Client-side Implementation

var braintreeClient: BTAPIClient!

override func viewDidLoad() {
    super.viewDidLoad();
    self.braintreeClient = BTAPIClient(authorization: "<#CLIENT_AUTHORIZATION#>");
    let customPayPalButton = UIButton(frame: CGRect(x: 0, y: 0, width: 60, height: 120));
    customPayPalButton.addTarget(self, action: #selector(customPayPalButtonTapped(button:)), for: UIControlEvents.touchUpInside);
    self.view.addSubview(customPayPalButton);
}

func customPayPalButtonTapped(button: UIButton) {
    let payPalDriver = BTPayPalDriver(apiClient: self.braintreeClient);
    // Start the Checkout flow
    let payPalRequest = BTPayPalCheckoutRequest(amount: "1.00");
    payPalRequest.currencyCode = "USD"; // Optional; see BTPayPalCheckoutRequest.h for more options
    // Specify the intent on the request object
    payPalRequest.intent = .order;
    payPalDriver.tokenizePayPalAccount(with: payPalRequest) { (tokenizedPayPalAccount, error) -> Void in
        // ...
    };
}