fix: deletion from slideshow for cases were user is not present

In that case, deleting the currently viewed file had the following issues:
1. When deleting the first image, the UI just stayed with it.
2. When deleting another image, the UI switched to the previous image,
   instead of the next one, which is inconsistent to the behavior when
   user is available
3. When switching to the next image, the title shown at the top did
   not correctly update

Signed-off-by: Philipp Hasper <[email protected]>

Author: PhilLab

app/src/main/java/com/owncloud/android/ui/preview/PreviewImageActivity.kt

@@ -55,6 +55,7 @@ import edu.umd.cs.findbugs.annotations.SuppressFBWarnings
 import java.io.Serializable
 import javax.inject.Inject
 import kotlin.math.max
+import kotlin.math.min
 
 /**
  * Holds a swiping gallery where image files contained in an Nextcloud directory are shown.
@@ -287,12 +288,19 @@ class PreviewImageActivity :
             }
 
             if (user.isPresent) {
+                // Re-init the view pager, which will advance to the next image
                 initViewPager(user.get())
-            } else {
+            } else if (result.isSuccess) {
+                // If deletion was successful, update adapter and display next image
                 val deletePosition = viewPager?.currentItem ?: return
-                val nextPosition = if (deletePosition > 0) deletePosition - 1 else 0
-                viewPager?.setCurrentItem(nextPosition, true)
-                previewImagePagerAdapter?.delete(deletePosition)
+                previewImagePagerAdapter?.let { adapter ->
+                    // advance to the next image if possible, since initViewPager() also advances forwards
+                    val nextPosition = min(deletePosition, adapter.itemCount - 1)
+                    viewPager?.setCurrentItem(nextPosition, true)
+                    adapter.delete(deletePosition)
+                    // Page needs to be reselected after the adapter has been updated. Otherwise, wrong title is shown
+                    selectPage(nextPosition)
+                }
             }
         } else if (operation is SynchronizeFileOperation) {
             onSynchronizeFileOperationFinish(result)

app/src/main/java/com/owncloud/android/ui/preview/PreviewImagePagerAdapter.kt

@@ -201,4 +201,13 @@ class PreviewImagePagerAdapter : FragmentStateAdapter {
     override fun createFragment(position: Int): Fragment = getItem(position)
 
     override fun getItemCount(): Int = imageFiles.size
+
+    override fun getItemId(position: Int): Long {
+        // Use the OCFile id, fallback to position if not available
+        return imageFiles.getOrNull(position)?.fileId ?: position.toLong()
+    }
+
+    override fun containsItem(itemId: Long): Boolean {
+        return imageFiles.any { it.fileId == itemId }
+    }
 }