cc.alcina.framework.gwt.client.widget.LazyPanel.java Source code

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Here is the source code for cc.alcina.framework.gwt.client.widget.LazyPanel.java

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/*
 * Copyright 2008 Google Inc.
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */
package cc.alcina.framework.gwt.client.widget;

import com.google.gwt.event.logical.shared.BeforeSelectionEvent;
import com.google.gwt.event.logical.shared.BeforeSelectionHandler;
import com.google.gwt.user.client.ui.SimplePanel;
import com.google.gwt.user.client.ui.TabPanel;
import com.google.gwt.user.client.ui.Widget;

/**
 * Convenience class to help lazy loading. A LazyPanel is only actually rendered
 * when setVisible(true) or ensureWidget is called.
 * 
 * @param <T>
 *            widget type
 */
public abstract class LazyPanel<T extends Widget> extends SimplePanel {
    public static void addBeforeSelectionHandler(final CustomisableTabPanel panel) {
        panel.addBeforeSelectionHandler(new BeforeSelectionHandler<Integer>() {
            public void onBeforeSelection(BeforeSelectionEvent<Integer> event) {
                Integer tabIndex = event.getItem();
                Widget widget = panel.getWidget(tabIndex);
                if (widget instanceof LazyPanel) {
                    ((LazyPanel) widget).ensureWidget();
                }
            }
        });
    }

    /**
     * Adds a tab listener to ensure that any {@link LazyPanel} instances are
     * loaded when a tab panel selects the lazy panel's tab.
     * 
     * @param panel
     *            to add listener to
     */
    public static void addBeforeSelectionHandler(final TabPanel panel) {
        panel.addBeforeSelectionHandler(new BeforeSelectionHandler<Integer>() {
            public void onBeforeSelection(BeforeSelectionEvent<Integer> event) {
                Integer tabIndex = event.getItem();
                Widget widget = panel.getWidget(tabIndex);
                if (widget instanceof LazyPanel) {
                    ((LazyPanel) widget).ensureWidget();
                }
            }
        });
    }

    public LazyPanel() {
    }

    /**
     * Create the widget contained within the {@link LazyPanel}.
     * 
     * @return the lazy widget
     */
    public abstract T createWidget();

    /**
     * Ensures the existence of the {@link LazyPanel} widget.
     * 
     * @return the widget
     */
    public T ensureWidget() {
        T widget = (T) getWidget();
        if (widget == null) {
            widget = createWidget();
            setWidget(widget);
        }
        return widget;
    }

    /**
     * Gets the panel's child widget. Will not create the widget, call
     * <code>ensureWidget</code> to ensure that the widget exists.
     * 
     * @return the child widget, or <code>null</code> if none is present
     */
    @SuppressWarnings("unchecked")
    public T getWidget() {
        return (T) super.getWidget();
    }

    @Override
    /**
     * Sets whether this object is visible. Creates the widget if necessary.
     * 
     * @param visible
     *            <code>true</code> to show the object, <code>false</code> to
     *            hide it
     */
    public void setVisible(boolean visible) {
        if (visible) {
            ensureWidget();
        }
        super.setVisible(visible);
    }
}