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).  Fields 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 = Ext.extend(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}.
     */
    
    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
	         * {@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);
    },

    /**
     * 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++){
                this.config[i].destroy();
            }
        }

        // 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 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 A function which, when passed a {@link Ext.grid.Column Column} object, must
     * return true if the column is to be included in the returned Array.
     * @param {Object} scope (optional) The scope (this reference) in which the function
     * is executed. Defaults to this ColumnModel.
     * @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;
    },
    
    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:
     * 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){
        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 true if the column is {@link Ext.grid.Column#hidden hidden},
     * false otherwise.
     * @param {Number} colIndex The column index
     * @return {Boolean}
     */
    isHidden : function(colIndex){
        return !!this.config[colIndex].hidden; // ensure returns boolean
    },

    /**
     * Returns true if the column is {@link Ext.grid.Column#fixed fixed},
     * false 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.

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){
        this.config[col].setEditor(editor);
    },

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

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