Move caption to the next album item on cancel.

After #4869 albums are sent with captions in the first media.
In case we cancel the first media leaving the rest of the album
the caption will be lost unless we move it to the new "first" media.

Telegram/SourceFiles/storage/localimageloader.cpp

@@ -11,6 +11,7 @@ https://github.com/telegramdesktop/tdesktop/blob/master/LEGAL
 #include "core/file_utilities.h"
 #include "core/mime_type.h"
 #include "media/media_audio.h"
+#include "history/history_item.h"
 #include "boxes/send_files_box.h"
 #include "media/media_clip_reader.h"
 #include "mainwidget.h"
@@ -24,6 +25,40 @@ namespace {
 
 constexpr auto kThumbnailQuality = 87;
 
+auto FindAlbumItem(
+		std::vector<SendingAlbum::Item> &items,
+		not_null<HistoryItem*> item) {
+	const auto result = ranges::find(
+		items,
+		item->fullId(),
+		&SendingAlbum::Item::msgId);
+
+	Ensures(result != end(items));
+	return result;
+}
+
+MTPInputSingleMedia PrepareAlbumItemMedia(
+		not_null<HistoryItem*> item,
+		const MTPInputMedia &media,
+		uint64 randomId) {
+	auto caption = item->originalText();
+	TextUtilities::Trim(caption);
+	auto sentEntities = TextUtilities::EntitiesToMTP(
+		caption.entities,
+		TextUtilities::ConvertOption::SkipLocal);
+	const auto flags = !sentEntities.v.isEmpty()
+		? MTPDinputSingleMedia::Flag::f_entities
+		: MTPDinputSingleMedia::Flag(0);
+
+	return MTP_inputSingleMedia(
+		MTP_flags(flags),
+		media,
+		MTP_long(randomId),
+		MTP_string(caption.text),
+		sentEntities);
+}
+
+
 } // namespace
 
 using Storage::ValidateThumbDimensions;
@@ -241,6 +276,44 @@ void TaskQueueWorker::onTaskAdded() {
 SendingAlbum::SendingAlbum() : groupId(rand_value<uint64>()) {
 }
 
+void SendingAlbum::fillMedia(
+		not_null<HistoryItem*> item,
+		const MTPInputMedia &media,
+		uint64 randomId) {
+	const auto i = FindAlbumItem(items, item);
+	Assert(!i->media);
+
+	i->media = PrepareAlbumItemMedia(item, media, randomId);
+}
+
+void SendingAlbum::refreshMediaCaption(not_null<HistoryItem*> item) {
+	const auto i = FindAlbumItem(items, item);
+	if (!i->media) {
+		return;
+	}
+	i->media = i->media->match([&](const MTPDinputSingleMedia &data) {
+		return PrepareAlbumItemMedia(item, data.vmedia, data.vrandom_id.v);
+	});
+}
+
+void SendingAlbum::removeItem(not_null<HistoryItem*> item) {
+	const auto localId = item->fullId();
+	const auto i = ranges::find(items, localId, &Item::msgId);
+	const auto moveCaption = (items.size() > 1) && (i == begin(items));
+	Assert(i != end(items));
+	items.erase(i);
+	if (moveCaption) {
+		const auto caption = item->originalText();
+		const auto firstId = items.front().msgId;
+		if (const auto first = App::histItemById(firstId)) {
+			// We don't need to finishEdition() here, because the whole
+			// album will be rebuilt after one item was removed from it.
+			first->setText(caption);
+			refreshMediaCaption(first);
+		}
+	}
+}
+
 FileLoadResult::FileLoadResult(
 	TaskId taskId,
 	uint64 id,