prevent calling discoverReaders if not completed (#351)

* prevent calling discoverReaders if not completed

* add busy message func

android/src/main/java/com/stripeterminalreactnative/Errors.kt

@@ -40,6 +40,16 @@ internal fun <T> requireCancelable(cancelable: T?, lazyMessage: () -> String): T
     )
 }
 
+@Throws(TerminalException::class)
+internal fun <T> throwIfBusy(command: T?, lazyMessage: () -> String): Unit? {
+    return command?.run {
+        throw TerminalException(
+            TerminalErrorCode.READER_BUSY,
+            lazyMessage()
+        )
+    }
+}
+
 @Throws(TerminalException::class)
 internal fun <T> requireParam(input: T?, lazyMessage: () -> String): T {
     return input ?: throw TerminalException(

android/src/main/java/com/stripeterminalreactnative/StripeTerminalReactNativeModule.kt

@@ -150,6 +150,10 @@ class StripeTerminalReactNativeModule(reactContext: ReactApplicationContext) :
 
         val listener = RNDiscoveryListener(context) { discoveredReadersList = it }
 
+        throwIfBusy(discoverCancelable) {
+            busyMessage("discoverReaders", "discoverReaders")
+        }
+
         discoverCancelable = terminal.discoverReaders(
             DiscoveryConfiguration(0, discoveryMethod, getBoolean(params, "simulated")),
             listener,
@@ -160,7 +164,9 @@ class StripeTerminalReactNativeModule(reactContext: ReactApplicationContext) :
     @ReactMethod
     @Suppress("unused")
     fun cancelDiscovering(promise: Promise) {
-        cancelOperation(promise, discoverCancelable, "discoverReaders")
+        cancelOperation(promise, discoverCancelable, "discoverReaders") {
+            discoverCancelable = null
+        }
     }
 
     private fun connectReader(
@@ -561,10 +567,16 @@ class StripeTerminalReactNativeModule(reactContext: ReactApplicationContext) :
         promise: Promise,
         cancelable: Cancelable?,
         operationName: String,
+        block: (() -> Unit)? = null
     ) = withExceptionResolver(promise) {
         val toCancel = requireCancelable(cancelable) {
             "$operationName could not be canceled because it has already been canceled or has completed."
         }
         toCancel.cancel(NoOpCallback(promise))
+        block?.invoke()
+    }
+
+    private fun busyMessage(command: String, busyBy: String): String {
+        return  "Could not execute $command because the SDK is busy with another command: $busyBy."
     }
 }

ios/Errors.swift

@@ -38,6 +38,10 @@ class Errors {
     }
 }
 
+func busyMessage(command: String, by busyCommand: String) -> String {
+    return "Could not execute \(command) because the SDK is busy with another command: \(busyCommand)."
+}
+
 extension ErrorCode.Code {
     var stringValue: String {
         switch self {

ios/StripeTerminalReactNative.swift

@@ -181,6 +181,12 @@ class StripeTerminalReactNative: RCTEventEmitter, DiscoveryDelegate, BluetoothRe
             discoveryMethod: Mappers.mapToDiscoveryMethod(discoveryMethod),
             simulated: simulated ?? false
         )
+
+        guard discoverCancelable == nil else {
+            let message = busyMessage(command: "discoverReaders", by: "discoverReaders")
+            resolve(Errors.createError(code: ErrorCode.busy, message: message))
+            return
+        }
 
         self.discoverCancelable = Terminal.shared.discoverReaders(config, delegate: self) { error in
             if let error = error as NSError? {