Prefire

Quick Start · Installation · Usage · AI agents · Configuration · Docs · Website

🔥 What is Prefire?

Your previews are already tests. Prefire turns the #Preview blocks you write anyway into snapshot tests, a Playbook catalog and images your AI agent can see. You don't write a line of test code.

📸 Snapshot tests Every preview becomes a snapshot test on the devices you pick. A changed padding fails the build, not the release.

How it works →

📚 Playbook A catalog of every component and user story inside your app, with their states and render times.

See Playbook →

🤖 Renders for AI agents Claude Code, Cursor and Codex render your previews through an MCP server and see the UI they just changed.

Set up →

🤖 Using an AI agent? brew install barredewe/prefire/prefire, then claude mcp add prefire -- prefire mcp in your project. The agent renders SwiftUI previews on any iPhone or iPad, light or dark, without Xcode Previews. Setup →

🚀 Key Features

• 🧠 Smart Preview Parsing — including #Preview, @Previewable
• 📸 Snapshot Testing — automatic test generation from previews (iOS, tvOS, macOS)
• 📚 Playbook View — auto-generated interactive component catalog (iOS)
• 🏃 Flow-aware — build user stories from multiple preview steps
• 🤖 AI-ready — render previews to PNG for agents with prefire render and an MCP server
• 🧩 UIKit + AppKit Support — support for UIView/UIViewController and NSView/NSViewController
• ⚙️ SPM + Xcode Plugins — works in CLI, Xcode build phases, or CI
• 🧠 Fast Caching — fingerprint-based AST and body caching avoids redundant work
• ✍️ Stencil Templates — customize output with your own templates

Why Prefire?

• 🔥 No second set of UI states. Tests come from the previews you already keep up to date.
• 🔥 Regressions fail CI. Snapshots are compared on every run, on every device you list.
• 🔥 Designers and QA see every state. Playbook shows components and user stories on a device.
• 🔥 Agents check their own UI work. They render the previews they changed before calling a task done.

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⚡ Quick Start

📦 Example project available at: Prefire Example

1. Add Prefire to Your Project

// Package.swift

dependencies: [ .package(url: "https://github.com/BarredEwe/Prefire.git", from: "6.3.0"), .package(url: "https://github.com/pointfreeco/swift-snapshot-testing", from: "1.18.0"), ], .testTarget( dependencies: [ .product(name: "Prefire", package: "Prefire"), .product(name: "SnapshotTesting", package: "swift-snapshot-testing"), ], plugins: [ .plugin(name: "PrefireTestsPlugin", package: "Prefire") ] )

2. Write #Preview

#Preview {

Button("Submit") }

3. Run tests

Just run the test target 🚀 — Prefire will auto-generate snapshots based on your previews.

💡 If your test target is empty, Prefire will still generate files and snapshot code during build.

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📦 Installation

Step-by-step setup is in the Installation guide.

Requirements

• Swift 6.0 or higher
• Xcode 16.0 or higher
• iOS 14 or higher; macOS 13 or higher (macOS: snapshot tests only, see Configuration)

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🛠 Usage

Choose previews

Prefire finds every `#Preview` automatically. For PreviewProvider, add the PrefireProvider protocol:

struct Text_Previews: PreviewProvider, PrefireProvider {

static var previews: some View { ... } }

Exclude a preview with .prefireIgnored():

#Preview {

Text("") .prefireIgnored() }

To opt in instead of opting out, set preview_default_enabled: false in .prefire.yml and mark the previews you want with .prefireEnabled():

#Preview {

Text("") .prefireEnabled() }

Snapshot tests

PrefireTestsPlugin generates the tests into the DerivedData folder when the test target builds. Run the test target as usual.

Tune the timing and precision of a single snapshot:

#Preview {

TestView() .snapshot(delay: 0.3, precision: 0.95, perceptualPrecision: 0.98) }

For previews that load or animate, wait instead of guessing a delay:

.snapshot(waitForIdle: true)                     // until nothing changes on screen

.snapshot(waitUntil: { viewModel.isLoaded }) // until a condition is true

How waiting works, its limits and how the modifiers combine: Snapshot timing and precision.

Snapshots are compared with swift-snapshot-testing.

Playbook (Demo) View

Show all previews as an interactive catalog with PlaybookView:

• isComponent: true lists all the Views
• isComponent: false groups them by UserStory

import Prefire 

struct ContentView: View { var body: some View { PlaybookView(isComponent: true, previewModels: PreviewModels.models) } }

To keep previews and PreviewModels out of release builds, see Excluding previews from release builds.

Flows

Connect previews into one Flow with .previewUserStory. Playbook shows them together, in order:

static var previews: some View {

PrefireView() .previewUserStory(.auth) }

static var previews: some View { AuthView() .previewUserStory(.auth) }

States

If a preview contains more than one View, mark the State of each:

static var previews: some View {

TestView("Default")

TestView("Loading") .previewState(.loading) }

Parameterized previews

Prefire expands each argument of a native SwiftUI parameterized preview into its own snapshot and Playbook preview:

#Preview("TextView", traits: .sizeThatFitsLayout, arguments: ["- A", "- B", "- C"]) { suffix in

Text("1 \(suffix)") }

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🧰 API

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💡 CLI Usage

# Generate snapshot tests

prefire tests

Generate playbook models

prefire playbook

Run prefire tests --help or prefire playbook --help for more options.

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🤖 Let AI agents see SwiftUI

Prefire renders previews in its snapshot tests, built anew and on the devices you choose, so it can show them to an AI agent as images, which Xcode Previews can't do reliably outside Xcode.

# Render previews to PNG + JSON

prefire render "Sign in" --device "iPhone 16 Pro" --color-scheme dark

An HTML board of the last render, grouped by user story

prefire board

Add the MCP server to your agent, from the folder with .prefire.yml:

An agent then calls list_previews, render_previews and build_board. Set the scheme to build in render_configuration of .prefire.yml; see the AI guide. Why not Xcode's own RenderPreview? Prefire needs no running Xcode, renders many previews in one build, on the devices, color scheme and Dynamic Type you ask for, and works in CI.

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🗂 Configuration: .prefire.yml

test_configuration:

target: MyApp

playbook_configuration: preview_default_enabled: true

All options are in the Configuration guide. To change the generated code, see Templates.

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🧠 How It Works

1. 🔍 Parses all source files — finds #Preview and PreviewProvider blocks and the .prefireEnabled() / .prefireIgnored() modifiers
2. 📂 Caches `Types` and `PreviewBodies` — keyed by file modification date + SHA-256 of inputs, so nothing is re-parsed if nothing changed
3. 🔢 Generates snapshot tests — from Stencil templates, respecting .prefire.yml
4. 📘 Generates Playbook models — groups by UserStory and State into PreviewModels.generated.swift

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