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203Chapter 15 .Generic HTML Element Objects Related Items: event.dataTransfer object; onDragEnd, onDragEnter, onDragLeave, onDragOver, onDragStart, onDrop event handlers. onDragEnter onDragLeave NN2 NN3 NN4 NN6 IE3/J1 IE3/J2 IE4 IE5 IE5.5 Compatibility These events (not implemented in IE5/Mac) fire during a drag operation. When the cursor enters the rectangular space of an element on the page, the onDragEnter event fires on that element. Immediately thereafter, the onDragLeaveevent fires on the element from which the cursor came. While this may seem to occur out of sequence from the physical action, the events always fire in this order. Depending on the speed of the client computer s CPU and the speed of the user s dragging action, one or the other of these events may not fire especially if the physical action outstrips the computer s capability to fire the events in time. See the discussion of the onDragevent handler earlier in this chapter for more details on the sequence of drag-related events. Example (with Listing 15-38) on the CD-ROM Related Items: onDrag, onDragEnd, onDragOver, onDragStart, onDrop event handlers. onDragOver NN2 NN3 NN4 NN6 IE3/J1 IE3/J2 IE4 IE5 IE5.5 Compatibility The onDragOver event (not implemented in IE5/Mac) fires continually while a dragged cursor is atop an element. In the course of dragging from one point on the page to another, the onDragOver event target changes with the element beneath the cursor. If no other drag-related events are firing (the mouse button is still down in the drag operation, but the cursor is not moving), the onDragand onDragOver events fire continually, alternating between the two. You should have the onDragOverevent handler of a drop target element set the event.returnValue property to false. See the discussion of the onDrag event handler earlier in this chapter for more details on the sequence of drag-related events. On the CD-ROM elementObject.onDragOver
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202 Part III .Document Objects Reference Table 15-10 (continued) Event Target Discussion [More of the same as the cursor progresses from cell B1 through the TABLE border again to cell B2, the TABLE again, cell B3, and the outermost edge of the TABLE.] onDragEnter BODY Dragging is free of the TABLE and is floating free on the bare BODY element. onDragLeave TABLE Yes, you just left the TABLE. onDrag cell A1 Still alive and receiving this event. onDragOver BODY That s where the cursor is now. Fires multiple times (depending on the speed of the computer and the user s drag action), alternating with the previous onDrag event. onDragEnter blank1 The cursor reaches the SPAN element whose ID is blank1, where the empty underline is. onDragLeave BODY Just left the BODY for the blank. onDrag cell A1 Still kicking. onDragOver blank1 That s where the cursor is now. Fires multiple times (depending on the speed of the computer and the user s drag action), alternating with the previous onDrag event. onDrop blank1 The SPAN element gets the notification of a recent drop. onDragEnd cell A1 The original source element gets the final word that dragging is complete. This event fires even if the drag does not succeed because the drag does not end on a drop target. In practice, some of the events shown in Table 15-10 may not fire. Much has to do with how many event handlers you trap that need to execute scripts along the way. The other major factor is the physical speed at which the user performs the drag-and-drop operation (which interacts with the CPU processing speed). The kinds of events that are most likely to be skipped are the onDragEnterand onDragLeave events, and perhaps some onDragOver events if the user flies over an object before its onDragOver event has a chance to fire. Despite this uncertainty about drag-related event reliability, you can count on several important ones to fire all the time. The onDragStart, onDrop (if over a drop target), and onDragEnd events as well some interstitial onDrag events will definitely fire in the course of dragging on the screen. All but onDropdirect their events to the source element, while onDropfires on the target. Example (with Listing 15-37) on the CD-ROM On the CD-ROM elementObject.onDrag
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201Chapter 15 .Generic HTML Element Objects It helps to imagine that the cells of the table with draggable content are named like spreadsheet cells: truck is cell A1; round is B1; doll is A2; and so on. During the drag operation, many objects are the targets of a variety of drag-related events. Table 15-10 lists the event sequence and the event targets. Table 15-10 Events and Their Targets During a Typical Drag-and-Drop Operation Event Target Discussion onDragStart cell A1 The very first event that fires during a drag-and-drop operation. onDrag cell A1 Fires continually on this target throughout the entire operation. Other events get interspersed, however. onDragEnter cell A1 Even though the cursor hasn t moved from cell A1 yet, the onDragEnter event fires upon first movement within the source element. onDragOver cell A1 Fires continually on whatever element the cursor rests on at that instant. If the user simply holds the mouse button down and does not move the cursor during a drag, the onDrag and onDragOver events fire continually, alternating between the two. (repetition) cell A1 onDrag and onDragOver events fire alternately while the cursor remains atop cell A1. onDragEnter TABLE The TABLE element, represented by the border and/or cell padding, receives the onDragEnter event when the cursor touches its space. onDragLeave cell A1 Notice that the onDragLeave event fires after the onDragEnter event fires on another element. onDrag cell A1 Still firing away. onDragOver TABLE The source element for this event shifts to the TABLE because that s what the cursor is over at this instant. If the cursor doesn t move from this spot, the onDrag (cell A1) and onDragOver (TABLE) events continue to fire in turn. onDragEnter cell B1 The drag is progressing from the TABLE border space to cell B1. onDragLeave TABLE onDrag cell A1 The onDrag event continues to fire on the cell A1 object. onDragOver cell B1 The cursor is atop cell B1 now, so the onDragOver event fires for that object. Fires multiple times (depending on the speed of the computer and the user s drag action), alternating with the previous onDrag event. Continued elementObject.onDrag
