GWT view navigation basic example - gwt

Question

I have a Navigation panel with Back/Forward buttons ( Function not important, just want to get my head around swapping Panels in and out of a GWT page )
NavigationButtonsPanel
public class NavigationButtonsPanel extends HTMLPanel
{
private Button btnA = new Button("Go back");
private Button btnB = new Button("Go forward");
public NavigationButtonsPanel( )
{
super("Navigation Panel");
add(btnA);
add(btnB);
}
public void setBackHandler( ClickHandler handler )
{
btnA.addClickHandler(handler);
}
public void setForwardHandler( ClickHandler handler )
{
btnB.addClickHandler(handler);
}
}
I then have 3 basic panels, A B and C which use this. My Navigation from A-B-C and back to B-A works fine.
I now simulate a user changing something before they go to Panel B. In this case a Label in PanelB.
My final app will obviously be more complex than this but would this be typical of how an app would work. You modify some fields in a Panel through a users interaction in the same panel or a different one, load the new panel, more interaction, load in different panel etc?
I will only want a user to use a very well defined navigation path so there is only one path to a certain panel.
PanelA
public class PanelA extends HTMLPanel
{
private static PanelA panel;
private PanelA()
{
super("Panel A");
final RootPanel rootPanel = RootPanel.get();
NavigationButtonsPanel btnPanel = new NavigationButtonsPanel();
btnPanel.setForwardHandler(new ClickHandler() {
public void onClick(ClickEvent event)
{
rootPanel.clear();
PanelB.setMessageFromPreviousPage("Message set in Panel AAAAA");
rootPanel.add( PanelB.getInstance() );
}
});
add( btnPanel );
}
public static PanelA getInstance()
{
if (panel == null)
{
panel = new PanelA();
}
return panel;
}
}
PanelB
public class PanelB extends HTMLPanel
{
private static PanelB panel;
private static String messageFromPreviousPage;
private PanelB()
{
super("Panel B");
final RootPanel rootPanel = RootPanel.get();
Label messageLabel = new Label(messageFromPreviousPage);
NavigationButtonsPanel btnPanel = new NavigationButtonsPanel();
btnPanel.setBackHandler(new ClickHandler() {
public void onClick(ClickEvent event)
{
rootPanel.clear();
rootPanel.add( PanelA.getInstance() );
}
});
btnPanel.setForwardHandler(new ClickHandler() {
public void onClick(ClickEvent event)
{
rootPanel.clear();
rootPanel.add( PanelC.getInstance() );
}
});
add( btnPanel );
add(messageLabel);
}
public static PanelB getInstance()
{
if (panel == null)
{
panel = new PanelB();
}
return panel;
}
public static void setMessageFromPreviousPage(
String message)
{
messageFromPreviousPage = message;
}
}
PanelC
public class PanelC extends HTMLPanel
{
private static PanelC panel;
private PanelC()
{
super("Panel C");
final RootPanel rootPanel = RootPanel.get();
NavigationButtonsPanel btnPanel = new NavigationButtonsPanel();
btnPanel.setBackHandler(new ClickHandler() {
public void onClick(ClickEvent event)
{
rootPanel.clear();
PanelB.setMessageFromPreviousPage("Message set in Panel CCCCCC");
rootPanel.add( PanelB.getInstance() );
}
});
add( btnPanel );
}
public static PanelC getInstance()
{
if (panel == null)
{
panel = new PanelC();
}
return panel;
}
}

Answer 1

Any idea why Panel B does not update its UI?
Because when first time PanelB instance created using PanelB.getInstance() , then messageLabel created and it also set value. while you are calling second time same method from PanelC after changing the message, At that time, it's using same instance of PanelB instead of new object. and as per your coding, you are changing the value of messageFromPreviousPage variable instead of label text. so you didn't find any changes on UI.
If you want to change the label text, then declare messageLabel as a globel variable static and change label. i.e.
private static Label messageLabel;
private PanelB()
{
//your code
}
public static PanelB getInstance()
{
if (panel == null)
{
panel = new PanelB();
}
else
{
messageLabel.setText(messageFromPreviousPage); //Require to set Text Again
}
return panel;
}