Rob Kerr's Blog

Filtering Collections in Swift, Kotlin and JavaScript

Extracting members of a collection that match some filtering predicate is a common pattern in programming. Traditionally we would iterate over all members (e.g. with a for or foreach loop), and save each member matching some condition. Most programming languages now provide some single step filter function to eliminate

Kotlin

Awaiting Kotlin suspend functions in serial, parallel and lazy startup modes

Kotlin allows a thread of execution to be suspended until some other asynchronous function completes and returns a result. Suspend functions are most commonly used when a response from an external resource is needed to complete the suspend function.

Swift

REPL Tool Reference

It's helpful to experiment with language syntax in a REPL tool before pasting code into your editor. REPL is an acryonym for Read, Evaluate, Print, Loop. REPL tools are simple interactive programming environments primarily used to validate syntax and algorithms. Swift The main Swift REPL tool is the Playground feature

Firebase

Using Firebase Auth claims with Swift, Kotlin and JavaScript

Firebase Authentication supports built-in and custom user claims that allow us to conditionally expose functionality to users based on authorization groups. This post demonstrates the syntax to add custom claims in a Firebase backend, and read claims from mobile apps. What are Claims? In identity systems, a "Claim" is is

Firebase

Using Firebase Auth claims with Swift, Kotlin and JavaScript

Firebase Authentication supports built-in and custom user claims that allow us to conditionally expose functionality to users based on authorization groups. This post demonstrates the syntax to add custom claims in a Firebase backend, and read claims from mobile apps. What are Claims? In identity systems, a "Claim" is is

Simple For Loop in Swift, Kotlin and JavaScript

A core feature of all programming languages is the trusty for loop. For loops execute a statement a predefined number of times with some way to exit the loop early. Swift Swift uses range operators to construct simple for loops. let start = 0 let end = 10 for i in start

Swift

URL Encoding GET parameters in Swift

When creating a GET URLRequest in Swift, it's often necessary to URL encode GET string parameters. Many languages include String functions to emit url encoded versions of Strings, but as of this writing Swift does not fully address this problem in the Foundation String class. However, Swift Foundation actually does

Swift

Creating async/await functions in a Swift Playground

Swift Playgrounds in Xcode are super useful for trying out Swift syntax either for learning, or for refining code before incorporating it into a real project. By default async functions can't be executed in a playground, but it is possible. Here's an example: import Foundation import _Concurrency import PlaygroundSupport var

Swift

Async/Await Syntax in Swift, Kotlin and JavaScript

Most modern languages support the ability for asynchronous function calls to be called from synchronous code blocks. This language feature is usually called async/await. This post is a brief syntax scratchpad comparing async/await in different languages. Swift In Swift, an await-able async function is created by adding the

Backend

Adding an X-Auth-Key to a Firebase Function stored in Google Secret Manager

Firebase cloud functions can be secured through various mechanisms supplied by Firebase with out-of-the-box features: 1. Functions can easily determine whether the user is being called by a user who has been authenticated by Firebase Auth 2. We can use Firebase App Check to ensure Functions can only be called

Backend

Sending a Push Notification from a Firebase Function

An essential part of maintaining user engagement with mobile applications is sending push notifications at the appropriate time. Push notifications come in two flavors: * Local push notifications: notifications scheduled by the app itself, based on internal events. For example, a simple timer app could schedule a push notification to alert

Kotlin

Android Jetpack Compose View Model Eventing Architecture

A common Android architectural pattern is to consume events in a UI View that are triggered from asynchronous web service code in a View Model. While there are many approaches to solving this problem, I wanted to share the approach I often use. You may have another approach that works

Swift

ForEach Syntax in Swift, Kotlin and JavaScript

ForEach is a common pattern in programming modern programming languages. Prior to ForEach, the standard approach to iterating over an array of objects was to use a for loop stepping from the first index to the last. ForEach syntax has extra utility in languages that support many types of collections:

SwiftUI

Using iOS 16 view modifiers with iOS 15 targets

With each new version of iOS, Apple introduces new SwiftUI features to add new features and plug holes in the framework. This article provides a solution for using new view modifiers that ship with new iOS versions--without raising the app's minimum iOS target setting. What are View Modifiers If you've

Android

Checking for Google Play Services with Kotlin

In many countries we can assume Android users use devices that have Google Play Services pre-installed, but not every Android device in the world actually does. It's possible that code written with the assumption that Play Services is available may crash--something we certainly don't want. What we do want is

Strategy

Accepting Credit Cards for In-App Purchase of Physical Goods

Whether your app is iOS, Android or web-based, selling products and services within your app requires back-end financial services integration. This article provides a business-level overview and best practices for the back-end services you'll need when selling goods and services in-app. Here I'll focus on selling physical goods and services

Strategy

Apple’s new Unlisted App Distribution model

How developers can distribute and monetize applications has been evolving quickly over this last year. Some of these changes have been in response to government or court mandates in various countries, but others appear to have been prompted by Apple itself. This week’s announcement of a new Unlisted App

SwiftUI

Check if a SwiftUI View is in Preview

SwiftUI Preview is a fantastic piece of development tooling to quickly iterate UI designs in Xcode. But sometimes the code in your view just doesn't work--or worse it crashes the preview engine. How can we work around this issue? I ran into a Preview crash issue in a SwiftUI view

Jetpack Compose

Adding a dropdown menu to a Jetpack Compose TopAppBar

Adding a dropdown menu to a Jetpack ComposeTopAppBar is quite easy, but simple examples aren't so easy to find! This post has a canonical example to use as a basis for your dropdown menu. Add a Scaffold that Includes a TopAppBar The first step is to create the Compose view

Bluetooth

Create an ESP32 BLE peripheral for iOS #2

This post is part two of a series targeted at helping mobile app developers build their own BLE Peripherals using inexpensive ESP32 hardware in less than 100 lines of code. This can help to more fully understand the BLE protocol. If you’re an iOS developer who would like to

Bluetooth

Create an ESP32 BLE peripheral for iOS #1

This post is part one of a series targeted at helping mobile app developers build their own BLE Peripherals using inexpensive ESP32 hardware in less than 100 lines of code. This can help to more fully understand the BLE protocol. If you’re an iOS developer who would like to

Swift

Migrating Swift API Calls to Async Await

Beginning in iOS 15 and macOS 13, Swift developers can begin using the Swift async/await syntax to to suspend and resume processing while waiting for long-running work to complete. This post walks through the process to upgrade a set of nested API calls from the familiar closure syntax to

SwiftUI

Observing ViewModel Changes in SwiftUI

SwiftUI uses declarative UI programming, where Views have immutable state, changing automatically as the data state they depend on changes. This is a mindset change for most iOS/macOS developers: instead of changing the UI with a “command and control” MVC mindset, data state changes indirectly change the UI. But

iOS

Bridging Javascript and WKWebkit

Many mobile applications incorporate remote web pages, either as passive (static) content — or as in this case as integral parts of the UI. Using the WebKit/WKWebView techniques presented here, your native apps can be better integrated with web content and provide a superior experience to end-users. Two-way Integration between

Google Cloud

Integrating Firebase with Google Big Query for Realtime Mobile App Analysis

Introduction Well designed mobile and web apps incorporate platforms such as Google Analytics, Amplitude, Mixpanel and similar services. These services are important tools to achieve one or more tactical objectives: * Tracking unique users, launch frequency, clickstream behavior and feature usage helps in planning which features to invest in and which