Actual code 🎉

Package.swift

@@ -4,25 +4,26 @@
 import PackageDescription
 
 let package = Package(
-    name: "ConstraintBuilder",
-    products: [
-        // Products define the executables and libraries a package produces, and make them visible to other packages.
-        .library(
-            name: "ConstraintBuilder",
-            targets: ["ConstraintBuilder"]),
-    ],
-    dependencies: [
-        // Dependencies declare other packages that this package depends on.
-        // .package(url: /* package url */, from: "1.0.0"),
-    ],
-    targets: [
-        // Targets are the basic building blocks of a package. A target can define a module or a test suite.
-        // Targets can depend on other targets in this package, and on products in packages this package depends on.
-        .target(
-            name: "ConstraintBuilder",
-            dependencies: []),
-        .testTarget(
-            name: "ConstraintBuilderTests",
-            dependencies: ["ConstraintBuilder"]),
-    ]
+	name: "ConstraintBuilder",
+	platforms: [
+		.iOS(.v11),
+		.tvOS(.v9),
+		.macCatalyst(.v13),
+		.macOS(.v10_11)
+	],
+	products: [
+		// Products define the executables and libraries a package produces, and make them visible to other packages.
+		.library(
+			name: "ConstraintBuilder",
+			targets: ["ConstraintBuilder"]),
+	],
+	dependencies: [],
+	targets: [
+		.target(
+			name: "ConstraintBuilder",
+			dependencies: []),
+		.testTarget(
+			name: "ConstraintBuilderTests",
+			dependencies: ["ConstraintBuilder"]),
+	]
 )

README.md

@@ -1,3 +1,10 @@
 # ConstraintBuilder
 
-A description of this package.
+Convenience Auto Layout methods, applying constraints through a function builder
+
+```swift
+someView.applyConstraints {
+	$0.leadingAnchor.constraint(equalTo: contentView.layoutMarginsGuide.leadingAnchor)
+	$0.centerYAnchor.constraint(equalTo: contentView.layoutMarginsGuide.centerYAnchor)
+}
+```

Sources/ConstraintBuilder/ConstraintBuilder.swift

@@ -1,6 +1,51 @@
+#if canImport(UIKit)
+import UIKit
+#elseif canImport(AppKit)
+import AppKit
+#endif
+
+@resultBuilder
 public struct ConstraintBuilder {
-    public private(set) var text = "Hello, World!"
+	static func buildBlock(_ components: NSLayoutConstraint...) -> [NSLayoutConstraint] {
+		Array(components)
+	}
+	static func buildBlock(_ components: [NSLayoutConstraint]) -> [NSLayoutConstraint] {
+		Array(components)
+	}
+}
+
+public extension NSLayoutConstraint {
+	/// Activates all constraints created in the `builder` closure;
+	/// - Parameter builder: Closure in which all constraints should be created or referenced
+	///
+	/// Typical usage:
+	/// ```swift
+	/// NSLayoutConstraint.build {
+	///     someView.leadingAnchor.constraint(equalTo: containerView.leadingAnchor)
+	///     someView.centerYAnchor.constraint(equalTo: containerView.centerYAnchor)
+	///     anotherView.leadingAnchor.constraint(equalTo: someView.trailingAnchor)
+	///     anotherView.centerYAnchor.constraint(equalTo: containerView.centerYAnchor)
+	/// }
+	/// ```
+	class func build(@ConstraintBuilder _ builder: () -> [NSLayoutConstraint]) {
+		activate(builder())
+	}
+}
 
-    public init() {
-    }
+public protocol ContraintBuildable {
+	/// Extends all edges to the edges of the provided view
+	/// - Parameter view: View of which edges should be extended to
+	func extend(to view: Self)
+
+	/// Aligns center with center of provided view
+	/// - Parameter view: View of which center should be aligned to
+	func center(in view: Self)
+
+	/// Extends all edges to the edges of the superview
+	/// Should result in `assertionFailure` when no superview is available
+	func extendToSuperview()
+
+	/// Aligns center with center of superview
+	/// Should result in `assertionFailure` when no superview is available
+	func centerInSuperview()
 }

