The source code

    /**
     * @class Ext.form.TriggerField
     * @extends Ext.form.TextField
     * Provides a convenient wrapper for TextFields that adds a clickable trigger button (looks like a combobox by default).
     * The trigger has no default action, so you must assign a function to implement the trigger click handler by
     * overriding {@link #onTriggerClick}. You can create a TriggerField directly, as it renders exactly like a combobox
     * for which you can provide a custom implementation.  For example:
     * <pre><code>
    var trigger = new Ext.form.TriggerField();
    trigger.onTriggerClick = myTriggerFn;
    trigger.applyToMarkup('my-field');
    </code></pre>
     *
     * However, in general you will most likely want to use TriggerField as the base class for a reusable component.
     * {@link Ext.form.DateField} and {@link Ext.form.ComboBox} are perfect examples of this.
     *
     * @constructor
     * Create a new TriggerField.
     * @param {Object} config Configuration options (valid {@Ext.form.TextField} config options will also be applied
     * to the base TextField)
     * @xtype trigger
     */
    Ext.form.TriggerField = Ext.extend(Ext.form.TextField,  {
        /**
         * @cfg {String} triggerClass
         * An additional CSS class used to style the trigger button.  The trigger will always get the
         * class <tt>'x-form-trigger'</tt> by default and <tt>triggerClass</tt> will be <b>appended</b> if specified.
         */
        /**
         * @cfg {Mixed} triggerConfig
         * <p>A {@link Ext.DomHelper DomHelper} config object specifying the structure of the
         * trigger element for this Field. (Optional).</p>
         * <p>Specify this when you need a customized element to act as the trigger button for a TriggerField.</p>
         * <p>Note that when using this option, it is the developer's responsibility to ensure correct sizing, positioning
         * and appearance of the trigger.  Defaults to:</p>
         * <pre><code>{tag: "img", src: Ext.BLANK_IMAGE_URL, cls: "x-form-trigger " + this.triggerClass}</code></pre>
         */
        /**
         * @cfg {String/Object} autoCreate <p>A {@link Ext.DomHelper DomHelper} element spec, or true for a default
         * element spec. Used to create the {@link Ext.Component#getEl Element} which will encapsulate this Component.
         * See <tt>{@link Ext.Component#autoEl autoEl}</tt> for details.  Defaults to:</p>
         * <pre><code>{tag: "input", type: "text", size: "16", autocomplete: "off"}</code></pre>
         */
        defaultAutoCreate : {tag: "input", type: "text", size: "16", autocomplete: "off"},
        /**
         * @cfg {Boolean} hideTrigger <tt>true</tt> to hide the trigger element and display only the base
         * text field (defaults to <tt>false</tt>)
         */
        hideTrigger:false,
        /**
         * @cfg {Boolean} editable <tt>false</tt> to prevent the user from typing text directly into the field,
         * the field will only respond to a click on the trigger to set the value. (defaults to <tt>true</tt>).
         */
        editable: true,
        /**
         * @cfg {Boolean} readOnly <tt>true</tt> to prevent the user from changing the field, and
         * hides the trigger.  Superceeds the editable and hideTrigger options if the value is true.
         * (defaults to <tt>false</tt>)
         */
        readOnly: false,
        /**
         * @cfg {String} wrapFocusClass The class added to the to the wrap of the trigger element. Defaults to
         * <tt>x-trigger-wrap-focus</tt>.
         */
        wrapFocusClass: 'x-trigger-wrap-focus',
        /**
         * @hide
         * @method autoSize
         */
        autoSize: Ext.emptyFn,
        // private
        monitorTab : true,
        // private
        deferHeight : true,
        // private
        mimicing : false,

        actionMode: 'wrap',

        defaultTriggerWidth: 17,

        // private
        onResize : function(w, h){
            Ext.form.TriggerField.superclass.onResize.call(this, w, h);
            var tw = this.getTriggerWidth();
            if(Ext.isNumber(w)){
                this.el.setWidth(w - tw);
            }
            this.wrap.setWidth(this.el.getWidth() + tw);
        },

        getTriggerWidth: function(){
            var tw = this.trigger.getWidth();
            if(!this.hideTrigger && !this.readOnly && tw === 0){
                tw = this.defaultTriggerWidth;
            }
            return tw;
        },

        // private
        alignErrorIcon : function(){
            if(this.wrap){
                this.errorIcon.alignTo(this.wrap, 'tl-tr', [2, 0]);
            }
        },

