How to fire event when scrolling up,javafx - javafx-8

Question

I am developing chat application on java and i want to use function used by facebook to retrieve chat history when user scroll up. I tried "on Scroll" action which fire event whenever scroll reach top or down of scroll bar. Now i want to Fire action event only when scroll bar reach top like in facbook chat box.

Answer 1

Assuming that you have a ScrollPane, you can observe vvalueProperty of it:
scrollPane.vvalueProperty().addListener(
(ObservableValue<? extends Number> observable, Number oldValue, Number newValue) -> {
if(newValue.doubleValue() == 0){
System.out.println( "AT TOP" );
// load more items
}
});
and you may also scroll to bottom with
scrollPane.setVvalue( scrollPane.getVmax() );
initially.

Answer 2

Here is the example of what you want. I hope this will help you.
import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.ScrollPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
public class ScrollUtil extends Application
{
private boolean scrollToBottom = false;
private boolean scrollToTop = false;
public static void main(String[] args)
{
launch(args);
}
#Override
public void start(final Stage stage) throws Exception
{
final VBox root = new VBox();
final ScrollPane scrollPane = new ScrollPane();
final VBox vBox = new VBox();
vBox.setAlignment(Pos.BOTTOM_CENTER);
scrollPane.setContent(vBox);
scrollPane.setHbarPolicy(ScrollPane.ScrollBarPolicy.NEVER);
Button button = new Button("add");
Button button3 = new Button("scroll up");
button3.setOnAction(new EventHandler<ActionEvent>()
{
#Override
public void handle(ActionEvent actionEvent)
{
scrollToTop = true;
scrollPane.setVvalue(scrollPane.getVmin());
System.out.println("vmin= "+scrollPane.getVmin() + "; vmax = " + scrollPane.getVmax() + "; vvalue" + scrollPane.getVvalue());
}
});
button.setOnAction(new EventHandler<ActionEvent>()
{
#Override
public void handle(ActionEvent actionEvent)
{
vBox.getChildren().add(new Label("hallo"));
scrollToBottom = true;
//System.out.println(scrollPane.getVmin() + "; max = " + scrollPane.getVmax() + "; " + scrollPane.getVvalue());
}
});
scrollPane.setVvalue(scrollPane.getVmax());
scrollPane.vvalueProperty().addListener(new ChangeListener<Number>()
{
#Override
public void changed(ObservableValue<? extends Number> observable, Number oldValue, Number newValue)
{
if(scrollToBottom)
{
scrollPane.setVvalue(scrollPane.getVmax());
scrollToBottom = false;
}
if(newValue.doubleValue() ==0)
{
vBox.getChildren().add(new Label("hallo"));
System.out.println("min value ="+scrollPane.getVmin());
//scrollPane.setVvalue(scrollPane.getVmin());
scrollToTop = true;
System.out.println("now get the chat history");
}
}
});
Button button2 = new Button("scroll");
button2.setOnAction(new EventHandler<ActionEvent>()
{
#Override
public void handle(ActionEvent actionEvent)
{
scrollPane.setVvalue(Double.MAX_VALUE);
}
});
root.getChildren().addAll(scrollPane, button, button2, button3);
Scene scene = new Scene(root);
stage.setScene(scene);
stage.show();
}
}