How to inject GWT Activity#start with a specific Widget?

I have the following parent container:

public class ParentContainer extends Composite {
    // Contains a bunch of TextButtons (RedButton, GreenButton, etc.).
    private LayoutPanel buttonPanel;

    // When user clicks a TextButton inside the buttonPanel,
    // it changes the content of this contentPanel.
    private LayoutPanel contentPanel;
}

So when the user clicks one of the TextButtons inside the buttonPanel, the contentPanel's contents change. I am trying to get each TextButton click to be remembered in history, using the Activities/Places framework. So, if the user clicks the "Red", "Green" and "Blue" buttons respectively, the contentPanel will change three times, and then they can click the Back/Forward browser history buttons and keep moving back and forth in history (and "replaying" the button clicks over and over again, etc.).

I also have the following classes:

com.mywebapp
    MainModule.gwt.xml
com.mywebapp.client
    MainModule
com.mywebapp.client.places
    RedButtonPlace
    GreenButtonPlace
    BlueButtonPlace
    ... 1 place for all buttons
com.mywebapp.client.activities
    RedButtonActivity
    GreenButtonActivity
    BlueButtonActivity
    ... 1 activity for all buttons
com.mywebapp.client.ui
    ParentContainer
    RedButton
    GreenButton
    BlueButton
    BlackButton
    PurpleButton
    OrangeButton

I am planning on wiring things up such that:

• PlaceController.goTo(new RedButtonPlace()) eventually routes to the RedButtonActivity
• PlaceController.goTo(new GreenButtonPlace()) eventually routes to the GreenButtonActivity
• etc. (every button has a place and activity per its color)

What I'm stuck on is: if I call PlaceController.goTo(new RedButtonPlace()) from inside a RedButton click handler, how and where do I instruct RedButtonActivity to update contentPanel? For instance:

public class RedButton extends TextButton {
    // ... bunch of stuff, nevermind why I am extending TextButton
    // this is just to help me connect all the major dots of GWT!

    public RedButton() {
        this.addClickHandler(new ClickHandler() {
            public void onClick(ClickEvent event) {
                // If the RedButton is clicked, we want all the content in RedButtonActivity#RedButtonView
                // to go inside ParentContainer#contentPanel.
                PlaceController.goto(new RedButtonPlace());
            }
        });
    }
}

public class RedButtonActivity extends AbstractActivity {
    public interface RedButtonView extends IsWidget {
        // Whatever the RedButton expects to be able to display.
    }

    private RedButtonView view;

    @Override
    public void start(AcceptsOneWidget panel, EventBus eventBus) {
        // Probably injected via GIN.
        view = somehowInjectTheView();

        panel.setWidget(view);
    }
}

That last line is the key here: panel.setWidget(view). How do we make sure that panel is the ParentContainer#contentPanel? Thanks in advance!

Edit: Per one answer suggests, here is a code update:

public class ParentContainer extends Composite {
    // All the stuff that's up above in the first parent container.

    public ParentContainer() {
        super();

        // Again, via GIN.
        ActivityManager redButtonActivityManager = getSomehow();
        redButtonActivityManager.setDisplay(contentPanel);
    }
}

If this is the correct way, then I assume when the start(AcceptsOneWidget panel, EventBus eventBus) method is called, the redButtonActivityManager knows to inject the correct display for the panel argument?

You would pass the ParentContainer#contentPanel to the setDisplay() method of your ActivityManager as part of the manager's initialization.