New: Add the concept of 'prepare'.

begin adding a step between the creation of a Synchronizer and starting it, called 'prepare' which is responsible for migrations and other steps to get the data ready for syncing.

src/main/java/cash/z/ecc/android/sdk/SdkSynchronizer.kt

@@ -233,6 +233,10 @@ class SdkSynchronizer internal constructor(
 
     override val latestBirthdayHeight: Int get() = processor.birthdayHeight
 
+    override fun prepare(): Synchronizer = apply {
+        storage.prepare()
+    }
+
     /**
      * Starts this synchronizer within the given scope. For simplicity, attempting to start an
      * instance that has already been started will throw a [SynchronizerException.FalseStart]
@@ -362,6 +366,9 @@ class SdkSynchronizer internal constructor(
     }
 
     private fun CoroutineScope.onReady() = launch(CoroutineExceptionHandler(::onCriticalError)) {
+        twig("Preparing to start...")
+        prepare()
+
         twig("Synchronizer (${this@SdkSynchronizer}) Ready. Starting processor!")
         var lastScanTime = 0L
         processor.onProcessorErrorListener = ::onProcessorError

src/main/java/cash/z/ecc/android/sdk/Synchronizer.kt

@@ -30,6 +30,13 @@ interface Synchronizer {
      */
     var isStarted: Boolean
 
+    /**
+     * Prepare the synchronizer to start. Must be called before start. This gives a clear point
+     * where setup and maintenance can occur for various Synchronizers. One that uses a database
+     * would take this opportunity to do data migrations or key migrations.
+     */
+    fun prepare(): Synchronizer
+
     /**
      * Starts this synchronizer within the given scope.
      *
@@ -358,6 +365,13 @@ interface Synchronizer {
          */
         DISCONNECTED,
 
+        /**
+         * Indicates that this Synchronizer is actively preparing to start, which usually involves
+         * setting up database tables, migrations or taking other maintenance steps that need to
+         * occur after an upgrade.
+         */
+        PREPARING,
+
         /**
          * Indicates that this Synchronizer is actively downloading new blocks from the server.
          */

src/main/java/cash/z/ecc/android/sdk/exception/Exceptions.kt

@@ -32,6 +32,11 @@ sealed class RepositoryException(message: String, cause: Throwable? = null) : Sd
         "The channel is closed. Note that once a repository has stopped it " +
             "cannot be restarted. Verify that the repository is not being restarted."
     )
+    object Unprepared : RepositoryException(
+        "Unprepared repository: Data cannot be accessed before the repository is prepared." +
+            " Ensure that things have been properly initialized. In most cases, this involves" +
+            " calling 'synchronizer.prepare' before 'synchronizer.start'"
+    )
 }
 
 /**

src/main/java/cash/z/ecc/android/sdk/transaction/TransactionRepository.kt

@@ -88,6 +88,8 @@ interface TransactionRepository {
 
     suspend fun getAccountCount(): Int
 
+    fun prepare()
+
     //
     // Transactions
     //