DevFright

Learn How to Create iOS Apps

Apple Sample Code for Three-Column Layout in SwiftUI

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For the last few weeks, I have been making plans for a new iPad/iPhone, and potentially macOS application. I haven’t used SwiftUI much in development so wasn’t sure how best to sort out the navigation on both mobile devices.

The app I am building, which is in very early stages, is for note-taking and meeting planning. My plan was to use a two-column layout, much like master/detail, but it seems that one good option is the three-column layout that has a menu on the far left column, a list view in the centre column, and then details on the rest of the view, as seen below.

Three-column Layout – Fruta by Apple
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Unable to use SwiftUI Preview due to Insufficient System Resources

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If you are getting an error saying that you cannot preview this file (while using SwiftUI), then perhaps like my experience, a reboot might fix it. That’s the short version to possibly get you going before reading on.

I created a new project today and switched the default ContentView.swift to use an iPad Pro 11 inch for the preview. An error appeared in right pane saying “Cannot preview in this file – Unable to boot device due to insufficient system resources”. Clicking the diagnostics button provided the following:

HumanReadableNSError: Unable to boot device due to insufficient system resources.

Please see Simulator Help for information on adjusting resource limits.

NSPOSIXErrorDomain (67):

==NSLocalizedFailureReason: The current system settings are not sufficient to allow booting additional simulators: maxFiles: 12288, openFiles: 11669, enforcedFilesBuffer: 1868

As mentioned, a reboot fixed the issue. It seems that Apple last recognised this happening back in 2017 in the Simulator Help section on the Apple help site that gives some recommendations on how to modify the number of processes and number of open files.

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What is a Swift Class

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We recently wrote about Swift structs. In this tutorial, we’ll look at classes. Classes and structures have a lot in common, although there are some important differences. Let’s take a look at what a class is and what you use them for.

  • A class is initialised as an object.
  • You need to create an initialiser for your class.
  • Classes can also inherit from other classes which bring to your class the abilities and properties of the other class. You can also override the default behaviour of a method of the parent class.
  • Unlike structs which are passed around by value, classes are passed by reference.
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What is a Swift Struct

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A struct, or structure, is a custom data type in Swift that allows you to add many properties to it. If you think of a regular string, you can store just one string in the variable, as follows:

var firstName = "Matthew"

What if we wanted to add Matthews height:

var firstName = "Matthew"
var height = 190

This rapidly becomes complicated because we now have two variables to manage and think about. Imagine if we needed to add their last name, and so on, and then have the need to pass this information somewhere else within the program. This is one benefit of using a struct.

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Using Codable to Encode and Decode JSON

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Codable provides a way to convert to and from formats such as JSON. In this tutorial we’ll just focus on converting JSON. The JSON format is popular among web services and provides a standard about how data can be represented. JSON formatted data can be understood by various systems making it possible to easily transfer data from one system to another regardless of operating system or programming language used.

On the iOS side, Codable is one way to convert an object or structure in to the JSON format so that the other end of the network can accept it and understand it as needed.

The Gmail API is one example of a service that provides data in JSON form. When you query the API the results come back as JSON. We don’t particularly care what programming language and operating systems are involved on the other side. All we need to know is that the response provided conforms to the JSON standard and thus, we can use Codable to convert that data in to whatever we need within our code.

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How to Use the SwiftUI List View with Dynamic Cells

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In a recent tutorial we covered how to combine views and briefly touched on dynamic cells within a List. In this tutorial we’ll look more closely at how you can bring in dynamic content in to a List.

Create a new project and make sure you select SwiftUI when the option is presented.

Open up ContentView.swift and just below the import line at the top of the file, add the following struct:

struct Message: Identifiable {
    var id = UUID()
    var from: String
    var date: String
    var subject: String
    var snippet: String
}
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Combining SwiftUI Views

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One of many things that make SwiftUI great is the ability to combine views. The template code when you create a new SwiftUI view contains the following:

struct ContentView: View {
    var body: some View {
        Text("Place Holder")
    }
}

The body variable is expecting some View. In the example above we are returning a Text view which contains a string “Place Holder”. If you preview this in the canvas you will see the words “Place Holder”, without quotes, in the centre of the view.

Swift provides the ability to combine views so that more views can be combined in to one and then returned. In our Stacks tutorial we explain this. In short, you can combine views in to a HStack, VStack, or ZStack and return the stack as one view.

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How to Use the SwiftUI List View with Static Cells

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SwiftUI allows us to make static table views by using a List view. The SwiftUI syntax makes the process extremely simple. Unlike UITableViewControllers that require you to manually specify that you are using static cells, and then have to assign the table view controller a class, you simply just add “List { }” to your code and insert all of the views you want in-between the curly braces.

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SwiftUI HStack, VStack, ZStack Tutorial

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When you create a SwiftUI view, the protocol defines that we need to return “some View”. The word “some” means that we are dealing with an opaque result type. This is a new feature added added in Swift 5.1. An opaque result type means that we must return one, and only one, of the specified type, in this case something that conforms to the “View” protocol.

This highlights a problem. Imagine if our view, ie… the part that the user interacts with on screen, was only able to show one Text field or perhaps one button, then interfaces would be incredibly boring.

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