The source code

    /**
     * @class Ext.grid.ColumnModel
     * @extends Ext.util.Observable
     * <p>After the data has been read into the client side cache (<b>{@link Ext.data.Store Store}</b>),
     * the ColumnModel is used to configure how and what parts of that data will be displayed in the
     * vertical slices (columns) of the grid. The Ext.grid.ColumnModel Class is the default implementation
     * of a ColumnModel used by implentations of {@link Ext.grid.GridPanel GridPanel}.</p>
     * <p>Data is mapped into the store's records and then indexed into the ColumnModel using the
     * <tt>{@link Ext.grid.Column#dataIndex dataIndex}</tt>:</p>
     * <pre><code>
    {data source} == mapping ==> {data store} == <b><tt>{@link Ext.grid.Column#dataIndex dataIndex}</tt></b> ==> {ColumnModel}
     * </code></pre>
     * <p>Each {@link Ext.grid.Column Column} in the grid's ColumnModel is configured with a
     * <tt>{@link Ext.grid.Column#dataIndex dataIndex}</tt> to specify how the data within
     * each record in the store is indexed into the ColumnModel.</p>
     * <p>There are two ways to initialize the ColumnModel class:</p>
     * <p><u>Initialization Method 1: an Array</u></p>
    <pre><code>
     var colModel = new Ext.grid.ColumnModel([
        { header: "Ticker", width: 60, sortable: true},
        { header: "Company Name", width: 150, sortable: true, id: 'company'},
        { header: "Market Cap.", width: 100, sortable: true},
        { header: "$ Sales", width: 100, sortable: true, renderer: money},
        { header: "Employees", width: 100, sortable: true, resizable: false}
     ]);
     </code></pre>
     * <p>The ColumnModel may be initialized with an Array of {@link Ext.grid.Column} column configuration
     * objects to define the initial layout / display of the columns in the Grid. The order of each
     * {@link Ext.grid.Column} column configuration object within the specified Array defines the initial
     * order of the column display.  A Column's display may be initially hidden using the
     * <tt>{@link Ext.grid.Column#hidden hidden}</tt></b> config property (and then shown using the column
     * header menu).  Fields that are not included in the ColumnModel will not be displayable at all.</p>
     * <p>How each column in the grid correlates (maps) to the {@link Ext.data.Record} field in the
     * {@link Ext.data.Store Store} the column draws its data from is configured through the
     * <b><tt>{@link Ext.grid.Column#dataIndex dataIndex}</tt></b>.  If the
     * <b><tt>{@link Ext.grid.Column#dataIndex dataIndex}</tt></b> is not explicitly defined (as shown in the
     * example above) it will use the column configuration's index in the Array as the index.</p>
     * <p>See <b><tt>{@link Ext.grid.Column}</tt></b> for additional configuration options for each column.</p>
     * <p><u>Initialization Method 2: an Object</u></p>
     * <p>In order to use configuration options from <tt>Ext.grid.ColumnModel</tt>, an Object may be used to
     * initialize the ColumnModel.  The column configuration Array will be specified in the <tt><b>{@link #columns}</b></tt>
     * config property. The <tt><b>{@link #defaults}</b></tt> config property can be used to apply defaults
     * for all columns, e.g.:</p><pre><code>
     var colModel = new Ext.grid.ColumnModel({
        columns: [
            { header: "Ticker", width: 60, menuDisabled: false},
            { header: "Company Name", width: 150, id: 'company'},
            { header: "Market Cap."},
            { header: "$ Sales", renderer: money},
            { header: "Employees", resizable: false}
        ],
        defaults: {
            sortable: true,
            menuDisabled: true,
            width: 100
        },
        listeners: {
            {@link #hiddenchange}: function(cm, colIndex, hidden) {
                saveConfig(colIndex, hidden);
            }
        }
    });
     </code></pre>
     * <p>In both examples above, the ability to apply a CSS class to all cells in a column (including the
     * header) is demonstrated through the use of the <b><tt>{@link Ext.grid.Column#id id}</tt></b> config
     * option. This column could be styled by including the following css:</p><pre><code>
     //add this css *after* the core css is loaded
    .x-grid3-td-company {
        color: red; // entire column will have red font
    }
    // modify the header row only, adding an icon to the column header
    .x-grid3-hd-company {
        background: transparent
            url(../../resources/images/icons/silk/building.png)
            no-repeat 3px 3px ! important;
            padding-left:20px;
    }
     </code></pre>
     * Note that the "Company Name" column could be specified as the
     * <b><tt>{@link Ext.grid.GridPanel}.{@link Ext.grid.GridPanel#autoExpandColumn autoExpandColumn}</tt></b>.
     * @constructor
     * @param {Mixed} config Specify either an Array of {@link Ext.grid.Column} configuration objects or specify
     * a configuration Object (see introductory section discussion utilizing Initialization Method 2 above).
     */
    Ext.grid.ColumnModel = Ext.extend(Ext.util.Observable, {
        /**
         * @cfg {Number} defaultWidth (optional) The width of columns which have no <tt>{@link #width}</tt>
         * specified (defaults to <tt>100</tt>).  This property shall preferably be configured through the
         * <tt><b>{@link #defaults}</b></tt> config property.
         */
        defaultWidth: 100,

