Handling ScrollEvent on TableView takes too much of the CPU Time - events

Question

I have a TableView named tableVerre and I want to have every row of it checked for a criteria ( stock column value ) and execute some code on them as I scroll so I wrote this code but it makes the program consume a lot of CPU time, I'm not familiar with Lambda expressions so is there a simpler way to write this ? :
tableVerre.addEventFilter(ScrollEvent.ANY, new EventHandler<ScrollEvent>() {
#Override
public void handle(ScrollEvent scrollEvent) {
stock.setCellFactory(column -> {
return new TableCell<VerreFX, Number>() {
#Override
protected void updateItem(Number item, boolean empty) {
super.updateItem(item, empty);
TableRow<VerreFX> currentRow = getTableRow();
if (empty || item == null) {
setText("");
setGraphic(null);
currentRow.setStyle(tableVerre.getStyle());
} else {
setText(getItem().toString());
}
if (!isEmpty()) {
if ((int) item == 0 && st.getVerresBOX()) currentRow.setStyle("-fx-background-color:lightcoral");
}
}
};
});
}
});

Answer 1

The table view will reuse cells as the user scrolls, and will automatically call updateItem on the cells when they are reused for new items. So you should set the cell factory only once, and then just let the table view take care of doing the job it is designed to do. You can set the cell factory in the initialize() method if you are using FXML, or just wherever you create the table and columns otherwise.
Your cell implementation isn't quite correct: because a cell may be reused to display any two different items arbitrarily, you need to account for all possible conditions. In your implementation, if a cell shows an item for which item.intValue()==0 and is then reused to show an item for which item.intValue() != 0, then the style will not be updated correctly.
Also note that you should "convert" a Number to an int by calling intValue().
TableColumn<VerreFX, Number> stock ;
// ...
stock.setCellFactory(column -> new TableCell<VerreFX, Number>() {
#Override
protected void updateItem(Number item, boolean empty) {
super.updateItem(item, empty);
TableRow<VerreFX> currentRow = getTableRow();
if (empty || item == null) {
setText("");
setGraphic(null);
currentRow.setStyle(tableVerre.getStyle());
} else {
setText(getItem().toString());
}
if (!isEmpty()) {
if (item.intValue() == 0 && st.getVerresBOX()) {
currentRow.setStyle("-fx-background-color:lightcoral");
} else {
currentRow.setStyle(tableVerre.getStyle());
}
}
}
});
You should be able to remove the scroll event handler entirely.

Answer 2

Firstly beside over CPU time using you didn't cover all the scrolling situations because if the user scrolls using the key Down/Up or using the scroll bar the scroll event will not be fired. So you have to add two more EventFilter, the first will handle the Scrolling using the Scrollbar.
tableVerre.addEventFilter(MouseEvent.MOUSE_CLICKED,(
MouseEvent event)->
{
if ((event.getTarget() instanceof TableColumnHeader) | event.isDragDetect()) {
System.err.println("Mouse Draged : " + event.toString());
stock.setCellFactory((TableColumn<VerreFX, Number> column) -> {
return new TableCell<VerreFX, Number>() {
#Override
protected void updateItem(Number item, boolean empty) {
super.updateItem(item, empty);
TableRow<VerreFX> currentRow = getTableRow();
if (empty || item == null) {
setText("");
setGraphic(null);
currentRow.setStyle(tableVerre.getStyle());
} else {
setText(getItem().toString());
}
if (!isEmpty()) {
if ((int) item == 0 && st.getVerresBOX()) {
currentRow.setStyle("-fx-background-color:lightcoral");
}
}
}
};
});
}
});
And the second one will handle the scrolling using the keyboard Key DOWN/UP.
tableVerre.addEventFilter(KeyEvent.KEY_PRESSED,new EventHandler<KeyEvent>(){
#Override
public void handle(KeyEvent event) {
if (event.getCode() == KeyCode.DOWN | event.getCode() == KeyCode.UP) {
stock.setCellFactory(column -> {
return new TableCell<VerreFX, Number>() {
#Override
protected void updateItem(Number item, boolean empty) {
super.updateItem(item, empty);
TableRow<VerreFX> currentRow = getTableRow();
if (empty || item == null) {
setText("");
setGraphic(null);
currentRow.setStyle(tableVerre.getStyle());
} else {
setText(getItem().toString());
}
if (!isEmpty()) {
if ((int) item == 0 && st.getVerresBOX()) {
currentRow.setStyle("-fx-background-color:lightcoral");
}
}
}
};
});
}
System.err.println("Key Pressed : " + event.toString());
}
});