Sources/ConstraintBuilder/UIView+ConstraintBuildable.swift

@@ -0,0 +1,94 @@
+#if canImport(UIKit)
+import UIKit
+
+public protocol UIViewConstraintBuildable: ContraintBuildable {
+	/// Create and activate constriants with this view as the main subject
+	/// ```swift
+	/// view.applyConstraints {
+	///     $0.leadingAnchor.constraint(equalTo: otherView.leadingAnchor)
+	///     $0.centerYAnchor.constraint(equalTo: otherView.centerYAnchor)
+	/// }
+	/// ```
+	/// - Parameter builder: Constraint builder to add the constriants from
+	func applyConstraints(@ConstraintBuilder _ builder: (UIView) -> [NSLayoutConstraint])
+
+	/// Extends all edges to provided layout guide
+	/// - Parameter view: UILayoutGuide of which edges should be extended to
+	func extend(to layoutGuide: UILayoutGuide)
+
+	/// Aligns center with center of layout guide
+	/// - Parameter view: UILayoutGuide of which center should be aligned to
+	func center(in layoutGuide: UILayoutGuide)
+
+	/// Extends all edges to the edges of the superview‘s safe area
+	/// Should result in `assertionFailure` when no superview is available
+	func extendToSuperviewSafeArea()
+
+	/// Extends all edges to the edges of the superview‘s layout margins
+	/// Should result in `assertionFailure` when no superview is available
+	func extendToSuperviewLayoutMargins()
+}
+
+extension UIView: UIViewConstraintBuildable {
+	public func applyConstraints(@ConstraintBuilder _ builder: (UIView) -> [NSLayoutConstraint]) {
+		NSLayoutConstraint.activate(builder(self))
+	}
+
+	public func extend(to view: UIView) {
+		NSLayoutConstraint.build {
+			leadingAnchor.constraint(equalTo: view.leadingAnchor)
+			trailingAnchor.constraint(equalTo: view.trailingAnchor)
+			topAnchor.constraint(equalTo: view.topAnchor)
+			bottomAnchor.constraint(equalTo: view.bottomAnchor)
+		}
+	}
+
+	public func center(in view: UIView) {
+		NSLayoutConstraint.build {
+			centerXAnchor.constraint(equalTo: view.centerXAnchor)
+			centerYAnchor.constraint(equalTo: view.centerYAnchor)
+		}
+	}
+
+	public func extend(to layoutGuide: UILayoutGuide) {
+		NSLayoutConstraint.build {
+			leadingAnchor.constraint(equalTo: layoutGuide.leadingAnchor)
+			trailingAnchor.constraint(equalTo: layoutGuide.trailingAnchor)
+			topAnchor.constraint(equalTo: layoutGuide.topAnchor)
+			bottomAnchor.constraint(equalTo: layoutGuide.bottomAnchor)
+		}
+	}
+
+	public func center(in layoutGuide: UILayoutGuide) {
+		NSLayoutConstraint.build {
+			centerXAnchor.constraint(equalTo: layoutGuide.centerXAnchor)
+			centerYAnchor.constraint(equalTo: layoutGuide.centerYAnchor)
+		}
+	}
+
+	/// Use superview if available,  `assertionFailure()` if not
+	private func withSuperview(_ method: (UIView) -> Void) {
+		guard let superview = superview else {
+			return assertionFailure()
+		}
+		method(superview)
+	}
+
+	public func extendToSuperview() {
+		withSuperview(extend(to:))
+	}
+
+	public func centerInSuperview() {
+		withSuperview(center(in:))
+	}
+
+	public func extendToSuperviewSafeArea() {
+		withSuperview { extend(to: $0.safeAreaLayoutGuide) }
+	}
+
+	public func extendToSuperviewLayoutMargins() {
+		withSuperview { extend(to: $0.layoutMarginsGuide) }
+	}
+}
+
+#endif

Tests/ConstraintBuilderTests/ConstraintBuilderTests.swift

@@ -2,10 +2,4 @@ import XCTest
 @testable import ConstraintBuilder
 
 final class ConstraintBuilderTests: XCTestCase {
-    func testExample() throws {
-        // This is an example of a functional test case.
-        // Use XCTAssert and related functions to verify your tests produce the correct
-        // results.
-        XCTAssertEqual(ConstraintBuilder().text, "Hello, World!")
-    }
 }