从 GWT 中的不同小部件触发处理程序事件

Question

I got two ListBoxes, assume customerListBox and timeListBox , and Label resultLabel . When timeListBox 's value is changed, it fires its ValueChangeHandler (let's call it recalculateValueHandler ) which recalculates result and put it into resultLabel . I need this to work vice versa - when customerListBox 's value is changed I need to recalculate new result for same time value but different customer. So I'd need something like customerListBox.onValueChange(fire(recalculateValueHandler)) , hope you do understand. Is there anything that could work for me this way? I try to avoid duplicating pretty much same code into both handlers.

Answer 1

You just need three things

• declare all elements at top level so they are accessible across all methods
• create a method that recalculates the value, let's call it recalculateValue
• add ChangeHandler s for both ListBox es ( ListBox don't have ValueChangeHandler ) that will call recalculateValue method.

Working example:

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.event.dom.client.ChangeEvent;
import com.google.gwt.event.dom.client.ChangeHandler;
import com.google.gwt.user.client.ui.Label;
import com.google.gwt.user.client.ui.ListBox;
import com.google.gwt.user.client.ui.RootPanel;

public class Test implements EntryPoint {

    // elements declared at top level are accessible across all methods
    private ListBox customerListBox;
    private ListBox timeListBox;
    private Label resultLabel;

    @Override
    public void onModuleLoad() {
        // add some data
        customerListBox = new ListBox();
        for(int i = 0; i < 10; i++)
            customerListBox.addItem("Customer " + (i + 1));

        // add some data
        timeListBox = new ListBox();
        for(int i = 0; i < 10; i++)
            timeListBox.addItem("Time " + (i + 1));

        resultLabel = new Label();
    
        // recalculateValue when customerListBox changes
        customerListBox.addChangeHandler(new ChangeHandler() {
            @Override
            public void onChange(ChangeEvent event) {
                recalculateValue();
            }
        });
    
        // recalculateValue when timeListBox changes
        timeListBox.addChangeHandler(new ChangeHandler() {
            @Override
            public void onChange(ChangeEvent event) {
                recalculateValue();
            }
        });

        // initial result (optional)
        recalculateValue();

        // show elements
        RootPanel.get().clear();
        RootPanel.get().add(customerListBox);
        RootPanel.get().add(timeListBox);
        RootPanel.get().add(resultLabel);
    }

    private void recalculateValue() {
        // use values from ListBoxes
        resultLabel.setText(customerListBox.getSelectedValue() + " / " + timeListBox.getSelectedValue());
    }
}

---

Notice that both handlers ares identical, so you can create only one handler and use it for both ListBox es like that:

ChangeHandler handler = new ChangeHandler() {
    @Override
    public void onChange(ChangeEvent event) {
        recalculateValue();
    }
};

// recalculateValue when customerListBox changes
customerListBox.addChangeHandler(handler);

// recalculateValue when timeListBox changes
timeListBox.addChangeHandler(handler);