Flutter: Get the position of all the letters. (Game) - flutter

Question

I'm making a game, but I can't change the letter that matches more than once.
I don't know what I'm doing wrong, I hope someone can help me, thanks.
I leave a demonstration video, the result of the word is DOMINGO
The problem I perceive is that when I press the key or it is assigned only at the beginning.
When it must be completed with all possible matches.
YouTube Video Here
This is my dart function.
String word = 'domingo';
String key = 'o'; // OnTap button "O"
List<Word> positionsFound = List<Word>();
_word(){
String wordEmpty = '';
for(int i=0; i<word.length; i++){
if(key.isEmpty){
wordEmpty = wordEmpty+'_';
} else {
wordEmpty = wordEmpty+'_';
int position = word.toLowerCase().indexOf(key.toLowerCase());
if(position == i){
positionsFound.add(Word(key, position));
print(position); // The position must be 1 and 6, it only gives me 1.
}
}
}
for(int i=0; i<positionsFound.length; i++){
wordEmpty = wordEmpty.replaceFirst('_', positionsFound[i].key, positionsFound[i].position);
}
return wordEmpty;
}
class Word {
String key;
int position;
Word(this.key, this.position);
}

Answer 1

After a while, I realized that indexof allows you to start from the last search you performed.
if(key.isNotEmpty){
int position = word.toLowerCase().indexOf(key.toLowerCase());
while(position != -1){
if(position != -1){
positionsFound.add(Word(key, position));
}
position = word.toLowerCase().indexOf(key.toLowerCase(), position+1);
}
}