        /**
         * @cfg {Boolean} defaultSortable (optional) Default sortable of columns which have no
         * sortable specified (defaults to <tt>false</tt>).  This property shall preferably be configured
         * through the <tt><b>{@link #defaults}</b></tt> config property.
         */
        defaultSortable: false,

        /**
         * @cfg {Array} columns An Array of object literals.  The config options defined by
         * <b>{@link Ext.grid.Column}</b> are the options which may appear in the object literal for each
         * individual column definition.
         */

        /**
         * @cfg {Object} defaults Object literal which will be used to apply {@link Ext.grid.Column}
         * configuration options to all <tt><b>{@link #columns}</b></tt>.  Configuration options specified with
         * individual {@link Ext.grid.Column column} configs will supersede these <tt><b>{@link #defaults}</b></tt>.
         */

        constructor : function(config) {
            /**
    	     * An Array of {@link Ext.grid.Column Column definition} objects representing the configuration
    	     * of this ColumnModel.  See {@link Ext.grid.Column} for the configuration properties that may
    	     * be specified.
    	     * @property config
    	     * @type Array
    	     */
    	    if (config.columns) {
    	        Ext.apply(this, config);
    	        this.setConfig(config.columns, true);
    	    } else {
    	        this.setConfig(config, true);
    	    }

    	    this.addEvents(
    	        /**
    	         * @event widthchange
    	         * Fires when the width of a column is programmaticially changed using
    	         * <code>{@link #setColumnWidth}</code>.
    	         * Note internal resizing suppresses the event from firing. See also
    	         * {@link Ext.grid.GridPanel}.<code>{@link #columnresize}</code>.
    	         * @param {ColumnModel} this
    	         * @param {Number} columnIndex The column index
    	         * @param {Number} newWidth The new width
    	         */
    	        "widthchange",

    	        /**
    	         * @event headerchange
    	         * Fires when the text of a header changes.
    	         * @param {ColumnModel} this
    	         * @param {Number} columnIndex The column index
    	         * @param {String} newText The new header text
    	         */
    	        "headerchange",

    	        /**
    	         * @event hiddenchange
    	         * Fires when a column is hidden or "unhidden".
    	         * @param {ColumnModel} this
    	         * @param {Number} columnIndex The column index
    	         * @param {Boolean} hidden true if hidden, false otherwise
    	         */
    	        "hiddenchange",

    	        /**
    	         * @event columnmoved
    	         * Fires when a column is moved.
    	         * @param {ColumnModel} this
    	         * @param {Number} oldIndex
    	         * @param {Number} newIndex
    	         */
    	        "columnmoved",

    	        /**
    	         * @event configchange
    	         * Fires when the configuration is changed
    	         * @param {ColumnModel} this
    	         */
    	        "configchange"
    	    );

    	    Ext.grid.ColumnModel.superclass.constructor.call(this);
        },

        /**
         * Returns the id of the column at the specified index.
         * @param {Number} index The column index
         * @return {String} the id
         */
        getColumnId : function(index) {
            return this.config[index].id;
        },

        getColumnAt : function(index) {
            return this.config[index];
        },

