isAvailable method
Checks if the message for the given messageIndex is already loaded.
Note that the messageIndex is expected to be based on
full messageSequence, where index 0 is newest message and
size-1 is the oldest message.
Implementation
bool isAvailable(int messageIndex) {
final index = messages.length - messageIndex - 1;
return index >= 0 && messageIndex >= 0;
}