/*
 * Copyright 2009 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 com.google.gwt.user.client.ui;

import com.google.gwt.dom.client.Document;
import com.google.gwt.dom.client.Element;
import com.google.gwt.dom.client.Style.Unit;
import com.google.gwt.layout.client.Layout;
import com.google.gwt.layout.client.Layout.Alignment;
import com.google.gwt.layout.client.Layout.AnimationCallback;
import com.google.gwt.layout.client.Layout.Layer;

/**
 * A panel that lays its children out in arbitrary
 * {@link com.google.gwt.layout.client.Layout.Layer layers} using the
 * {@link Layout} class.
 * 
 * <p>
 * This widget will <em>only</em> work in standards mode, which requires that
 * the HTML page in which it is run have an explicit &lt;!DOCTYPE&gt;
 * declaration.
 * </p>
 * 
 * <p>
 * <h3>Example</h3>
 * {@example com.google.gwt.examples.LayoutPanelExample}
 * </p>
 * 
 * <h3>Use in UiBinder Templates</h3>
 * <p>
 * LayoutPanel elements in {@link com.google.gwt.uibinder.client.UiBinder
 * UiBinder} templates lay out their children with arbitrary constraints, using
 * &lt;g:layer> elements. Each layer may have any of the following constraint
 * attributes specified as CSS length attributes: <code>left</code>,
 * <code>top</code>, <code>right</code>, <code>bottom</code>, <code>width</code>
 * , and <code>height</code>.
 * 
 * <p>
 * Precisely zero or two constraints are required for each axis (horizontal and
 * vertical). Specifying no constraints implies that the child should fill that
 * axis completely.
 * 
 * <p>
 * The valid sets of horizontal constraints are:
 * <dl>
 * <dt>(none)
 * <dd>Fill the parent's horizontal axis
 * <dt>left, width
 * <dd>Fixed width, positioned from parent's left edge
 * <dt>right, width
 * <dd>Fixed width, positioned from parent's right edge
 * <dt>left, right
 * <dd>Width implied by fixed distance from parent's left and right edges
 * </dl>
 * 
 * <p>
 * The valid sets of vertical constraints are:
 * <dl>
 * <dt>(none)
 * <dd>Fill the parent's vertical axis
 * <dt>top, height
 * <dd>Fixed height, positioned from parent's top edge
 * <dt>bottom, height
 * <dd>Fixed height, positioned from parent's bottom edge
 * <dt>top, bottom
 * <dd>Height implied by fixed distance from parent's top and bottom edges
 * </dl>
 * 
 * <p>
 * The values of constraint attributes can be any valid <a
 * href='http://www.w3.org/TR/CSS21/syndata.html#length-units'>CSS length</a>,
 * such as <code>1px</code>, <code>3em</code>, or <code>0</code> (zero lengths require no
 * units).
 * 
 * <p>
 * For example:
 * 
 * <pre>
 * &lt;g:LayoutPanel>
 *   &lt;!-- No constraints causes the layer to fill the parent -->
 *   &lt;g:layer>
 *     &lt;g:Label>Lorem ipsum...&lt;/g:Label>
 *   &lt;/g:layer>
 *   &lt;!-- Position horizontally 25% from each edge;
 *        Vertically 4px from the top and 10em tall. -->
 *   &lt;g:layer left='25%' right='25%' top='4px' height='10em'>
 *     &lt;g:Label>Header&lt;/g:Label>
 *   &lt;/g:layer>
 * &lt;/g:LayoutPanel>
 * </pre>
 */
public class LayoutPanel extends ComplexPanel implements AnimatedLayout,
    RequiresResize, ProvidesResize {

  private final Layout layout;
  private final LayoutCommand layoutCmd;

  /**
   * Creates an empty layout panel.
   */
  public LayoutPanel() {
    setElement(Document.get().createDivElement());
    layout = new Layout(getElement());
    layoutCmd = new LayoutCommand(layout);
  }

  /**
   * Adds a widget to this panel.
   * 
   * <p>
   * By default, each child will fill the panel. To build more interesting
   * layouts, set child widgets' layout constraints using
   * {@link #setWidgetLeftRight(Widget, double, Style.Unit, double, Style.Unit)}
   * and related methods.
   * </p>
   * 
   * @param widget the widget to be added
   */
  @Override
  public void add(Widget widget) {
    insert(widget, getWidgetCount());
  }

  public void animate(int duration) {
    animate(duration, null);
  }

  public void animate(final int duration, final AnimationCallback callback) {
    layoutCmd.schedule(duration, callback);
  }

  public void forceLayout() {
    layoutCmd.cancel();
    layout.layout();
    onResize();
  }

  /**
   * Gets the container element wrapping the given child widget.
   * 
   * @param child
   * @return the widget's container element
   */
  public Element getWidgetContainerElement(Widget child) {
    assertIsChild(child);
    return getLayer(child).getContainerElement();
  }

  /**
   * Inserts a widget before the specified index.
   * 
   * <p>
   * By default, each child will fill the panel. To build more interesting
   * layouts, set child widgets' layout constraints using
   * {@link #setWidgetLeftRight(Widget, double, Style.Unit, double, Style.Unit)}
   * and related methods.
   * </p>
   * 
   * <p>
   * Inserting a widget in this way has no effect on the DOM structure, but can
   * be useful for other panels that wrap LayoutPanel to maintain insertion
   * order.
   * </p>
   * 
   * @param widget the widget to be inserted
   * @param beforeIndex the index before which it will be inserted
   * @throws IndexOutOfBoundsException if <code>beforeIndex</code> is out of
   *           range
   */
  public void insert(Widget widget, int beforeIndex) {
    // Detach new child.
    widget.removeFromParent();