        /**
         * <p>Reconfigures this column model according to the passed Array of column definition objects.
         * For a description of the individual properties of a column definition object, see the
         * <a href="#Ext.grid.ColumnModel-configs">Config Options</a>.</p>
         * <p>Causes the {@link #configchange} event to be fired. A {@link Ext.grid.GridPanel GridPanel}
         * using this ColumnModel will listen for this event and refresh its UI automatically.</p>
         * @param {Array} config Array of Column definition objects.
         * @param {Boolean} initial Specify <tt>true</tt> to bypass cleanup which deletes the <tt>totalWidth</tt>
         * and destroys existing editors.
         */
        setConfig : function(config, initial) {
            var i, c, len;

            if (!initial) { // cleanup
                delete this.totalWidth;

                for (i = 0, len = this.config.length; i < len; i++) {
                    c = this.config[i];

                    if (c.setEditor) {
                        //check here, in case we have a special column like a CheckboxSelectionModel
                        c.setEditor(null);
                    }
                }
            }

            // backward compatibility
            this.defaults = Ext.apply({
                width: this.defaultWidth,
                sortable: this.defaultSortable
            }, this.defaults);

            this.config = config;
            this.lookup = {};

            for (i = 0, len = config.length; i < len; i++) {
                c = Ext.applyIf(config[i], this.defaults);

                // if no id, create one using column's ordinal position
                if (Ext.isEmpty(c.id)) {
                    c.id = i;
                }

                if (!c.isColumn) {
                    var Cls = Ext.grid.Column.types[c.xtype || 'gridcolumn'];
                    c = new Cls(c);
                    config[i] = c;
                }

                this.lookup[c.id] = c;
            }

            if (!initial) {
                this.fireEvent('configchange', this);
            }
        },

        /**
         * Returns the column for a specified id.
         * @param {String} id The column id
         * @return {Object} the column
         */
        getColumnById : function(id) {
            return this.lookup[id];
        },

        /**
         * Returns the index for a specified column id.
         * @param {String} id The column id
         * @return {Number} the index, or -1 if not found
         */
        getIndexById : function(id) {
            for (var i = 0, len = this.config.length; i < len; i++) {
                if (this.config[i].id == id) {
                    return i;
                }
            }
            return -1;
        },

        /**
         * Moves a column from one position to another.
         * @param {Number} oldIndex The index of the column to move.
         * @param {Number} newIndex The position at which to reinsert the coolumn.
         */
        moveColumn : function(oldIndex, newIndex) {
            var config = this.config,
                c      = config[oldIndex];

            config.splice(oldIndex, 1);
            config.splice(newIndex, 0, c);
            this.dataMap = null;
            this.fireEvent("columnmoved", this, oldIndex, newIndex);
        },

        /**
         * Returns the number of columns.
         * @param {Boolean} visibleOnly Optional. Pass as true to only include visible columns.
         * @return {Number}
         */
        getColumnCount : function(visibleOnly) {
            var length = this.config.length,
                c = 0,
                i;

            if (visibleOnly === true) {
                for (i = 0; i < length; i++) {
                    if (!this.isHidden(i)) {
                        c++;
                    }
                }

                return c;
            }

            return length;
        },

        /**
         * Returns the column configs that return true by the passed function that is called
         * with (columnConfig, index)
    <pre><code>
    // returns an array of column config objects for all hidden columns
    var columns = grid.getColumnModel().getColumnsBy(function(c){
      return c.hidden;
    });
    </code></pre>
         * @param {Function} fn A function which, when passed a {@link Ext.grid.Column Column} object, must
         * return <code>true</code> if the column is to be included in the returned Array.
         * @param {Object} scope (optional) The scope (<code>this</code> reference) in which the function
         * is executed. Defaults to this ColumnModel.
         * @return {Array} result
         */
        getColumnsBy : function(fn, scope) {
            var config = this.config,
                length = config.length,
                result = [],
                i, c;

            for (i = 0; i < length; i++){
                c = config[i];

                if (fn.call(scope || this, c, i) === true) {
                    result[result.length] = c;
                }
            }

            return result;
        },

        /**
         * Returns true if the specified column is sortable.
         * @param {Number} col The column index
         * @return {Boolean}
         */
        isSortable : function(col) {
            return !!this.config[col].sortable;
        },

        /**
         * Returns true if the specified column menu is disabled.
         * @param {Number} col The column index
         * @return {Boolean}
         */
        isMenuDisabled : function(col) {
            return !!this.config[col].menuDisabled;
        },

        /**
         * Returns the rendering (formatting) function defined for the column.
         * @param {Number} col The column index.
         * @return {Function} The function used to render the cell. See {@link #setRenderer}.
         */
        getRenderer : function(col) {
            return this.config[col].renderer || Ext.grid.ColumnModel.defaultRenderer;
        },

        getRendererScope : function(col) {
            return this.config[col].scope;
        },

