form.java

j2me polish学习的经典代码

//#condition polish.usePolishGui

// generated by de.enough.doc2java.Doc2Java (www.enough.de) on Sat Dec 06 15:06:44 CET 2003
/*
 * Copyright (c) 2003, 2004 Robert Virkus / Enough Software
 *
 * This file is part of J2ME Polish.
 *
 * J2ME Polish is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 * 
 * J2ME Polish is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with J2ME Polish; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 * 
 * Commercial licenses are also available, please
 * refer to the accompanying LICENSE.txt or visit
 * http://www.j2mepolish.org for details.
 */
package de.enough.polish.ui;

import de.enough.polish.util.ArrayList;

import javax.microedition.lcdui.Image;

/**
 * A <code>Form</code> is a <code>Screen</code> that contains
 * an arbitrary mixture of items: images,
 * read-only text fields, editable text fields, editable date fields, gauges,
 * choice groups, and custom items. In general, any subclass of the
 * <A HREF="../../../javax/microedition/lcdui/Item.html"><CODE>Item</CODE></A> class may be contained within a form.
 * The implementation handles layout, traversal, and scrolling.
 * The entire contents of the <code>Form</code> scrolls together.
 * 
 * <h2>Item Management</h2>
 * <p>
 * The items contained within a <code>Form</code> may be edited
 * using append, delete,
 * insert, and set methods.  <code>Items</code> within a
 * <code>Form</code> are referred to by their
 * indexes, which are consecutive integers in the range from zero to
 * <code>size()-1</code>,
 * with zero referring to the first item and <code>size()-1</code>
 * to the last item.  </p>
 * 
 * <p> An item may be placed within at most one
 * <code>Form</code>. If the application
 * attempts to place an item into a <code>Form</code>, and the
 * item is already owned by this
 * or another <code>Form</code>, an
 * <code>IllegalStateException</code> is thrown.
 * The application must
 * remove the item from its currently containing <code>Form</code>
 * before inserting it into
 * the new <code>Form</code>. </p>
 * 
 * <p> If the <code>Form</code> is visible on the display when
 * changes to its contents are
 * requested by the application, updates to the display take place as soon
 * as it is feasible for the implementation to do so.
 * Applications need not take any special action to refresh a
 * <code>Form's</code> display
 * after its contents have been modified. </p>
 * 
 * <a name="layout"></a>
 * <h2>Layout</h2>
 * 
 * <p>Layout policy in <code>Form</code> is organized around
 * rows. Rows are typically
 * related to the width of the screen, respective of margins, scroll bars, and
 * such.  All rows in a particular <code>Form</code> will have the
 * same width.  Rows do not
 * vary in width based on the <code>Items</code> contained within
 * the <code>Form</code>, although they
 * may all change width in certain circumstances, such as when a scroll bar
 * needs to be added or removed. <code>Forms</code> generally do not scroll
 * horizontally.</p>
 * 
 * <p><code>Forms</code> grow vertically and scroll vertically as
 * necessary. The height
 * of a <code>Form</code> varies depending upon the number of rows
 * and the height of
 * each row. The height of each row is determined by the items that are
 * positioned on that row. Rows need not all have the same height.
 * Implementations may also vary row heights to provide proper padding or
 * vertical alignment of <code>Item</code> labels.</p>
 * 
 * <p>An implementation may choose to lay out <code>Items</code> in a
 * left-to-right or right-to-left direction depending upon the language
 * conventions in use.  The same choice of layout direction must apply to all
 * rows within a particular <code>Form</code>.</p>
 * 
 * <p>Prior to the start of the layout algorithm, the
 * <code>Form</code> is considered to
 * have one empty row at the top. The layout algorithm considers each Item
 * in turn, starting at <code>Item</code> zero and proceeding in
 * order through each <code>Item</code>
 * until the last <code>Item</code> in the <code>Form</code>
 * has been processed.
 * If the layout direction (as described above) is left-to-right, the
 * beginning of the row is the left edge of the <code>Form</code>.  If the
