path: root/libjava/classpath/javax/swing/plaf/ComponentUI.java

/* ComponentUI.java --
   Copyright (C) 2002, 2003, 2004  Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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permission to link this library with independent modules to produce an
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package javax.swing.plaf;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;

import javax.accessibility.Accessible;
import javax.swing.JComponent;

/**
 * The abstract base class for all delegates that provide the
 * pluggable look and feel for Swing components. User applications
 * should not need to access this class; it is internal to Swing
 * and the look-and-feel implementations.
 *
 * <p><img src="doc-files/ComponentUI-1.png" width="700" height="550"
 * alt="[UML diagram illustrating the architecture for pluggable
 * look and feels]" /></p>
 *
 * <p>Components such as {@link javax.swing.JSlider} do not directly
 * implement operations related to the look and feel of the user
 * interface, such as painting or layout. Instead, they use a delegate
 * object for all such tasks. In the case of <code>JSlider</code>, the
 * user interface would be provided by some concrete subclass of
 * {@link javax.swing.plaf.SliderUI}.
 *
 * <p>Soon after its creation, a <code>ComponentUI</code> will be sent
 * an {@link #installUI} message. The <code>ComponentUI</code> will
 * react by setting properties such as the border or the background
 * color of the <code>JComponent</code> for which it provides its
 * services. Soon before the end of its lifecycle, the
 * <code>ComponentUI</code> will receive an {@link #uninstallUI}
 * message, at which time the <code>ComponentUI</code> is expected to
 * undo any changes.</p>
 *
 * <p>Note that the <code>ui</code> of a <code>JComponent</code>
 * changes whenever the user switches between look and feels.  For
 * example, the <code>ui</code> property of a <code>JSlider</code>
 * could change from an instance of <code>MetalSliderUI</code> to an
 * instance of <code>FooSliderUI</code>. This switch can happen at any
 * time, but it will always be performed from inside the Swing thread.</p>
 *
 * @author Sascha Brawer (brawer@dandelis.ch)
 */
public abstract class ComponentUI
{
  /**
   * Constructs a new UI delegate.
   */
  public ComponentUI()
  {
    // Nothing to do here.
  }


  /**
   * Sets up the specified component so it conforms the the design
   * guidelines of the implemented look and feel. When the look and
   * feel changes, a <code>ComponentUI</code> delegate is created.
   * The delegate object then receives an <code>installUI</code>
   * message.
   *
   * <p>This method should perform the following tasks:</p>
   *
   * <ul>
   * <li>Set visual properties such as borders, fonts, colors, or
   *     icons. However, no change should be performed for those
   *     properties whose values have been directly set by the client
   *     application. To allow the distinction, LookAndFeels are expected
   *     to use values that implement the {@link UIResource} marker
   *     interface, such as {@link BorderUIResource} or {@link
   *     ColorUIResource}.</li>
   * <li>If necessary, install a {@link java.awt.LayoutManager}.</li>
   * <li>Embed custom sub-components. For instance, the UI delegate
   *     for a {@link javax.swing.JSplitPane} might install a special
   *     component for the divider.</li>
   * <li>Register event listeners.</li>
   * <li>Set up properties related to keyborad navigation, such as
   *     mnemonics or focus traversal policies.</li>
   * </ul>
   *
   * @param c the component for which this delegate will provide
   *        services.
   *
   * @see #uninstallUI
   * @see javax.swing.JComponent#setUI
   * @see javax.swing.JComponent#updateUI
   */
  public void installUI(JComponent c)
  {
    // The default implementation does not change any properties.
  }


  /**
   * Puts the specified component into the state it had before
   * {@link #installUI} was called.
   *
   * @param c the component for which this delegate has provided
   *        services.
   *
   * @see #installUI
   * @see javax.swing.JComponent#setUI
   * @see javax.swing.JComponent#updateUI
   */
  public void uninstallUI(JComponent c)
  {
    // The default implementation does not change any properties.
  }


  /**
   * Paints the component according to the design guidelines
   * of the look and feel. Most subclasses will want to override
   * this method.
   *
   * @param g the graphics for painting.
   *
   * @param c the component for which this delegate performs
   *          services.
   */
  public void paint(Graphics g, JComponent c)
  {
    // Nothing is done here. This method is meant to be overridden by
    // subclasses.
  }


