The source code

    /**
     * @class Ext.grid.ColumnModel
     * @extends Ext.util.Observable
     * After the data has been read into the client side cache ({@link Ext.data.Store Store}),
     * 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}.
     * Data is mapped into the store's records and then indexed into the ColumnModel using the
     * {@link Ext.grid.Column#dataIndex dataIndex}:
     *
    {data source} == mapping ==> {data store} == {@link Ext.grid.Column#dataIndex dataIndex} ==> {ColumnModel}
     *
     * Each {@link Ext.grid.Column Column} in the grid's ColumnModel is configured with a
     * {@link Ext.grid.Column#dataIndex dataIndex} to specify how the data within
     * each record in the store is indexed into the ColumnModel.
     * There are two ways to initialize the ColumnModel class:
     * Initialization Method 1: an Array

     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}
     ]);

     * 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
     * {@link Ext.grid.Column#hidden hidden} config property (and then shown using the column
     * header menu).  Field's that are not included in the ColumnModel will not be displayable at all.
     * 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
     * {@link Ext.grid.Column#dataIndex dataIndex}.  If the
     * {@link Ext.grid.Column#dataIndex dataIndex} is not explicitly defined (as shown in the
     * example above) it will use the column configuration's index in the Array as the index.
     * See {@link Ext.grid.Column} for additional configuration options for each column.
     * Initialization Method 2: an Object
     * In order to use configuration options from Ext.grid.ColumnModel, an Object may be used to
     * initialize the ColumnModel.  The column configuration Array will be specified in the {@link #columns}
     * config property. The {@link #defaults} config property can be used to apply defaults
     * for all columns, e.g.:
     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);
            }
        }
    });

     * 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 {@link Ext.grid.Column#id id} config
     * option. This column could be styled by including the following css:
     //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;
    }

     * Note that the "Company Name" column could be specified as the
     * {@link Ext.grid.GridPanel}.{@link Ext.grid.GridPanel#autoExpandColumn autoExpandColumn}.
     * @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 = 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
             * {@link #setColumnWidth}.
             * Note internal resizing suppresses the event from firing. See also
             * {@link Ext.grid.GridPanel}.{@link #columnresize}.
             * @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);
    };
    Ext.extend(Ext.grid.ColumnModel, Ext.util.Observable, {
        /**
         * @cfg {Number} defaultWidth (optional) The width of columns which have no {@link #width}
         * specified (defaults to 100).  This property shall preferably be configured through the
         * {@link #defaults} config property.
         */
        defaultWidth: 100,
        /**
         * @cfg {Boolean} defaultSortable (optional) Default sortable of columns which have no
         * sortable specified (defaults to false).  This property shall preferably be configured
         * through the {@link #defaults} config property.
         */
        defaultSortable: false,
        /**
         * @cfg {Array} columns An Array of object literals.  The config options defined by
         * {@link Ext.grid.Column} 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 {@link #columns}.  Configuration options specified with
         * individual {@link Ext.grid.Column column} configs will supersede these {@link #defaults}.
         */

        /**
         * 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];
        },

        /**
         * 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
         * Config Options.
         * 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.
         * @param {Array} config Array of Column definition objects.
         * @param {Boolean} initial Specify true to bypass cleanup which deletes the totalWidth
         * 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.editor){
                        c.editor.destroy();
                    }
                }
            }

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

            this.config = config;
            this.lookup = {};
            // if no id, create one
            for(i = 0, len = config.length; i < len; i++){
                c = Ext.applyIf(config[i], this.defaults);
                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 c = this.config[oldIndex];
            this.config.splice(oldIndex, 1);
            this.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){
            if(visibleOnly === true){
                var c = 0;
                for(var i = 0, len = this.config.length; i < len; i++){
                    if(!this.isHidden(i)){
                        c++;
                    }
                }
                return c;
            }
            return this.config.length;
        },

        /**
         * Returns the column configs that return true by the passed function that is called
         * with (columnConfig, index)

    // returns an array of column config objects for all hidden columns
    var columns = grid.getColumnModel().getColumnsBy(function(c){
      return c.hidden;
    });

         * @param {Function} fn
         * @param {Object} scope (optional)
         * @return {Array} result
         */
        getColumnsBy : function(fn, scope){
            var r = [];
            for(var i = 0, len = this.config.length; i < len; i++){
                var c = this.config[i];
                if(fn.call(scope||this, c, i) === true){
                    r[r.length] = c;
                }
            }
            return r;
        },

        /**
         * 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){
            if(!this.config[col].renderer){
                return Ext.grid.ColumnModel.defaultRenderer;
            }
            return this.config[col].renderer;
        },

        /**
         * 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:
         * value : ObjectThe data value for the cell.
         * metadata : ObjectAn object in which you may set the following attributes:
         * css : StringA CSS class name to add to the cell's TD element.
         * attr : StringAn HTML attribute definition string to apply to the data container element within the table cell
         * (e.g. 'style="color:red;"').
         * record : Ext.data.recordThe {@link Ext.data.Record} from which the data was extracted.
         * rowIndex : NumberRow index
         * colIndex : NumberColumn index
         * store : Ext.data.StoreThe {@link Ext.data.Store} object from which the Record was extracted.
         */
        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){
            return this.config[col].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 {@link #widthchange}
         * 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.

    // Get field name for the column
    var fieldName = grid.getColumnModel().getDataIndex(columnIndex);

         * @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.

    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,
      ...
    });

         * @param {Number} colIndex The column index
         * @param {Number} rowIndex The row index
         * @return {Boolean}
         */
        isCellEditable : function(colIndex, rowIndex){
            return (this.config[colIndex].editable || (typeof this.config[colIndex].editable == "undefined" && this.config[colIndex].editor)) ? true : false;
        },

        /**
         * 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 true if the column is hidden.
         * @param {Number} colIndex The column index
         * @return {Boolean}
         */
        isHidden : function(colIndex){
            return this.config[colIndex].hidden;
        },

        /**
         * Returns true if the column width cannot be changed
         */
        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.

    myGrid.getColumnModel().setHidden(0, true); // hide column 0 (0 = the first column).

         * @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){
            Ext.destroy(this.config[col].editor);
            this.config[col].editor = editor;
        },

        /**
         * Destroys this column model by purging any event listeners, and removing any editors.
         */
        destroy : function(){
            for(var i = 0, c = this.config, len = c.length; i < len; i++){
                Ext.destroy(c[i].editor);
            }
            this.purgeListeners();
        }
    });

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