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200 Part III .Document Objects Reference onDrag NN2 NN3 NN4 NN6 IE3/J1 IE3/J2 IE4 IE5 IE5.5 Compatibility The onDrag event fires after the onDragStart event and continues firing repeatedly while the user drags a selection or object on the screen. Unlike the onMouseMove event, which fires only as the cursor moves on the screen, the onDrag event continues to fire even when the cursor is stationary. In the IE5+ environment, users can drag objects to other browser windows or other applications. The event fires while the dragging extends beyond the browser window. Because the event fires regardless of what is underneath the dragged object, you can use it in a game or training environment in which the user has only a fixed amount of time to complete a dragging operation (for example, matching similar pairs of objects). If future versions of the browser accommodate downloadable cursors, the onDragevent could cycle the cursor through a series of cursor versions to resemble an animated cursor. Understanding the sequence of drag-related events during a user drag operation can be helpful if your scripts need to micromanage the actions (usually not necessary for basic drag-and-drop operations). Consider the drag-and-drop operation shown in Figure 15-2. Figure 15-2: A typical drag-and-drop operation blank1 Table A1 B1 B2 B3 elementObject.onDrag
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199Chapter 15 .Generic HTML Element Objects method in your onCopy or onCutevent handler, you must also set the event. returnValue property to false in the handler function to avoid the default copy or cut action from wiping out your clipboard contents. Because you are in charge of what data is stored in the clipboard, you are not limited to a direct copy of the data. For example, you might wish to store the value of the src property of an image object so that the user can paste it elsewhere on the page. In the case of the onCut event handler, your script is also responsible for cutting the element or selected content from the page. To eliminate all of the content of an element, you can set the element s innerHTML or innerText property to an empty string. For a selection, use the selection.createRange() method to generate a TextRange object whose contents you can manipulate through the TextRange object s methods. Example (with Listing 15-36) on the CD-ROM Related Items: onBeforeCopy, onBeforeCut, onBeforePaste, and onPaste event handlers. onDblClick NN2 NN3 NN4 NN6 IE3/J1 IE3/J2 IE4 IE5 IE5.5 Compatibility The onDblClick event fires after the second click of a double-click sequence. The timing between clicks depends on the client s mouse control panel settings. The onClick event also fires, but only after the first of the two clicks. NN4 implements the onDblClickevent handler only for link objects (but not at all on the Macintosh version of NN4). IE4 introduced the event to virtually every HTML element. In general, it is rarely a good design to have an element perform one task when the mouse is single-clicked and a different task if double-clicked. With the event sequence employed in modern browsers, this isn t practical anyway (the onClick event always fires, even when the user double-clicks). But it is not uncommon to have the mouse down action perform some helper action. You see this in most icon- based file systems: if you click a file icon, it is highlighted at mouse down to select the item; you can double-click the item to launch it. In either case, one event s action does not impede the other nor confuse the user. Example on the CD-ROM Related Items: onClick, onMouseDown, onMouseUp event handlers. On the CD-ROM On the CD-ROM elementObject.onDblClick
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198 Part III .Document Objects Reference property). It is not uncommon to wish to block the context menu from appearing so that users are somewhat inhibited from downloading copies of images or viewing the source code of a frame. Be aware, however, that if a user turns Active Scripting off in IE5+, the event handler cannot prevent the context menu from appearing. Another possibility for this event is to trigger the display of a custom context menu constructed with other DHTML facilities. In this case, you must also disable the intrinsic context menu so that both menus do not display at the same time. Example on the CD-ROM Related Items: releaseCapture(), setCapture() methods. onControlSelect NN2 NN3 NN4 NN6 IE3/J1 IE3/J2 IE4 IE5 IE5.5 Compatibility The onControlSelect event should fire just before a user makes a selection on what Microsoft calls a control selection. Microsoft is less than clear in explaining what a control selection is, but it appears to have something to do with a user edit mode. I have not been able to have this event fire naturally in IE5.5. If I receive further details, they will appear at the JavaScript Bible Support Center (http://www.dannyg.com). Related Items: onResizeEnd, onResizeStart event handlers. onCopy onCut NN2 NN3 NN4 NN6 IE3/J1 IE3/J2 IE4 IE5 IE5.5 Compatibility The onCopy and onCutevents (not implemented in IE5/Mac) fire immediately after the user or script initiates a copy or cut edit action on the current object. Each event is preceded by its associated before event, which fires before any Edit or context menu appears (or before the copy or cut action, if initiated by keyboard shortcut). Use these event handlers to provide edit functionality to elements that don t normally allow copying or cutting. In such circumstances, you need to enable the Copy or Cut menu items in the context or Edit menu by setting the event.returnValue for the onBeforeCopy or onBeforeCut event handlers to false. Then your onCopy or onCut event handlers must manually stuff a value into the clipboard by way of the setData()method of the clipboardData object. If you use the setData() On the CD-ROM elementObject.onCopy
