Support caching via "Last-Modified" & "If-Modified-Since" HTTP headers.

+ Add "schedule_last_modified" column to "meta" table.
  Increase "meta" database version.
+ Ensure default value is empty string. Otherwise, the app would crash on
  the next update in RealMetaDatabaseRepository/query/getString with a NPE.
+ Docs:
  + https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching
  + https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-Modified-Since
+ Related:
  + FOSDEM/website#235
  + https://git.cccv.de/hub/hub/-/issues/564

app/src/main/java/nerd/tuxmobil/fahrplan/congress/dataconverters/HttpHeaderExtensions.kt

@@ -6,12 +6,15 @@ import info.metadude.android.eventfahrplan.network.models.HttpHeader as HttpHead
 
 fun HttpHeaderAppModel.toHttpHeaderNetworkModel() = HttpHeaderNetworkModel(
     eTag = eTag,
+    lastModified = lastModified,
 )
 
 fun HttpHeaderDatabaseModel.toHttpHeaderAppModel() = HttpHeaderAppModel(
     eTag = eTag,
+    lastModified = lastModified,
 )
 
 fun HttpHeaderNetworkModel.toHttpHeaderDatabaseModel() = HttpHeaderDatabaseModel(
     eTag = eTag,
+    lastModified = lastModified,
 )

app/src/main/java/nerd/tuxmobil/fahrplan/congress/models/HttpHeader.kt

@@ -5,4 +5,5 @@ package nerd.tuxmobil.fahrplan.congress.models
  */
 data class HttpHeader(
     val eTag: String = "",
+    val lastModified: String = "",
 )

app/src/test/java/nerd/tuxmobil/fahrplan/congress/dataconverters/FetchScheduleResultExtensionsTest.kt

@@ -15,7 +15,10 @@ class FetchScheduleResultExtensionsTest {
         val networkFetchScheduleResult = NetworkFetchScheduleResult(
                 httpStatus = NetworkHttpStatus.HTTP_NOT_MODIFIED,
                 scheduleXml = "<xml></xml>",
-                httpHeader = HttpHeader("mno456"),
+                httpHeader = HttpHeader(
+                    eTag = "mno456",
+                    lastModified = "2023-12-31T23:59:59+01:00",
+                ),
                 hostName = "example.com",
                 exceptionMessage = "SSLException"
         )

app/src/test/java/nerd/tuxmobil/fahrplan/congress/dataconverters/MetaExtensionsTest.kt

@@ -13,7 +13,10 @@ import nerd.tuxmobil.fahrplan.congress.models.Meta as MetaAppModel
 class MetaExtensionsTest {
 
     private val metaAppModel = MetaAppModel(
-            httpHeader = HttpHeaderAppModel("abc123"),
+            httpHeader = HttpHeaderAppModel(
+                eTag = "abc123",
+                lastModified = "2019-12-31T23:59:59+01:00",
+            ),
             numDays = 23,
             subtitle = "My subtitle",
             timeZoneId = ZoneId.of("Europe/Berlin"),
@@ -22,7 +25,10 @@ class MetaExtensionsTest {
     )
 
     private val metaDatabaseModel = MetaDatabaseModel(
-            httpHeader = HttpHeaderDatabaseModel("abc123"),
+            httpHeader = HttpHeaderDatabaseModel(
+                eTag = "abc123",
+                lastModified = "2019-12-31T23:59:59+01:00",
+            ),
             numDays = 23,
             subtitle = "My subtitle",
             timeZoneName = "Europe/Berlin",
@@ -31,7 +37,10 @@ class MetaExtensionsTest {
     )
 
     private val metaNetworkModel = MetaNetworkModel(
-            httpHeader = HttpHeaderNetworkModel("abc123"),
+            httpHeader = HttpHeaderNetworkModel(
+                eTag = "abc123",
+                lastModified = "2019-12-31T23:59:59+01:00",
+            ),
             numDays = 23,
             subtitle = "My subtitle",
             timeZoneName = "Europe/Berlin",

app/src/test/java/nerd/tuxmobil/fahrplan/congress/repositories/AppRepositoryLoadAndParseScheduleTest.kt

