import org.eclipse.swt.*;
import org.eclipse.swt.layout.*;
import org.eclipse.swt.widgets.*;

/**
 * @author Thomas Singer
 */
public class LazyTableLoading {

	public static void main(String[] args) {
		final Display display = new Display();

		final Shell shell = new Shell(display);
		shell.setLayout(new FillLayout());

		final Table table = new Table(shell, SWT.BORDER | SWT.V_SCROLL);
		table.setHeaderVisible(true);

		final TableColumn column = new TableColumn(table, SWT.LEFT);
		column.setText("Column");
		column.setWidth(150);

		for (int i = 0; i < 4; i++) {
			createItem(table);
		}

		final boolean[] loading = new boolean[1];
		table.addListener(SWT.Paint, new Listener() {
			@Override
			public void handleEvent(Event event) {
				if (table.isDisposed()) {
					return;
				}

				if (!shouldLoad()) {
					return;
				}

				if (loading[0]) {
					return;
				}

				loading[0] = true;
				display.timerExec(1000, () -> {
					for (int i = 0; i < 50; i++) {
						if (table.getItemCount() == 300) {
							break;
						}

						createItem(table);
					}
					loading[0] = false;
				});
			}

			private boolean shouldLoad() {
				final ScrollBar scrollBar = table.getVerticalBar();
				if (!scrollBar.isVisible()) {
					return true;
				}

				final int maximum = scrollBar.getMaximum();
				if (maximum == 0) {
					return true;
				}

				final int scrollBarPosition = scrollBar.getSelection() + scrollBar.getThumb();
				System.out.println(scrollBarPosition + " of " + maximum);
				return scrollBarPosition > maximum * 0.75;
			}
		});

		shell.setSize(400, 300);
		shell.open();

		while (!shell.isDisposed()) {
			if (!display.readAndDispatch()) {
				display.sleep();
			}
		}

		display.dispose();
	}

	private static void createItem(Table table) {
		final TableItem item = new TableItem(table, SWT.LEFT);
		item.setText(0, "Row " + table.getItemCount());
	}
}