SwiftLint is the best tool to enforce coding standards and best practices. It comes with pre-defined rules that can be customized to suit individual project needs. However, sometimes the built-in rules fall short of project requirements. In such cases, we write regular expressions to fill the gap. While regex is a powerful tool, creating complex rules can be difficult or even impossible. This is where native SwiftLint rules come in. They're just like the rules that come with SwiftLint - they're written in Swift, efficient, and can catch more violations compared to regex. https://twitter.com/simjp/status/1558168307756318720?s=20&embedable=true The final project can be found at https://github.com/jpsim/swiftlint-bazel-example Setting Up the Project Install Bazel if not installed. brew list --formula | grep -q "bazel" || brew install bazel Start by creating a new directory. mkdir MySwiftLintRules && cd MySwiftLintRules Next, set up a new Bazel workspace. This involves creating several files and enabling bzlmod: touch .bazelrc WORKSPACE MODULE.bazel BUILD && echo "common --enable_bzlmod" > .bazelrc Add the boilerplate code to the MODULE.bazel module(
    name = "swiftlint-bazel-example",
    version = "0.0.0",
    compatibility_level = 1
)

bazel_dep(name = "swiftlint", version = "0.51.0", repo_name = "SwiftLint")
extra_rules = use_extension("@SwiftLint//bazel:extensions.bzl", "extra_rules")
extra_rules.setup(srcs = "@swiftlint-bazel-example//swiftlint_extra_rules:extra_rules")

bazel_dep(name = "rules_xcodeproj", version = "1.4.0") Add the boilerplate code to the file. This will add Xcode support for our Bazel project. BUILD load("@rules_xcodeproj//xcodeproj:defs.bzl", "xcodeproj")

xcodeproj(
    name = "swiftlint_xcodeproj",
    project_name = "SwiftLint",
    tags = ["manual"],
    top_level_targets = [
        "@SwiftLint//:swiftlint",
        "@SwiftLint//Tests:ExtraRulesTests",
    ],
) Create a directory to store the source code for your rules. mkdir swiftlint_extra_rules && touch swiftlint_extra_rules/Rules.swift && touch swiftlint_extra_rules/BUILD And for the last time, paste the boilerplate code to the file swiftlint_extra_rules/BUILD filegroup(
    name = "extra_rules",
    srcs = glob(["**/*.swift"]),
    visibility = ["//visibility:public"],
) At this point, you should be able to build SwiftLint using the command. bazel build Developing Rules With Xcode We’ve added dependency to our project to make developing your custom rules easier. So, let’s generate our project: rules_xcodeproj bazel run swiftlint_xcodeproj && open SwiftLint.xcodeproj -a Xcode Now, we can build and debug the scheme. You can use this app as usual with swiftlint binary installed from the homebrew. swiftlint Try running it on your project: Click the scheme name in the , click “Edit Scheme…“, and set the "Working Directory" in the "Options" tab to the path where you would like to execute SwiftLint. toolbar of your project window For more information on developing native SwiftLint rules, check out SwiftLint’s CONTRIBUTING.md Adding a Custom Rule In this example, we are creating the "forbidden var rule," as shown in the video tutorial at . https://vimeo.com/819268038 Copy and paste this code into the file, then run the scheme in Xcode. Rules.swift swiftlint import SwiftSyntax

// This function should return an array containing all of your custom rules
func extraRules() -> [Rule.Type] {
	[ForbiddenVarRule.self]
}

struct ForbiddenVarRule: ConfigurationProviderRule, SwiftSyntaxRule {
	var configuration = SeverityConfiguration(.error)

	init() {}

	static let description = RuleDescription(
		identifier: "forbidden_var",
		name: "Forbidden Var",
		description: "Variables should not be called 'forbidden'",
		kind: .idiomatic,
		nonTriggeringExamples: [
			Example("let notForbidden = 0")
		],
		triggeringExamples: [
			Example("let ↓forbidden = 0")
		]
	)

	func makeVisitor(file: SwiftLintFile) -> ViolationsSyntaxVisitor {
		Visitor(viewMode: .sourceAccurate)
	}
}

private extension ForbiddenVarRule {
	final class Visitor: ViolationsSyntaxVisitor {
		override func visitPost(_ node: IdentifierPatternSyntax) {
			if node.identifier.text == "forbidden" {
				violations.append(node.identifier.positionAfterSkippingLeadingTrivia)
			}
		}
	}
} And that's it! You've created a custom SwiftLint rule and integrated it into your Swift project using Bazel. With this setup, you can create and test as many custom rules as you need, helping to ensure that your code stays consistent and conforms to your team's coding standards. Happy linting!

Meet the Writer: HackerNoon Contributor Misha Kharytonchyk - Lead iOS Developer

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