        // private
        onRender : function(ct, position){
            this.doc = Ext.isIE ? Ext.getBody() : Ext.getDoc();
            Ext.form.TriggerField.superclass.onRender.call(this, ct, position);

            this.wrap = this.el.wrap({cls: 'x-form-field-wrap x-form-field-trigger-wrap'});
            this.trigger = this.wrap.createChild(this.triggerConfig ||
                    {tag: "img", src: Ext.BLANK_IMAGE_URL, alt: "", cls: "x-form-trigger " + this.triggerClass});
            this.initTrigger();
            if(!this.width){
                this.wrap.setWidth(this.el.getWidth()+this.trigger.getWidth());
            }
            this.resizeEl = this.positionEl = this.wrap;
        },

        getWidth: function() {
            return(this.el.getWidth() + this.trigger.getWidth());
        },

        updateEditState: function(){
            if(this.rendered){
                if (this.readOnly) {
                    this.el.dom.readOnly = true;
                    this.el.addClass('x-trigger-noedit');
                    this.mun(this.el, 'click', this.onTriggerClick, this);
                    this.trigger.setDisplayed(false);
                } else {
                    if (!this.editable) {
                        this.el.dom.readOnly = true;
                        this.el.addClass('x-trigger-noedit');
                        this.mon(this.el, 'click', this.onTriggerClick, this);
                    } else {
                        this.el.dom.readOnly = false;
                        this.el.removeClass('x-trigger-noedit');
                        this.mun(this.el, 'click', this.onTriggerClick, this);
                    }
                    this.trigger.setDisplayed(!this.hideTrigger);
                }
                this.onResize(this.width || this.wrap.getWidth());
            }
        },

        /**
         * Changes the hidden status of the trigger.
         * @param {Boolean} hideTrigger True to hide the trigger, false to show it.
         */
        setHideTrigger: function(hideTrigger){
            if(hideTrigger != this.hideTrigger){
                this.hideTrigger = hideTrigger;
                this.updateEditState();
            }
        },

        /**
         * Allow or prevent the user from directly editing the field text.  If false is passed,
         * the user will only be able to modify the field using the trigger.  Will also add
         * a click event to the text field which will call the trigger. This method
         * is the runtime equivalent of setting the {@link #editable} config option at config time.
         * @param {Boolean} value True to allow the user to directly edit the field text.
         */
        setEditable: function(editable){
            if(editable != this.editable){
                this.editable = editable;
                this.updateEditState();
            }
        },

        /**
         * Setting this to true will supersede settings {@link #editable} and {@link #hideTrigger}.
         * Setting this to false will defer back to {@link #editable} and {@link #hideTrigger}. This method
         * is the runtime equivalent of setting the {@link #readOnly} config option at config time.
         * @param {Boolean} value True to prevent the user changing the field and explicitly
         * hide the trigger.
         */
        setReadOnly: function(readOnly){
            if(readOnly != this.readOnly){
                this.readOnly = readOnly;
                this.updateEditState();
            }
        },

        afterRender : function(){
            Ext.form.TriggerField.superclass.afterRender.call(this);
            this.updateEditState();
        },

        // private
        initTrigger : function(){
            this.mon(this.trigger, 'click', this.onTriggerClick, this, {preventDefault:true});
            this.trigger.addClassOnOver('x-form-trigger-over');
            this.trigger.addClassOnClick('x-form-trigger-click');
        },

        // private
        onDestroy : function(){
            Ext.destroy(this.trigger, this.wrap);
            if (this.mimicing){
                this.doc.un('mousedown', this.mimicBlur, this);
            }
            delete this.doc;
            Ext.form.TriggerField.superclass.onDestroy.call(this);
        },

        // private
        onFocus : function(){
            Ext.form.TriggerField.superclass.onFocus.call(this);
            if(!this.mimicing){
                this.wrap.addClass(this.wrapFocusClass);
                this.mimicing = true;
                this.doc.on('mousedown', this.mimicBlur, this, {delay: 10});
                if(this.monitorTab){
                    this.on('specialkey', this.checkTab, this);
                }
            }
        },

        // private
        checkTab : function(me, e){
            if(e.getKey() == e.TAB){
                this.triggerBlur();
            }
        },

        // private
        onBlur : Ext.emptyFn,

        // private
        mimicBlur : function(e){
            if(!this.isDestroyed && !this.wrap.contains(e.target) && this.validateBlur(e)){
                this.triggerBlur();
            }
        },

        // private
        triggerBlur : function(){
            this.mimicing = false;
            this.doc.un('mousedown', this.mimicBlur, this);
            if(this.monitorTab && this.el){
                this.un('specialkey', this.checkTab, this);
            }
            Ext.form.TriggerField.superclass.onBlur.call(this);
            if(this.wrap){
                this.wrap.removeClass(this.wrapFocusClass);
            }
        },

        beforeBlur : Ext.emptyFn,

        // private
        // This should be overriden by any subclass that needs to check whether or not the field can be blurred.
        validateBlur : function(e){
            return true;
        },

        /**
         * The function that should handle the trigger's click event.  This method does nothing by default
         * until overridden by an implementing function.  See Ext.form.ComboBox and Ext.form.DateField for
         * sample implementations.
         * @method
         * @param {EventObject} e
         */
        onTriggerClick : Ext.emptyFn