        /**
         * Sets the rendering (formatting) function for a column.  See {@link Ext.util.Format} for some
         * default formatting functions.
         * @param {Number} col The column index
         * @param {Function} fn The function to use to process the cell's raw data
         * to return HTML markup for the grid view. The render function is called with
         * the following parameters:<ul>
         * <li><b>value</b> : Object<p class="sub-desc">The data value for the cell.</p></li>
         * <li><b>metadata</b> : Object<p class="sub-desc">An object in which you may set the following attributes:<ul>
         * <li><b>css</b> : String<p class="sub-desc">A CSS class name to add to the cell's TD element.</p></li>
         * <li><b>attr</b> : String<p class="sub-desc">An HTML attribute definition string to apply to the data container element <i>within</i> the table cell
         * (e.g. 'style="color:red;"').</p></li></ul></p></li>
         * <li><b>record</b> : Ext.data.record<p class="sub-desc">The {@link Ext.data.Record} from which the data was extracted.</p></li>
         * <li><b>rowIndex</b> : Number<p class="sub-desc">Row index</p></li>
         * <li><b>colIndex</b> : Number<p class="sub-desc">Column index</p></li>
         * <li><b>store</b> : Ext.data.Store<p class="sub-desc">The {@link Ext.data.Store} object from which the Record was extracted.</p></li></ul>
         */
        setRenderer : function(col, fn) {
            this.config[col].renderer = fn;
        },

        /**
         * Returns the width for the specified column.
         * @param {Number} col The column index
         * @return {Number}
         */
        getColumnWidth : function(col) {
            var width = this.config[col].width;
            if(typeof width != 'number'){
                width = this.defaultWidth;
            }
            return width;
        },

        /**
         * Sets the width for a column.
         * @param {Number} col The column index
         * @param {Number} width The new width
         * @param {Boolean} suppressEvent True to suppress firing the <code>{@link #widthchange}</code>
         * event. Defaults to false.
         */
        setColumnWidth : function(col, width, suppressEvent) {
            this.config[col].width = width;
            this.totalWidth = null;

            if (!suppressEvent) {
                 this.fireEvent("widthchange", this, col, width);
            }
        },

        /**
         * Returns the total width of all columns.
         * @param {Boolean} includeHidden True to include hidden column widths
         * @return {Number}
         */
        getTotalWidth : function(includeHidden) {
            if (!this.totalWidth) {
                this.totalWidth = 0;
                for (var i = 0, len = this.config.length; i < len; i++) {
                    if (includeHidden || !this.isHidden(i)) {
                        this.totalWidth += this.getColumnWidth(i);
                    }
                }
            }
            return this.totalWidth;
        },

        /**
         * Returns the header for the specified column.
         * @param {Number} col The column index
         * @return {String}
         */
        getColumnHeader : function(col) {
            return this.config[col].header;
        },

        /**
         * Sets the header for a column.
         * @param {Number} col The column index
         * @param {String} header The new header
         */
        setColumnHeader : function(col, header) {
            this.config[col].header = header;
            this.fireEvent("headerchange", this, col, header);
        },

        /**
         * Returns the tooltip for the specified column.
         * @param {Number} col The column index
         * @return {String}
         */
        getColumnTooltip : function(col) {
                return this.config[col].tooltip;
        },
        /**
         * Sets the tooltip for a column.
         * @param {Number} col The column index
         * @param {String} tooltip The new tooltip
         */
        setColumnTooltip : function(col, tooltip) {
                this.config[col].tooltip = tooltip;
        },

        /**
         * Returns the dataIndex for the specified column.
    <pre><code>
    // Get field name for the column
    var fieldName = grid.getColumnModel().getDataIndex(columnIndex);
    </code></pre>
         * @param {Number} col The column index
         * @return {String} The column's dataIndex
         */
        getDataIndex : function(col) {
            return this.config[col].dataIndex;
        },

        /**
         * Sets the dataIndex for a column.
         * @param {Number} col The column index
         * @param {String} dataIndex The new dataIndex
         */
        setDataIndex : function(col, dataIndex) {
            this.config[col].dataIndex = dataIndex;
        },

