Precondition, Assert, Fatal Error, or Guard in your Swift Code

Similar, but not quite the same

If you want to stop your code from crashing, or if you don’t want a function to complete if it does not have a variable set as you desire. But what about whether you want to perform actions in production or development code?

For the lowdown, read on!

Photo by Jean Carlo Emer on Unsplash

Difficulty: Beginner | Easy | Normal | Challenging

Prerequisites

Some basic understanding of functions and methods in Swift would be useful (guide HERE).

Terminology

Assert: A function used for internal sanity checks that are active during testing, but are not used in production code

Function: The process of removing unnecessary detail from a problem

Guard: A statement used to transfer program control out of a scope if the condition is not met

Precondition: A function that checks a condition

Scope: The region of a program where a variable is valid

Variable: A named value, that potentially can be changed as the program runs

Precondition

precondition()

Precondition allows us to check a condition before proceeding with code. That is, if a Condition stated evaluates to false the program execution stops.

The one big condition in Precondition (LOLOLOL) is that it works in production code as well as development code. If the chosen Condition stated evaluates to false precondition will stop the executing code, and log a chosen message

func testPrecondition(willRun: Bool) {
    precondition(willRun)
    print ("The function /(#function) completed")
}
testPrecondition(willRun: true) // The function testPrecondition(willRun:) completed
testPrecondition(willRun: false) // Precondition failed: file PreconditionAssertGuard.playground, line 6

We can also add a message to allow

func testPrecondition(willRun: Bool) {
    precondition(willRun, "Must set willRun to true")
    print ("The function /(#function) completed")
}
testPrecondition(willRun: true)…