 * layout direction is right-to-left, the beginning of the row is the right
 * edge of the <code>Form</code>.  <code>Items</code> are laid out at the
 * beginning of each row, proceeding across each row in the chosen layout
 * direction, packing as many <code>Items</code> onto each row as will fit,
 * unless a condition occurs that causes the packing of a row to be terminated
 * early.
 * A new row is then added, and
 * <code>Items</code> are packed onto it
 * as described above. <code>Items</code> are packed onto rows,
 * and new rows are added
 * below existing rows as necessary until all <code>Items</code>
 * have been processed by
 * the layout algorithm.</p>
 * 
 * <p>The layout algorithm has a concept of a <em>current alignment</em>.
 * It can have the value <code>LAYOUT_LEFT</code>,
 * <code>LAYOUT_CENTER</code>, or <code>LAYOUT_RIGHT</code>.
 * The value of the current alignment at the start of the layout algorithm
 * depends upon the layout direction in effect for this <code>Form</code>.  If
 * the layout direction is left-to-right, the initial alignment value must be
 * <code>LAYOUT_LEFT</code>.  If the layout direction is right-to-left, the
 * initial alignment value must be <code>LAYOUT_RIGHT</code>.
 * The current alignment changes when the layout
 * algorithm encounters an <code>Item</code> that has one of the layout
 * directives <code>LAYOUT_LEFT</code>, <code>LAYOUT_CENTER</code>, or
 * <code>LAYOUT_RIGHT</code>.  If none of these directives is present on an
 * <code>Item</code>, the current layout directive does not change.  This
 * rule has the effect of grouping the contents of the
 * <code>Form</code> into sequences of consecutive <code>Items</code>
 * sharing an alignment value.  The alignment value of each <code>Item</code>
 * is maintained internally to the <code>Form</code> and does not affect the
 * <code>Items'</code> layout value as reported by the
 * <A HREF="../../../javax/microedition/lcdui/Item.html#getLayout()"><CODE>Item.getLayout</CODE></A> method.</p>
 * 
 * <p>The layout algorithm generally attempts to place an item on the same
 * row as the previous item, unless certain conditions occur that cause a
 * &quot;row break.&quot; When there is a row break, the current item
 * will be placed
 * at the beginning of a new row instead of being placed after
 * the previous item, even if there is room.</p>
 * 
 * <p>A row break occurs before an item if any of the following
 * conditions occurs:</p>
 * 
 * <ul>
 * <li>the previous item has a row break after it;</li>
 * <li>it has the <code>LAYOUT_NEWLINE_BEFORE</code> directive; or</li>
 * <li>it is a <code>StringItem</code> whose contents starts with
 * &quot;\n&quot;;</li>
 * <li>it is a
 * <code>ChoiceGroup</code>, <code>DateField</code>,
 * <code>Gauge</code>, or a <code>TextField</code>, and the
 * <code>LAYOUT_2</code> directive is not set; or</li>
 * <li>this <code>Item</code> has a <code>LAYOUT_LEFT</code>,
 * <code>LAYOUT_CENTER</code>, or <code>LAYOUT_RIGHT</code> directive
 * that differs from the <code>Form's</code> current alignment.</li>
 * </ul>
 * 
 * <p>A row break occurs after an item if any of the following
 * conditions occurs:</p>
 * 
 * <ul>
 * <li>it is a <code>StringItem</code> whose contents ends with
 * &quot;\n&quot;; or</li>
 * <li>it has the <code>LAYOUT_NEWLINE_AFTER</code> directive; or</li>
 * <li>it is a
 * <code>ChoiceGroup</code>, <code>DateField</code>,
 * <code>Gauge</code>, or a <code>TextField</code>, and the
 * <code>LAYOUT_2</code> directive is not set.</li>
 * </ul>
 * 
 * <p>The presence of the <code>LAYOUT_NEWLINE_BEFORE</code> or
 * <code>LAYOUT_NEWLINE_AFTER</code> directive does not cause
 * an additional row break if there is one already present.  For example,
 * if a <code>LAYOUT_NEWLINE_BEFORE</code> directive appears on a
 * <code>StringItem</code> whose contents starts with &quot;\n&quot;,
 * there is only a single row break.  A similar rule applies with a
 * trailing &quot;\n&quot; and <code>LAYOUT_NEWLINE_AFTER</code>.
 * Also, there is only a single row
 * break if an item has the <code>LAYOUT_NEWLINE_AFTER</code> directive
 * and the next item has the <code>LAYOUT_NEWLINE_BEFORE</code> directive.
 * However, the presence of consecutive &quot;\n&quot; characters,
