Capacitor Plugin Development

Capacitor Plugin Development

Create and maintain Capacitor plugins — scaffolding, native API bridging (iOS/Android), type definitions, testing, and publishing.

Prerequisites

Agent Behavior

• Auto-detect before asking. Read package.json, inspect the directory structure, and infer the plugin's current state before prompting the user.
• Guide step-by-step. Walk the user through one phase at a time. Do not present multiple unrelated tasks simultaneously.
• Skip what exists. If the plugin is already scaffolded, skip scaffolding. If the iOS implementation already exists, skip to the next platform.
• Ask which platforms to target. If the user does not specify, implement all three (iOS, Android, Web).

Procedures

Step 1: Determine the Task

Ask the user what they want to do. Common tasks:

1. Create a new plugin from scratch — proceed to Step 2.
2. Add a new method to an existing plugin — skip to Step 5.
3. Add a new platform implementation — skip to the relevant platform step (Step 6, 7, or 8).
4. Set up plugin configuration — read references/plugin-configuration.md and apply.
5. Set up plugin hooks — read references/testing-and-workflow.md (Plugin Hooks section) and apply.
6. Publish the plugin — skip to Step 10.

If the user's intent is clear from context, skip this question and proceed directly.

Step 2: Scaffold the Plugin

Read references/scaffolding.md and guide the user through generating a new plugin project using the Capacitor plugin generator.

Gather the following from the user (or infer from context):

• Plugin npm package name
• Android package ID
• Plugin class name
• Repository URL
• License
• Description

Run the generator and verify the scaffold with npm run verify.

Step 3: Design the TypeScript API

Read references/designing-api.md and guide the user through defining the plugin interface.

1. Ask the user what methods the plugin should expose and what data each method accepts/returns.
2. Define the plugin interface and all supporting types in src/definitions.ts.
3. Register the plugin in src/index.ts using registerPlugin().

Ensure:

• Every method and property has JSDoc comments with @since tags.
• Options and result types are defined as separate interfaces.
• String union types are used instead of TypeScript enums.

Step 4: Implement the Web Layer

Read references/web-guide.md and implement the web layer in src/web.ts.

1. Extend WebPlugin and implement the plugin interface.
2. For methods that use Web APIs, implement the browser-based logic.
3. For methods that have no web equivalent, throw this.unimplemented().
4. For methods that require a web API not available in all browsers, check for the API's existence and throw this.unavailable() if missing.

After implementation, build and verify:

npm run build
npm run verify:web

Step 5: Define Method Signatures Across Platforms

Before implementing native code, determine the method types for each plugin method:

Most methods use the "returns a value" type. Use "callback" only for continuous data streams (e.g., geolocation watching).

Step 6: Implement the iOS Plugin

Read references/ios-guide.md and implement the iOS layer.

1. Create or update the implementation class (e.g., ios/Sources/<ClassName>Plugin/Example.swift) with the platform logic. Extend NSObject and mark methods with @objc.
2. Create or update the plugin class (e.g., ios/Sources/<ClassName>Plugin/ExamplePlugin.swift):
   • Extend CAPPlugin and conform to CAPBridgedPlugin.
   • Set identifier, jsName, and pluginMethods properties.
   • Implement each @objc func method, reading data from CAPPluginCall and calling resolve(), reject(), unavailable(), or unimplemented().
3. If the plugin requires third-party iOS dependencies, add them to the .podspec file.
4. If the plugin requires permissions, implement checkPermissions() and requestPermissions(), and document the required Info.plist keys.

After implementation, verify:

npm run verify:ios

Step 7: Implement the Android Plugin

Read references/android-guide.md and implement the Android layer.

1. Create or update the implementation class (e.g., android/src/main/java/<package-path>/Example.java) with the platform logic.
2. Create or update the plugin class (e.g., android/src/main/java/<package-path>/ExamplePlugin.java):
   • Extend Plugin and add @CapacitorPlugin(name = "<JSName>").
   • Implement each @PluginMethod method, reading data from PluginCall and calling resolve(), reject(), unavailable(), or unimplemented().
3. If the plugin requires third-party Android dependencies, add them to android/build.gradle.
4. If the plugin requires permissions:
   • Define permission aliases in the @CapacitorPlugin annotation.
   • Implement checkPermissions() and requestPermissions().
   • Document which permissions the app developer must add to AndroidManifest.xml.

After implementation, verify:

npm run verify:android

Step 8: Add Events (If Needed)

If the plugin emits events to JavaScript:

1. TypeScript: Add addListener() and removeAllListeners() methods to the plugin interface in src/definitions.ts. Define an event interface for each event type.
2. Web: Call this.notifyListeners('eventName', data) when the event occurs.
3. iOS: Call self.notifyListeners("eventName", data: [...]) when the event occurs. Register observers in load() if listening to system notifications.
4. Android: Call notifyListeners("eventName", jsObject) when the event occurs. Override handleOnConfigurationChanged() or register broadcast receivers as needed.

Ensure the event name string is identical across all platforms and matches the eventName parameter in addListener().

Step 9: Generate Documentation and Verify

1. Add JSDoc comments to all methods and interfaces in src/definitions.ts if not already present.
2. Generate documentation:

npm run docgen

3. Run the full verification:

npm run verify

4. Lint and format:

npm run lint
npm run fmt

Step 10: Publish

Read references/publishing.md and guide the user through the publishing process.

1. Verify package.json fields are correct (name, version, description, files, capacitor).
2. Run the pre-publish checklist.
3. Publish to npm:

npm publish --access public

Error Handling

• npm init @capacitor/plugin@latest fails: Ensure Node.js LTS and npm 6+ are installed. Run node -v and npm -v to check.
• npm run verify:ios fails with "module not found": Run cd ios && pod install --repo-update && cd .. to install CocoaPods dependencies.
• npm run verify:android fails with build errors: Open android/ in Android Studio, sync Gradle, and check for missing dependencies or SDK version mismatches.
• npm run build fails with TypeScript errors: Check src/definitions.ts for type mismatches. Ensure the web implementation in src/web.ts correctly implements the plugin interface.
• registerPlugin() name mismatch: The first argument to registerPlugin() in src/index.ts must exactly match the jsName property on iOS and the @CapacitorPlugin(name = "...") value on Android. A mismatch causes the plugin to not load.
• iOS: Methods not callable from JavaScript: Ensure all plugin methods are marked with @objc and listed in the pluginMethods array of CAPBridgedPlugin.
• Android: Methods not callable from JavaScript: Ensure all plugin methods have the @PluginMethod() annotation and are public.
• Events not received in JavaScript: Verify the event name string is identical in the native notifyListeners() call and the TypeScript addListener() definition. Verify addListener() is called before the event fires.
• Plugin configuration values not reading: Verify the key names in getConfig().getString("key") match the keys in the Capacitor config file under plugins.<PluginJSName>.
• npm run docgen produces empty output: Ensure JSDoc comments are present on the plugin interface methods in src/definitions.ts, not on the implementation classes.

Related Skills

• capacitor-plugin-spm-support — Add Swift Package Manager support to a Capacitor plugin.
• capacitor-plugin-upgrades — Upgrade a Capacitor plugin to a newer Capacitor major version.
• capacitor-plugins — Install and configure existing Capacitor plugins in an app project.