Use an explicit SCHEMA_VERSION instead of BuildConfig.VERSION_CODE (#3324)

* Use an explicit SCHEMA_VERSION instead of BuildConfig.VERSION_CODE

Every nightly release has a new BuildConfig.VERSION_CODE, so the previous
code would not do the right thing.

Require the schema version to be explicitly set. While I'm here, provide
a clear set of guidelines as to what has to happen when the schema changes.

* Improve documentation links

app/src/main/java/com/keylesspalace/tusky/TuskyApplication.kt

@@ -23,6 +23,7 @@ import autodispose2.AutoDisposePlugins
 import com.keylesspalace.tusky.components.notifications.NotificationWorkerFactory
 import com.keylesspalace.tusky.di.AppInjector
 import com.keylesspalace.tusky.settings.PrefKeys
+import com.keylesspalace.tusky.settings.SCHEMA_VERSION
 import com.keylesspalace.tusky.util.APP_THEME_DEFAULT
 import com.keylesspalace.tusky.util.LocaleManager
 import com.keylesspalace.tusky.util.setAppNightMode
@@ -69,10 +70,10 @@ class TuskyApplication : Application(), HasAndroidInjector {
         AppInjector.init(this)
 
         // Migrate shared preference keys and defaults from version to version. The last
-        // version that did not have a SCHEMA_VERSION was 97, so that's the default.
+        // version that did not have a SCHEMA_VERSION was 100, so that's the default.
-        val oldVersion = sharedPreferences.getInt(PrefKeys.SCHEMA_VERSION, 97)
+        val oldVersion = sharedPreferences.getInt(PrefKeys.SCHEMA_VERSION, 100)
-        if (oldVersion != BuildConfig.VERSION_CODE) {
+        if (oldVersion != SCHEMA_VERSION) {
-            upgradeSharedPreferences(oldVersion, BuildConfig.VERSION_CODE)
+            upgradeSharedPreferences(oldVersion, SCHEMA_VERSION)
         }
 
         // In this case, we want to have the emoji preferences merged with the other ones

app/src/main/java/com/keylesspalace/tusky/settings/SettingsConstants.kt

@@ -12,6 +12,37 @@ enum class AppTheme(val value: String) {
     }
 }
 
+/**
+ * Current preferences schema version. Format is 4-digit year + 2 digit month (zero padded) + 2
+ * digit day (zero padded) + 2 digit counter (zero padded).
+ *
+ * If you make an incompatible change to the preferences schema you must:
+ *
+ * - Update this value
+ * - Update the code in
+ *   [TuskyApplication.upgradeSharedPreferences][com.keylesspalace.tusky.TuskyApplication.upgradeSharedPreferences]
+ *   to migrate from the old schema version to the new schema version.
+ *
+ * An incompatible change is:
+ *
+ * - Deleting a preference. The migration should delete the old preference.
+ * - Changing a preference's default value (e.g., from true to false, or from one enum value to
+ *   another). The migration should check to see if the user had set an explicit value for
+ *   that preference ([SharedPreferences.contains][android.content.SharedPreferences.contains]);
+ *   if they hadn't then the migration should set the *old* default value as the preference's
+ *   value, so the app behaviour does not unexpectedly change.
+ * - Changing a preference's type (e.g,. from a boolean to an enum). If you do this you may want
+ *   to give the preference a different name, but you still need to migrate the user's previous
+ *   preference value to the new preference.
+ * - Renaming a preference key. The migration should copy the user's previous value for the
+ *   preference under the old key to the value for the new, and delete the old preference.
+ *
+ * A compatible change is:
+ *
+ * - Adding a new preference that does not change the interpretation of an existing preference
+ */
+const val SCHEMA_VERSION = 2023021501
+
 object PrefKeys {
     // Note: not all of these keys are actually used as SharedPreferences keys but we must give
     // each preference a key for it to work.