        /**
         * @cfg {Boolean} grow @hide
         */
        /**
         * @cfg {Number} growMin @hide
         */
        /**
         * @cfg {Number} growMax @hide
         */
    });

    /**
     * @class Ext.form.TwinTriggerField
     * @extends Ext.form.TriggerField
     * TwinTriggerField is not a public class to be used directly.  It is meant as an abstract base class
     * to be extended by an implementing class.  For an example of implementing this class, see the custom
     * SearchField implementation here:
     * <a href="http://extjs.com/deploy/ext/examples/form/custom.html">http://extjs.com/deploy/ext/examples/form/custom.html</a>
     */
    Ext.form.TwinTriggerField = Ext.extend(Ext.form.TriggerField, {
        /**
         * @cfg {Mixed} triggerConfig
         * <p>A {@link Ext.DomHelper DomHelper} config object specifying the structure of the trigger elements
         * for this Field. (Optional).</p>
         * <p>Specify this when you need a customized element to contain the two trigger elements for this Field.
         * Each trigger element must be marked by the CSS class <tt>x-form-trigger</tt> (also see
         * <tt>{@link #trigger1Class}</tt> and <tt>{@link #trigger2Class}</tt>).</p>
         * <p>Note that when using this option, it is the developer's responsibility to ensure correct sizing,
         * positioning and appearance of the triggers.</p>
         */
        /**
         * @cfg {String} trigger1Class
         * An additional CSS class used to style the trigger button.  The trigger will always get the
         * class <tt>'x-form-trigger'</tt> by default and <tt>triggerClass</tt> will be <b>appended</b> if specified.
         */
        /**
         * @cfg {String} trigger2Class
         * An additional CSS class used to style the trigger button.  The trigger will always get the
         * class <tt>'x-form-trigger'</tt> by default and <tt>triggerClass</tt> will be <b>appended</b> if specified.
         */

        initComponent : function(){
            Ext.form.TwinTriggerField.superclass.initComponent.call(this);

            this.triggerConfig = {
                tag:'span', cls:'x-form-twin-triggers', cn:[
                {tag: "img", src: Ext.BLANK_IMAGE_URL, alt: "", cls: "x-form-trigger " + this.trigger1Class},
                {tag: "img", src: Ext.BLANK_IMAGE_URL, alt: "", cls: "x-form-trigger " + this.trigger2Class}
            ]};
        },

        getTrigger : function(index){
            return this.triggers[index];
        },

        afterRender: function(){
            Ext.form.TwinTriggerField.superclass.afterRender.call(this);
            var triggers = this.triggers,
                i = 0,
                len = triggers.length;

            for(; i < len; ++i){
                if(this['hideTrigger' + (i + 1)]){
                        triggers[i].hide();
                    }

            }
        },

        initTrigger : function(){
            var ts = this.trigger.select('.x-form-trigger', true),
                triggerField = this;

            ts.each(function(t, all, index){
                var triggerIndex = 'Trigger'+(index+1);
                t.hide = function(){
                    var w = triggerField.wrap.getWidth();
                    this.dom.style.display = 'none';
                    triggerField.el.setWidth(w-triggerField.trigger.getWidth());
                    triggerField['hidden' + triggerIndex] = true;
                };
                t.show = function(){
                    var w = triggerField.wrap.getWidth();
                    this.dom.style.display = '';
                    triggerField.el.setWidth(w-triggerField.trigger.getWidth());
                    triggerField['hidden' + triggerIndex] = false;
                };
                this.mon(t, 'click', this['on'+triggerIndex+'Click'], this, {preventDefault:true});
                t.addClassOnOver('x-form-trigger-over');
                t.addClassOnClick('x-form-trigger-click');
            }, this);
            this.triggers = ts.elements;
        },

        getTriggerWidth: function(){
            var tw = 0;
            Ext.each(this.triggers, function(t, index){
                var triggerIndex = 'Trigger' + (index + 1),
                    w = t.getWidth();
                if(w === 0 && !this['hidden' + triggerIndex]){
                    tw += this.defaultTriggerWidth;
                }else{
                    tw += w;
                }
            }, this);
            return tw;
        },

        // private
        onDestroy : function() {
            Ext.destroy(this.triggers);
            Ext.form.TwinTriggerField.superclass.onDestroy.call(this);
        },

        /**
         * The function that should handle the trigger's click event.  This method does nothing by default
         * until overridden by an implementing function. See {@link Ext.form.TriggerField#onTriggerClick}
         * for additional information.
         * @method
         * @param {EventObject} e
         */
        onTrigger1Click : Ext.emptyFn,
        /**
         * The function that should handle the trigger's click event.  This method does nothing by default
         * until overridden by an implementing function. See {@link Ext.form.TriggerField#onTriggerClick}
         * for additional information.
         * @method
         * @param {EventObject} e
         */
        onTrigger2Click : Ext.emptyFn
    });
    Ext.reg('trigger', Ext.form.TriggerField);
    Ext.reg('twintrigger', Ext.form.TwinTriggerField);