 * either within a single <code>StringItem</code> or in adjacent
 * <code>StringItems</code>, will cause as many row breaks as there are
 * &quot;\n&quot; characters.  This will cause empty rows to be present.
 * The height of an empty row is determined by the prevailing font height of
 * the <code>StringItem</code> within which the &quot;\n&quot; that ends the
 * row occurs.</p>
 * 
 * <p>Implementations may provide additional conditions under which a row
 * break occurs.  For example, an implementation's layout policy may lay out
 * labels specially, implicitly causing a break before every
 * <code>Item</code> that has a
 * label.  Or, as another example, a particular implementation's user
 * interface style may dictate that a DateField item always appears on a row
 * by itself.  In this case, this implementation may cause row breaks to occur
 * both before and after every <code>DateField</code> item.</p>
 * 
 * <p>Given two items with adjacent <code>Form</code> indexes, if
 * none of the specified
 * or implementation-specific conditions for a row break between them
 * occurs, and if space permits, these items should be placed on the same
 * row.</p>
 * 
 * <p>When packing <code>Items</code> onto a row, the width of the
 * item is compared with
 * the remaining space on the row. For this purpose, the width used is the
 * <code>Item's</code> preferred width, unless the
 * <code>Item</code> has the <code>LAYOUT_SHRINK</code>
 * directive,
 * in which case the <code>Item's</code> minimum width is used. If
 * the <code>Item</code> is too wide
 * to fit in the space remaining on the row, the row is considered to be
 * full, a new row is added beneath this one, and the
 * <code>Item</code> is laid out on
 * this new row.</p>
 * 
 * <p>Once the contents of a row have been determined, the space available on
 * the row is distributed by expanding items and by adding space between
 * items. If any items on this row have the
 * <code>LAYOUT_SHRINK</code> directive (that is,
 * they are shrinkable), space is first distributed to these items. Space is
 * distributed to each of these items proportionally to the difference between
 * the each <code>Item's</code> preferred size and its minimum
 * size.  At this stage, no
 * shrinkable item is expanded beyond its preferred width.</p>
 * 
 * <p>For example, consider a row that has <code>30</code> pixels
 * of space available and
 * that has two shrinkable items <code>A</code> and
 * <code>B</code>. Item <code>A's</code> preferred size is
 * <code>15</code> and
 * its minimum size is <code>10</code>. Item <code>B's</code>
 * preferred size is <code>30</code> and its minimum
 * size is <code>20</code>. The difference between
 * <code>A's</code> preferred and minimum size is
 * <code>5</code>,
 * and <code>B's</code> difference is <code>10</code>. The
 * <code>30</code> pixels are distributed to these items
 * proportionally to these differences. Therefore, <code>10</code>
 * pixels are
 * distributed to item <code>A</code> and <code>20</code>
 * pixels to item <code>B</code>.</p>
 * 
 * <p>If after expanding all the shrinkable items to their preferred widths,
 * there is still space left on the row, this remaining space is distributed
 * equally among the Items that have the
 * <code>LAYOUT_EXPAND</code> directive (the
 * stretchable <code>Items</code>).  The presence of any
 * stretchable items on a row will
 * cause the <code>Items</code> on this row to occupy the full
 * width of the row.</p>
 * 
 * <p>If there are no stretchable items on this row, and there is still space
 * available on this row, the <code>Items</code> are packed as tightly as
 * possible and are placed on the row according to the alignment value shared
 * by the <code>Items</code> on this row.  (Since changing the current
 * alignment causes a row break, all <code>Items</code> on the same row must
 * share the same alignment value.)  If the alignment value is
 * <code>LAYOUT_LEFT</code>, the <code>Items</code> are positioned at the left
 * end of the row and the remaining space is placed at the right end of the
 * row.  If the alignment value is <code>LAYOUT_RIGHT</code>, the
 * <code>Items</code> are positioned at the right end of the row and the
 * remaining space is placed at the left end of the row.  If the alignment