  /**
   * Fills the specified component with its background color
   * (unless the <code>opaque</code> property is <code>false</code>)
   * before calling {@link #paint}.
   *
   * <p>It is unlikely that a subclass needs to override this method.
   * The actual rendering should be performed by the {@link #paint}
   * method.
   *
   * @param g the graphics for painting.
   *
   * @param c the component for which this delegate performs
   *          services.
   *
   * @see #paint
   * @see javax.swing.JComponent#paintComponent
   */
  public void update(Graphics g, JComponent c)
  {
    if (c.isOpaque())
      {
        Color oldColor = g.getColor();
        g.setColor(c.getBackground());
        g.fillRect(0, 0, c.getWidth(), c.getHeight());
        g.setColor(oldColor);
      }
    paint(g, c);
  }

  /**
   * Determines the preferred size of a component. The default
   * implementation returns <code>null</code>, which means that
   * <code>c</code>&#x2019;s layout manager should be asked to
   * calculate the preferred size.
   *
   * @param c the component for which this delegate performs services.
   *
   * @return the preferred size, or <code>null</code> to indicate that
   *         <code>c</code>&#x2019;s layout manager should be asked
   *         for the preferred size.
   */
  public Dimension getPreferredSize(JComponent c)
  {
    return null;
  }


  /**
   * Determines the minimum size of a component. The default
   * implementation calls {@link #getPreferredSize}, but subclasses
   * might want to override this.
   *
   * @param c the component for which this delegate performs services.
   *
   * @return the minimum size, or <code>null</code> to indicate that
   *         <code>c</code>&#x2019;s layout manager should be asked
   *         to calculate the minimum size.
   */
  public Dimension getMinimumSize(JComponent c)
  {
    return getPreferredSize(c);
  }


  /**
   * Determines the maximum size of a component. The default
   * implementation calls {@link #getPreferredSize}, but subclasses
   * might want to override this.
   *
   * @param c the component for which this delegate performs services.
   *
   * @return the maximum size, or <code>null</code> to indicate that
   *         <code>c</code>&#x2019;s layout manager should be asked
   *         to calculate the maximum size.
   */
  public Dimension getMaximumSize(JComponent c)
  {
    return getPreferredSize(c);
  }


  /**
   * Determines whether a click into the component at a specified
   * location is considered as having hit the component. The default
   * implementation checks whether the point falls into the
   * component&#x2019;s bounding rectangle. Some subclasses might want
   * to override this, for example in the case of a rounded button.
   *
   * @param c the component for which this delegate performs services.
   *
   * @param x the x coordinate of the point, relative to the local
   *        coordinate system of the component. Zero would be be
   *        component&#x2019;s left edge, irrespective of the location
   *        inside its parent.
   *
   * @param y the y coordinate of the point, relative to the local
   *        coordinate system of the component. Zero would be be
   *        component&#x2019;s top edge, irrespective of the location
   *        inside its parent.
   */
  public boolean contains(JComponent c, int x, int y)
  {
    /* JComponent.contains calls the ui delegate for hit
     * testing. Therefore, endless mutual recursion would result if we
     * called c.contains(x, y) here.
     *
     * The previous Classpath implementation called the deprecated
     * method java.awt.Component.inside. In the Sun implementation, it
     * can be observed that inside, other than contains, does not call
     * the ui delegate.  But that inside() behaves different to
     * contains() clearly is in violation of the method contract, and
     * it is not something that a good implementation should rely upon
     * -- even if Classpath ends up being forced to replicate this
     * apparent bug of the Sun implementation.
     */
    return (x >= 0) && (x < c.getWidth())
      && (y >= 0) && (y < c.getHeight());
  }


  /**
   * Creates a delegate object for the specified component.  Users
   * should use the <code>createUI</code> method of a suitable
   * subclass. The implementation of <code>ComponentUI</code>
   * always throws an error.
   *
   * @param c the component for which a UI delegate is requested.
   */
  public static ComponentUI createUI(JComponent c)
  {
    throw new Error(
      "javax.swing.plaf.ComponentUI does not implement createUI; call "
      + "createUI on a subclass.");
  }


  /**
   * Counts the number of accessible children in the component.  The
   * default implementation delegates the inquiry to the {@link
   * javax.accessibility.AccessibleContext} of <code>c</code>.
   *
   * @param c the component whose accessible children
   *        are to be counted.
   */
  public int getAccessibleChildrenCount(JComponent c)
  {
    return c.getAccessibleContext().getAccessibleChildrenCount();
  }


  /**
   * Returns the specified accessible child of the component. The
   * default implementation delegates the inquiry to the {@link
   * javax.accessibility.AccessibleContext} of <code>c</code>.
   *
   * @param i the index of the accessible child, starting at zero.
   *
   * @param c the component whose <code>i</code>-th accessible child
   *        is requested.
   */
  public Accessible getAccessibleChild(JComponent c, int i)
  {
    return c.getAccessibleContext().getAccessibleChild(i);
  }
}