    // Logical attach.
    getChildren().insert(widget, beforeIndex);

    // Physical attach.
    Layer layer = layout.attachChild(widget.getElement(), widget);
    widget.setLayoutData(layer);

    // Adopt.
    adopt(widget);

    animate(0);
  }

  public void onResize() {
    for (Widget child : getChildren()) {
      if (child instanceof RequiresResize) {
        ((RequiresResize) child).onResize();
      }
    }
  }

  @Override
  public boolean remove(Widget w) {
    boolean removed = super.remove(w);
    if (removed) {
      layout.removeChild((Layer) w.getLayoutData());
    }
    return removed;
  }

  /**
   * Sets the child widget's bottom and height values.
   * 
   * @param child
   * @param bottom
   * @param bottomUnit
   * @param height
   * @param heightUnit
   */
  public void setWidgetBottomHeight(Widget child, double bottom,
      Unit bottomUnit, double height, Unit heightUnit) {
    assertIsChild(child);
    getLayer(child).setBottomHeight(bottom, bottomUnit, height, heightUnit);
    animate(0);
  }

  /**
   * Sets the child widget's horizontal position within its layer.
   * 
   * @param child
   * @param position
   */
  public void setWidgetHorizontalPosition(Widget child, Alignment position) {
    assertIsChild(child);
    getLayer(child).setChildHorizontalPosition(position);
    animate(0);
  }

  /**
   * Sets the child widget's left and right values.
   * 
   * @param child
   * @param left
   * @param leftUnit
   * @param right
   * @param rightUnit
   */
  public void setWidgetLeftRight(Widget child, double left, Unit leftUnit,
      double right, Unit rightUnit) {
    assertIsChild(child);
    getLayer(child).setLeftRight(left, leftUnit, right, rightUnit);
    animate(0);
  }

  /**
   * Sets the child widget's left and width values.
   * 
   * @param child
   * @param left
   * @param leftUnit
   * @param width
   * @param widthUnit
   */
  public void setWidgetLeftWidth(Widget child, double left, Unit leftUnit,
      double width, Unit widthUnit) {
    assertIsChild(child);
    getLayer(child).setLeftWidth(left, leftUnit, width, widthUnit);
    animate(0);
  }

  /**
   * Sets the child widget's right and width values.
   * 
   * @param child
   * @param right
   * @param rightUnit
   * @param width
   * @param widthUnit
   */
  public void setWidgetRightWidth(Widget child, double right, Unit rightUnit,
      double width, Unit widthUnit) {
    assertIsChild(child);
    getLayer(child).setRightWidth(right, rightUnit, width, widthUnit);
    animate(0);
  }

  /**
   * Sets the child widget's top and bottom values.
   * 
   * @param child
   * @param top
   * @param topUnit
   * @param bottom
   * @param bottomUnit
   */
  public void setWidgetTopBottom(Widget child, double top, Unit topUnit,
      double bottom, Unit bottomUnit) {
    assertIsChild(child);
    getLayer(child).setTopBottom(top, topUnit, bottom, bottomUnit);
    animate(0);
  }

  /**
   * Sets the child widget's top and height values.
   * 
   * @param child
   * @param top
   * @param topUnit
   * @param height
   * @param heightUnit
   */
  public void setWidgetTopHeight(Widget child, double top, Unit topUnit,
      double height, Unit heightUnit) {
    assertIsChild(child);
    getLayer(child).setTopHeight(top, topUnit, height, heightUnit);
    animate(0);
  }

  /**
   * Sets the child widget's vertical position within its layer.
   * 
   * @param child
   * @param position
   */
  public void setWidgetVerticalPosition(Widget child, Alignment position) {
    assertIsChild(child);
    getLayer(child).setChildVerticalPosition(position);
    animate(0);
  }

  /**
   * Shows or hides the given widget and its layer. This method explicitly
   * calls {@link UIObject#setVisible(boolean)} on the child widget and ensures
   * that its associated layer is shown/hidden.
   * 
   * @param child
   * @param visible
   */
  public void setWidgetVisible(Widget child, boolean visible) {
    assertIsChild(child);
    getLayer(child).setVisible(visible);
    child.setVisible(visible);
    animate(0);
  }

  @Override
  protected void onLoad() {
    layout.onAttach();
  }

  @Override
  protected void onUnload() {
    layout.onDetach();
  }

  /**
   * Gets the {@link Layout} instance associated with this widget. This is made
   * package-protected for use by {@link RootLayoutPanel}.
   * 
   * @return this widget's layout instance
   */
  Layout getLayout() {
    return layout;
  }

  private void assertIsChild(Widget widget) {
    assert (widget == null) || (widget.getParent() == this) : "The specified widget is not a child of this panel";
  }

  private Layout.Layer getLayer(Widget child) {
    assert child.getParent() == this : "The requested widget is not a child of this panel";
    return (Layout.Layer) child.getLayoutData();
  }
}