        /**
         * Finds the index of the first matching column for the given dataIndex.
         * @param {String} col The dataIndex to find
         * @return {Number} The column index, or -1 if no match was found
         */
        findColumnIndex : function(dataIndex) {
            var c = this.config;
            for(var i = 0, len = c.length; i < len; i++){
                if(c[i].dataIndex == dataIndex){
                    return i;
                }
            }
            return -1;
        },

        /**
         * Returns true if the cell is editable.
    <pre><code>
    var store = new Ext.data.Store({...});
    var colModel = new Ext.grid.ColumnModel({
      columns: [...],
      isCellEditable: function(col, row) {
        var record = store.getAt(row);
        if (record.get('readonly')) { // replace with your condition
          return false;
        }
        return Ext.grid.ColumnModel.prototype.isCellEditable.call(this, col, row);
      }
    });
    var grid = new Ext.grid.GridPanel({
      store: store,
      colModel: colModel,
      ...
    });
    </code></pre>
         * @param {Number} colIndex The column index
         * @param {Number} rowIndex The row index
         * @return {Boolean}
         */
        isCellEditable : function(colIndex, rowIndex) {
            var c = this.config[colIndex],
                ed = c.editable;

            //force boolean
            return !!(ed || (!Ext.isDefined(ed) && c.editor));
        },

        /**
         * Returns the editor defined for the cell/column.
         * @param {Number} colIndex The column index
         * @param {Number} rowIndex The row index
         * @return {Ext.Editor} The {@link Ext.Editor Editor} that was created to wrap
         * the {@link Ext.form.Field Field} used to edit the cell.
         */
        getCellEditor : function(colIndex, rowIndex) {
            return this.config[colIndex].getCellEditor(rowIndex);
        },

        /**
         * Sets if a column is editable.
         * @param {Number} col The column index
         * @param {Boolean} editable True if the column is editable
         */
        setEditable : function(col, editable) {
            this.config[col].editable = editable;
        },

        /**
         * Returns <tt>true</tt> if the column is <code>{@link Ext.grid.Column#hidden hidden}</code>,
         * <tt>false</tt> otherwise.
         * @param {Number} colIndex The column index
         * @return {Boolean}
         */
        isHidden : function(colIndex) {
            return !!this.config[colIndex].hidden; // ensure returns boolean
        },

        /**
         * Returns <tt>true</tt> if the column is <code>{@link Ext.grid.Column#fixed fixed}</code>,
         * <tt>false</tt> otherwise.
         * @param {Number} colIndex The column index
         * @return {Boolean}
         */
        isFixed : function(colIndex) {
            return !!this.config[colIndex].fixed;
        },

        /**
         * Returns true if the column can be resized
         * @return {Boolean}
         */
        isResizable : function(colIndex) {
            return colIndex >= 0 && this.config[colIndex].resizable !== false && this.config[colIndex].fixed !== true;
        },

        /**
         * Sets if a column is hidden.
    <pre><code>
    myGrid.getColumnModel().setHidden(0, true); // hide column 0 (0 = the first column).
    </code></pre>
         * @param {Number} colIndex The column index
         * @param {Boolean} hidden True if the column is hidden
         */
        setHidden : function(colIndex, hidden) {
            var c = this.config[colIndex];
            if(c.hidden !== hidden){
                c.hidden = hidden;
                this.totalWidth = null;
                this.fireEvent("hiddenchange", this, colIndex, hidden);
            }
        },

        /**
         * Sets the editor for a column and destroys the prior editor.
         * @param {Number} col The column index
         * @param {Object} editor The editor object
         */
        setEditor : function(col, editor) {
            this.config[col].setEditor(editor);
        },

        /**
         * Destroys this column model by purging any event listeners. Destroys and dereferences all Columns.
         */
        destroy : function() {
            var length = this.config.length,
                i = 0;

            for (; i < length; i++){
                this.config[i].destroy(); // Column's destroy encapsulates all cleanup.
            }
            delete this.config;
            delete this.lookup;
            this.purgeListeners();
        },

        /**
         * @private
         * Setup any saved state for the column, ensures that defaults are applied.
         */
        setState : function(col, state) {
            state = Ext.applyIf(state, this.defaults);
            Ext.apply(this.config[col], state);
        }
    });

    // private
    Ext.grid.ColumnModel.defaultRenderer = function(value) {
        if (typeof value == "string" && value.length < 1) {
            return "&#160;";
        }
        return value;
    };