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# Core SDK Functions
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Essential SDK initialization and configuration functions for app startup and user lifecycle management.
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## Capabilities
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### SDK Initialization
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Initialize the OneSignal SDK with your application ID. This must be called during app startup.
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```typescript { .api }
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/**
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 * Initializes the OneSignal SDK. This should be called during startup of the application.
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 * @param appId - Your OneSignal application ID
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 */
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function initialize(appId: string): void;
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```
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**Usage Example:**
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```typescript
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import { OneSignal } from "react-native-onesignal";
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// Initialize during app startup
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OneSignal.initialize("your-onesignal-app-id");
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```
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### User Authentication
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Manage user sessions for cross-device user identification and tracking.
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```typescript { .api }
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/**
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 * If your integration is user-centric, or you want the ability to identify the user beyond 
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 * the current device, the login method should be called to identify the user.
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 * @param externalId - External user identifier from your system
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 */
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function login(externalId: string): void;
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/**
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 * Once (or if) the user is no longer identifiable in your app (i.e. they logged out), 
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 * the logout method should be called.
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 */
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function logout(): void;
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```
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**Usage Examples:**
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```typescript
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// User logs in - identify them for cross-device tracking
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OneSignal.login("user123");
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// User logs out - clear identification
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OneSignal.logout();
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```
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### GDPR Compliance
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Manage privacy consent for GDPR compliance and data protection regulations.
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```typescript { .api }
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/**
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 * For GDPR users, your application should call this method before setting the App ID.
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 * @param required - Whether privacy consent is required
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 */
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function setConsentRequired(required: boolean): void;
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/**
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 * If your application is set to require the user's privacy consent, you can provide this 
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 * consent using this method. Indicates whether privacy consent has been granted.
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 * @param granted - Whether consent has been granted
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 */
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function setConsentGiven(granted: boolean): void;
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```
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**Usage Example:**
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```typescript
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// For GDPR compliance - call before initialize()
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OneSignal.setConsentRequired(true);
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// After user grants consent
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OneSignal.setConsentGiven(true);
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// Then initialize
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OneSignal.initialize("your-app-id");
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```
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### Debug Configuration
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Configure logging and debugging output for development and troubleshooting.
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```typescript { .api }
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namespace Debug {
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  /**
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   * Enable logging to help debug if you run into an issue setting up OneSignal.
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   * @param nsLogLevel - Sets the logging level to print to the Android LogCat log or Xcode log
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   */
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  function setLogLevel(nsLogLevel: LogLevel): void;
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  /**
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   * Enable logging to help debug if you run into an issue setting up OneSignal.
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   * @param visualLogLevel - Sets the logging level to show as alert dialogs
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   */  
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  function setAlertLevel(visualLogLevel: LogLevel): void;
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}
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```
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**Usage Example:**
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```typescript
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import { OneSignal, LogLevel } from "react-native-onesignal";
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// Enable verbose logging for development
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OneSignal.Debug.setLogLevel(LogLevel.Verbose);
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// Show errors as alert dialogs
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OneSignal.Debug.setAlertLevel(LogLevel.Error);
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```