PayPal Spec
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If a merchant wants to integrate with the PayPal module, these are the steps they would take.

1. Import `PayPalPayments` at the top of their file
2. Create a `PaymentsConfig` object using their client Id and return Url
3. Create a `PayPalClient` object using that configuration
4. Set the delegate of the `PayPalClient` to an object that should receive messages from the SDK, and handle the events (approval, cancellation, errors, and shipping updates)
5. Start the checkout experience using an `orderId` (which would have been retrieved by the merchant outside the SDK)

#### Swift Interface

public protocol PayPalClientDelegate {
    // Invoked when a transaction is approved by the 
    // user and ready for authorization or capture by
    // the merchant
    func paypalDidApprove(client: PayPalClient, data: ApprovalData)
    
    // Invoked when the checkout experience has been canceled
    func paypalDidCancel(client: PayPalClient)
    
    // Invoked when the SDK encounters an unrecoverable error
    func paypalDidError(client: PayPalClient, error: PayPalError)
    
    // Invoked when the user wants to change the shipping or 
    // pickup information. Requires merchant response to 
    // confirm the new selection 
    func paypalShippingChange(client: PayPalClient, ...)
} 

public class PayPalClient {
    public weak var delegate: PayPalClientDelegate?
    
    public init(config: PaymentsConfig) {
        ...
    }
    
    public func checkout(orderId: String) {
        
    }
    
    public func checkout(billingAgreement: String) {
    
    }
}

let config = PaymentsConfig(clientId: "", returnUrl: "")
let client = PayPalClient(config: config)

client.delegate = self
client.checkout(orderId)

#### Kotlin Interface

interface PayPalClientListener {
    fun onPayPalApprove(data: ApprovalData)
    fun onPayPalCancel()
    fun onPayPalError(error: PayPalError)
    fun onPayPalShippingChange(...)
}

class PayPalClient(val config: PaymentsConfig) {
    
    var listener: PayPalClientListener? = null
    
    fun checkout(orderId: String) {
    
    }
    
    fun checkout(billingAgreement: String) {
        
    }
}

val config = PaymentsConfig(clientId = "", returnUrl = "")
val client = PayPalClient(config = config)

### Improvements

- Fetch merchant configuration from a PayPal backend and infer `returnUrl` to reduce the number of parameters needed when constructing a `PayPalClient`

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