title: How to identify the paymentMethod Token for selected payment option using braintree BTDropInController to create subcriptions?
body: <p>How to know which card did the user choose from the dropIn UI (which is stores in the vault)?</p>
  
  <p><strong>Swift code:</strong></p>
  
  <pre><code>let dropIn = BTDropInController(authorization: clientTokenOrTokenizationKey, request: request)
  { (controller, result, error) in
      if (error != nil) {
          print("ERROR")
      } else if (result?.isCancelled == true) {
          print("CANCELLED")
      } else if let result = result {
          // Use the BTDropInResult properties to update your UI
          // result.paymentOptionType
          // result.paymentMethod
          // result.paymentIcon
          // result.paymentDescription
          let urlString = fullURLString(baseURL: baseURL(), apiPath: CHECKOUT_API_PATH)
          let parameters: [String: Any] = [
             "payment_method_nonce" : result.paymentMethod?.nonce ?? ""
          ]
  
          // ??????????????????? how to get paymentMethod Token?????
      }
  }
  </code></pre>
  
  <p>For <strong><em>transaction</em></strong> we can use result.paymentMethod?.nonce.</p>
  
  <p><strong>Node.js:</strong></p>
  
  <pre><code>gateway.transaction.sale({
      amount: "1.00",
      paymentMethodNonce: nonceFromTheClient, // result.paymentMethod?.nonce ?? from iOS
      options: {
        submitForSettlement: true
      }
    }, function (err, result) {
  });
  </code></pre>
  
  <p>However for <strong><em>subscriptions</em></strong>, we need to use paymentMethodToken, but how do we know which one did the user select? </p>
  
  <p><strong>Node.js:</strong></p>
  
  <pre><code>gateway.subscription.create({
      paymentMethodToken: ???,
      planId: "goldPlanID"
  }, function (err, result) {
      console.log('subscription result:', result);
      console.log('subscription err:', err);
  
      res.send(result)
  });
  </code></pre>
  
  <p>By using find function in customer, we can get the paymentMethods (but we don't know which one the user selected at client/iOS side:</p>
  
  <p><strong>Node.js:</strong></p>
  
  <pre><code>gateway.customer.find(customerBraintreeID, function(err, customer) {
  ...
  }
  </code></pre>
  
  <p><strong>PaymentMethods.token</strong></p>
  
  <pre><code>{
  "paymentMethods":
  [ CreditCard {
  ...
  token: 'jxxxxx',
  uniqueNumberIdentifier: 'xxxxxxxxxx',
  updatedAt: 
  venmoSdk: false,
  verifications: [],
  maskedNumber: '411111******1111',
  expirationDate: '01/2020' }
  ...
  }]
  </code></pre>