@@ -306,7 +306,7 @@ class AppRepositoryLoadAndParseScheduleTest {
         NetworkFetchScheduleResult(
             httpStatus = httpStatus,
             scheduleXml = "some fahrplan xml",
-            httpHeader = HttpHeader("a1b2bc3"),
+            httpHeader = HttpHeader(eTag = "a1b2bc3", lastModified = "2023-12-31T23:59:59+01:00"),
             hostName = HOST_NAME
         )
 

database/src/androidTest/java/info/metadude/android/eventfahrplan/database/extensions/MetaExtensionsTest.kt

@@ -3,6 +3,7 @@ package info.metadude.android.eventfahrplan.database.extensions
 import androidx.test.ext.junit.runners.AndroidJUnit4
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.ETAG
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.NUM_DAYS
+import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.SCHEDULE_LAST_MODIFIED
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.SUBTITLE
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.TIME_ZONE_NAME
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.TITLE
@@ -19,7 +20,7 @@ class MetaExtensionsTest {
     @Test
     fun toContentValues() {
         val meta = Meta(
-                httpHeader = HttpHeader(eTag = "abc123"),
+                httpHeader = HttpHeader(eTag = "abc123", lastModified = "2023-12-31T23:59:59+01:00"),
                 numDays = 23,
                 subtitle = "My subtitle",
                 timeZoneName = "Europe/Berlin",
@@ -28,6 +29,7 @@ class MetaExtensionsTest {
         )
         val values = meta.toContentValues()
         assertThat(values.getAsString(ETAG)).isEqualTo("abc123")
+        assertThat(values.getAsString(SCHEDULE_LAST_MODIFIED)).isEqualTo("2023-12-31T23:59:59+01:00")
         assertThat(values.getAsInteger(NUM_DAYS)).isEqualTo(23)
         assertThat(values.getAsString(SUBTITLE)).isEqualTo("My subtitle")
         assertThat(values.getAsString(TIME_ZONE_NAME)).isEqualTo("Europe/Berlin")

database/src/main/java/info/metadude/android/eventfahrplan/database/contract/FahrplanContract.java

@@ -18,6 +18,7 @@ interface Columns {
             /* 5 */ String ETAG = "etag";
             /* 6 */ String NUM_DAYS = "numdays";
             /* 7 */ String TIME_ZONE_NAME = "time_zone_name";
+            /* 8 */ String SCHEDULE_LAST_MODIFIED = "schedule_last_modified";
         }
 
         interface Defaults {

database/src/main/java/info/metadude/android/eventfahrplan/database/extensions/MetaExtensions.kt

@@ -3,6 +3,7 @@ package info.metadude.android.eventfahrplan.database.extensions
 import androidx.core.content.contentValuesOf
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.ETAG
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.NUM_DAYS
+import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.SCHEDULE_LAST_MODIFIED
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.SUBTITLE
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.TIME_ZONE_NAME
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.TITLE
@@ -11,6 +12,7 @@ import info.metadude.android.eventfahrplan.database.models.Meta
 
 fun Meta.toContentValues() = contentValuesOf(
         ETAG to httpHeader.eTag,
+        SCHEDULE_LAST_MODIFIED to httpHeader.lastModified,
         NUM_DAYS to numDays,
         SUBTITLE to subtitle,
         TIME_ZONE_NAME to timeZoneName,

database/src/main/java/info/metadude/android/eventfahrplan/database/models/HttpHeader.kt

@@ -5,5 +5,6 @@ package info.metadude.android.eventfahrplan.database.models
  */
 data class HttpHeader(
     val eTag: String = "",
+    val lastModified: String = "",
 )
 

database/src/main/java/info/metadude/android/eventfahrplan/database/repositories/RealMetaDatabaseRepository.kt

@@ -5,6 +5,7 @@ import android.database.sqlite.SQLiteException
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.ETAG
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.NUM_DAYS
+import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.SCHEDULE_LAST_MODIFIED
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.SUBTITLE
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.TIME_ZONE_NAME
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns.TITLE
@@ -54,6 +55,7 @@ class RealMetaDatabaseRepository(
                         subtitle = cursor.getString(SUBTITLE),
                         httpHeader = HttpHeader(
                             eTag = cursor.getString(ETAG),
+                            lastModified = cursor.getString(SCHEDULE_LAST_MODIFIED),
                         ),
                 )
             } else {

database/src/main/java/info/metadude/android/eventfahrplan/database/sqliteopenhelper/MetaDBOpenHelper.java

