add hasAccount api

KinEcosystem/Core/Kin.swift

@@ -256,6 +256,36 @@ public class Kin {
 
     }
 
+    public func hasAccount(peer: String, handler: @escaping (Bool?, Error?) -> ()) {
+        guard let core = core else {
+            logError("Kin not started")
+            DispatchQueue.main.async {
+                handler(nil, KinEcosystemError.client(.notStarted, nil))
+            }
+            return
+        }
+        _ = core.network.dataAtPath("users/exists",
+                                    method: .get,
+                                    contentType: .json,
+                                    parameters: ["user_id" : peer])
+            .then { data in
+                if  let response = String(data: data, encoding: .utf8),
+                    let ans = Bool(response) {
+                    DispatchQueue.main.async {
+                        handler(ans, nil)
+                    }
+                } else {
+                    DispatchQueue.main.async {
+                        handler(nil, KinEcosystemError.service(.response, nil))
+                    }
+                }
+            }.error { error in
+                DispatchQueue.main.async {
+                    handler(nil, KinEcosystemError.transform(error))
+                }
+        }
+    }
+
     public func payToUser(offerJWT: String, completion: @escaping KinCallback) -> Bool {
         return purchase(offerJWT: offerJWT, completion: completion)
     }

README.md

@@ -429,6 +429,27 @@ _ = Kin.shared.payToUser(offerJWT: encoded, completion: handler)
 
 3.	Complete the pay to user offer after you receive confirmation from the Kin Server that the funds were transferred successfully.
 
+### Finding out if another user has a kin account ###
+
+Before paying to a user, you might want to check if this user actually exists. to do that:
+
+```swift
+// from sample app:
+Kin.shared.hasAccount(peer: otherUserId) { [weak self] response, error in
+    if let response = response {
+        guard response else {
+            self?.presentAlert("User Not Found", body: "User \(otherUserId) could not be found. Make sure the receiving user has activated kin, and in on the same environment as this user")
+            return
+        }
+        // Proceed with payment (transferKin is an internal function in the sample app)
+        self?.transferKin(to: otherUserId, appId: id, pKey: jwtPKey)
+    } else if let error = error {
+        self?.presentAlert("An Error Occurred", body: "\(error.localizedDescription)")
+    } else {
+        self?.presentAlert("An Error Occurred", body: "unknown error")
+    }
+}
+```
 
 ## License
 The kin-ecosystem-ios-sdk library is licensed under [MIT license](LICENSE.md).