistency. To be honest, I believe it is Xcode's responsibility to handle such tasks. Many popular IDEs do.

Loading an image from an asset catalog, for example, should be easier and less prone to errors. It shouldn't involve string literals if the image is included in the application bundle. It could look something like this.

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A type that can be initialized using an array literal.

An array literal is a simple way of expressing a list of values. Simply surround a comma-separated list of values, instances, or literals with square brackets to create an array literal. You can use an array literal anywhere an instance of an ExpressibleByArrayLiteral type is expected: as a value assigned to a variable or constant, as a parameter to a method or initializer, or even as the subject of a nonmutating operation like map(_:) or filter(_:).

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Arrays, sets, and option sets all conform to ExpressibleByArrayLiteral, and your own custom types can as well. Here's an example of creating a set and an array using array literals:

The Set and Array types each handle array literals in their own way to create new instances. In this case, the newly created set drops the duplicate value ('Dave') and doesn't maintain the order of the array literal's elements. The new array, on the other hand, matches the order and number of elements provided.

Note: An array literal is not the same as an Array instance. You can't initialize a type that conforms to ExpressibleByArrayLiteral simply by assigning an existing array.

Whenever possible, Swift's compiler infers the full intended type of your array literal. Because Array is the default type for an array literal, without writing any other code, you can declare an array with a particular element type by providing one or more values.

In this example, the compiler infers the full type of each array literal.

An empty array literal alone doesn't provide enough information for the compiler to infer the intended type of the Array instance. When using an empty array literal, specify the type of the variable or constant.

Because many functions and initializers fully specify the types of their parameters, you can often use an array literal with or without elements as a parameter. Xcode 11 download mac. For example, the sum(_:) function shown here takes an Int array as a parameter:

When you call a function that does not fully specify its parameters' types, use the type-cast operator (as) to specify the type of an array literal. For example, the log(name:value:) function shown here has an unconstrained generic value parameter.

Add the capability to be initialized with an array literal to your own custom types by declaring an init(arrayLiteral:) initializer. The following example shows the array literal initializer for a hypothetical OrderedSet type, which has setlike semantics but maintains the order of its elements.

Inheritance	View Protocol Hierarchy →
Associated Types	Element
Import	import Swift

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Initializers

init(arrayLiteral:)Required

Creates an instance initialized with the given elements.

Declaration

init(arrayLiteral elements: Self.Element..)