@@ -9,10 +9,11 @@
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable;
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Columns;
 import info.metadude.android.eventfahrplan.database.contract.FahrplanContract.MetasTable.Defaults;
+import info.metadude.android.eventfahrplan.database.extensions.SQLiteDatabaseExtensions;
 
 public class MetaDBOpenHelper extends SQLiteOpenHelper {
 
-    private static final int DATABASE_VERSION = 8;
+    private static final int DATABASE_VERSION = 9;
 
     private static final String DATABASE_NAME = "meta";
 
@@ -23,7 +24,8 @@ public class MetaDBOpenHelper extends SQLiteOpenHelper {
                     Columns.TITLE + " TEXT, " +
                     Columns.SUBTITLE + " TEXT, " +
                     Columns.ETAG + " TEXT, " +
-                    Columns.TIME_ZONE_NAME + " TEXT);";
+                    Columns.TIME_ZONE_NAME + " TEXT, " +
+                    Columns.SCHEDULE_LAST_MODIFIED + " TEXT DEFAULT '');";
 
     public MetaDBOpenHelper(@NonNull Context context) {
         super(context.getApplicationContext(), DATABASE_NAME, null, DATABASE_VERSION);
@@ -64,5 +66,12 @@ public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
             db.execSQL("DROP TABLE IF EXISTS " + MetasTable.NAME);
             onCreate(db);
         }
+        if (oldVersion < 9) {
+            boolean columnExists = SQLiteDatabaseExtensions.columnExists(db, MetasTable.NAME, Columns.SCHEDULE_LAST_MODIFIED);
+            if (!columnExists) {
+                db.execSQL("ALTER TABLE " + MetasTable.NAME + " ADD COLUMN " +
+                        Columns.SCHEDULE_LAST_MODIFIED + " TEXT DEFAULT ''");
+            }
+        }
     }
 }

network/src/main/java/info/metadude/android/eventfahrplan/network/fetching/FetchFahrplan.kt

@@ -25,7 +25,7 @@ internal class FetchFahrplan(private val logging: Logging) {
 
     fun fetch(okHttpClient: OkHttpClient, url: String, httpHeader: HttpHeader) {
         task = FetchFahrplanTask(okHttpClient, logging, onFetchScheduleResult)
-        task.execute(url, httpHeader.eTag)
+        task.execute(url, httpHeader.eTag, httpHeader.lastModified)
     }
 
     fun cancel() {
@@ -55,10 +55,13 @@ internal class FetchFahrplanTask(
         const val LOG_TAG = "FetchFahrplan"
         const val HTTP_HEADER_NAME_ETAG = "ETag"
         const val HTTP_HEADER_NAME_IF_NONE_MATCH = "If-None-Match"
+        const val HTTP_HEADER_NAME_LAST_MODIFIED = "Last-Modified"
+        const val HTTP_HEADER_NAME_IF_MODIFIED_SINCE = "If-Modified-Since"
     }
 
     private var responseStr = EMPTY_RESPONSE_STRING
     private var eTagStr = ""
+    private var lastModifiedStr = ""
     private var completed = false
     private lateinit var status: HttpStatus
     private var host = ""
@@ -75,8 +78,9 @@ internal class FetchFahrplanTask(
     override fun doInBackground(vararg args: String?): HttpStatus {
         val url = args[0]!!
         val eTag = args[1]!!
+        val lastModified = args[2]!!
         host = Uri.parse(url).host ?: throw NullPointerException("Host is null for url = '$url'")
-        return fetch(url, eTag)
+        return fetch(url, eTag, lastModified)
     }
 
     @Deprecated("Deprecated in Java")
@@ -95,27 +99,31 @@ internal class FetchFahrplanTask(
         if (status == HttpStatus.HTTP_OK) {
             logging.d(LOG_TAG, "Fetch done successfully")
             onFetchScheduleResult(
-                FetchScheduleResult(status, responseStr, HttpHeader(eTagStr), host, exceptionMessage)
+                FetchScheduleResult(status, responseStr, HttpHeader(eTagStr, lastModifiedStr), host, exceptionMessage)
             )
         } else {
             logging.d(LOG_TAG, "Fetch failed")
             onFetchScheduleResult(
                 FetchScheduleResult(
-                    status, EMPTY_RESPONSE_STRING, HttpHeader(eTagStr), host, exceptionMessage
+                    status, EMPTY_RESPONSE_STRING, HttpHeader(eTagStr, lastModifiedStr), host, exceptionMessage
                 )
             )
         }
         completed = false // notify only once
     }
 
