[React-Native] Text Input in a FlatList, can't modify value

Hello.
I’m currently trying to make my own board with some data that I fetch in my componentWillMount(), then, I store this data in a state. Until here, all is normal, I can display my data with some FlatList. But, in one of them, I use a TextInput to change a quantity value.
When the value is null, I can modify it without any problem. But, when there is a value (store with the fetch), I can’t modify it. When I try, the TextInput is replacing it by the default value. I don’t understand because in my onChangeText, I’m modifying the value in the array that I use.
Someone got an idea? Thanks ! (Sorry for my english :rofl:)

Here is my FlatList code :

                       <FlatList
                          data={state.tableData}
                          keyExtractor={item => item.product_id + ""}
                          renderItem={({ item, index }) =>
                          <View style={[styles.columnRow, {backgroundColor: "#dff2ff"}]}>
                          <TextInput
                            style={styles.textInput}
                            maxLength={10}
                            textAlign={'center'}
                            keyboardType={'numeric'}
                            returnKeyType="next"
                            blurOnSubmit={false}
                            onChangeText={text => {
                              let { tableData } = this.state;
                              let newQte = '';
                              let numbers = '0123456789';

                              for (var i=0; i < text.length; i++) {
                                  if(numbers.indexOf(text[i]) > -1 ) {
                                      newQte = newQte + text[i];
                                  }
                                  else {
                                      alert("Veuillez saisir uniquement des chiffres.");
                                  }
                              }
                              tableData[index].quantity = newQte;
                              this.setState({
                                tableData,
                              });

                            }}
                           value={item.quantity}
                          />
                          </View>}
                          scrollEnabled={false}
                        />