-    private fun fetch(url: String, eTag: String): HttpStatus {
+    private fun fetch(url: String, eTag: String, lastModified: String): HttpStatus {
         logging.d(LOG_TAG, url)
         logging.d(LOG_TAG, "$HTTP_HEADER_NAME_ETAG: '$eTag'")
+        logging.d(LOG_TAG, "$HTTP_HEADER_NAME_LAST_MODIFIED: '$lastModified'")
         val requestBuilder = Request.Builder().apply {
             url(url)
             if (eTag.isNotEmpty()) {
                 addHeader(HTTP_HEADER_NAME_IF_NONE_MATCH, eTag)
             }
+            if (lastModified.isNotEmpty()) {
+                addHeader(HTTP_HEADER_NAME_IF_MODIFIED_SINCE, lastModified)
+            }
         }
 
         val response = try {
@@ -162,6 +170,13 @@ internal class FetchFahrplanTask(
             logging.d(LOG_TAG, "$HTTP_HEADER_NAME_ETAG: '$eTagStr'")
         }
 
+        lastModifiedStr = response.header(HTTP_HEADER_NAME_LAST_MODIFIED).orEmpty()
+        if (lastModifiedStr.isEmpty()) {
+            logging.d(LOG_TAG, "$HTTP_HEADER_NAME_LAST_MODIFIED is missing")
+        } else {
+            logging.d(LOG_TAG, "$HTTP_HEADER_NAME_LAST_MODIFIED: '$lastModifiedStr'")
+        }
+
         responseStr = try {
             response.body!!.string()
         } catch (e: NullPointerException) {

network/src/main/java/info/metadude/android/eventfahrplan/network/models/HttpHeader.kt

@@ -8,4 +8,5 @@ import info.metadude.android.eventfahrplan.network.fetching.FetchFahrplan
  */
 data class HttpHeader(
     val eTag: String = "",
+    val lastModified: String = "",
 )

network/src/main/java/info/metadude/android/eventfahrplan/network/serialization/FahrplanParser.java

@@ -50,7 +50,7 @@ public FahrplanParser(@NonNull Logging logging) {
 
     public void parse(String fahrplan, HttpHeader httpHeader) {
         task = new ParserTask(logging, listener);
-        task.execute(fahrplan, httpHeader.getETag());
+        task.execute(fahrplan, httpHeader.getETag(), httpHeader.getLastModified());
     }
 
     public void cancel() {
@@ -98,7 +98,7 @@ public void setListener(FahrplanParser.OnParseCompleteListener listener) {
 
     @Override
     protected Boolean doInBackground(String... args) {
-        boolean parsingSuccessful = parseFahrplan(args[0], args[1]);
+        boolean parsingSuccessful = parseFahrplan(args[0], args[1], args[2]);
         if (parsingSuccessful) {
             DateFieldValidation dateFieldValidation = new DateFieldValidation(logging);
             dateFieldValidation.validate(sessions);
@@ -126,7 +126,7 @@ protected void onPostExecute(Boolean result) {
         }
     }
 
-    private Boolean parseFahrplan(String fahrplan, String eTag) {
+    private Boolean parseFahrplan(String fahrplan, String eTag, String lastModified) {
         XmlPullParser parser = Xml.newPullParser();
         try {
             parser.setInput(new StringReader(fahrplan));
@@ -148,7 +148,7 @@ private Boolean parseFahrplan(String fahrplan, String eTag) {
                     case XmlPullParser.START_DOCUMENT:
                         sessions = new ArrayList<>();
                         meta = new Meta();
-                        meta.setHttpHeader(new HttpHeader(eTag));
+                        meta.setHttpHeader(new HttpHeader(eTag, lastModified));
                         break;
                     case XmlPullParser.END_TAG:
                         name = parser.getName();