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197Chapter 15 .Generic HTML Element Objects The event model in IE5+ provides one more way to prevent the intrinsic action of an object from firing when a user clicks it. If the onClick event handler function sets the returnValue property of the event object to false, the intrinsic action is cancelled. Simply include the following statement in the function invoked by the event handler: event.returnValue = false The event model of the W3C DOM has a different approach to cancelling the default action. In the event handler function for an event, invoke the eventObj. cancelDefault() method. A common mistake made by scripting beginners is to use a submit type input button as a button intended to perform some script action rather than submitting a form. The typical scenario is an INPUT element of type submitassigned an onClick event handler to perform some local action. The submit input button has an intrinsic behavior, just like links and areas. While you can block the intrinsic behavior, as just described, you should use an INPUT element of type button. If you are experiencing difficulty with an implementation of the onClick event handler (such as trying to find out which mouse button was used for the click), it may be that the operating system or default browser behavior is getting in the way of your scripting. But you can usually get what you need via the onMouseDown event handler. (The onMouseUp event may not fire when you use the secondary mouse button to click an object.) Use the onClickevent handler whenever possible to capture user clicks because this event behaves most like users are accustomed to in their daily computing work. But fall back on onMouseDownin an emergency. Example (with Listing 15-35) on the CD-ROM Related Items: click() method; onContextMenu, onDblClick, onMouseDown, onMouseUp event handlers. onContextMenu NN2 NN3 NN4 NN6 IE3/J1 IE3/J2 IE4 IE5 IE5.5 Compatibility The onContextMenu event (not implemented in IE5/Mac) fires when the user clicks an object with the secondary (usually the right-hand) mouse button. The only click-related events that fire with the secondary button are onMouseDownand onContextMenu. To block the intrinsic application menu display of the onContextMenuevent, use any of the three event cancellation methodologies available in IE5+ (as just described in the onClick event handler description: two variations of evaluating the event handler to return false; assigning false to the event.returnValue On the CD-ROM elementObject.onContextMenu
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196 Part III .Document Objects Reference The availability of the onClick event has expanded with succeeding generations of script-capable browsers. In the earlier versions, the event was limited primarily to button style input elements (including checkbox and radio input elements) and links (Aelements with HREFattributes assigned to them). You can safely use this event handler for elements that date back to the earliest scriptable browsers. In Navigator 4, the AREA element gained the onClickevent (and window, document, and layer objects could capture onClick events, as described in Chapter 29). In IE4+, virtually every element that you can see on a page can have an onClick event handler defined for it and thereby respond to user clicks. Beginning with version 4 browsers, scripters could access more mouse-related events. It is important to know the sequence of these incremental events as a user clicks or double-clicks an element. The other related events are onMouseDown, onMouseUp, and onDoubleClick. The onMouseDown event fires when the user makes contact with the mouse switch on the downstroke of a click action. Next comes the onMouseUpevent (when the contact breaks). Only then does the onClick event fire provided that the onMouseDown and onMouseUp events have fired in the same object. See the discussions on the onMouseDown and onMouseUp events later in this chapter for examples of their usage. Interaction with the onDblClick event is simple: the onClick event fires first (after the first click), followed by the onDblClick event (after the second click). See the discussion of the onDblClick event handler later in this chapter for more about the interaction of these two event handlers. When used with objects that have intrinsic actions when users click them (namely links and areas), the onClick event handler can perform all of the action including navigating to the destination normally assigned to the HREFattribute of the element. For example, to be compatible with all scriptable browsers, you can make an image clickable if you surround its tag with an link tag. This lets the onClickevent of that tag substitute for the missing onClickevent handler of earlier tags. If you assign an onClickevent handler without special protection, the event handler will execute and the intrinsic action of the element will be carried out. Therefore, you need to block the intrinsic action. To accomplish this, the event handler must evaluate to the statement return false. You can do this in two ways. The first is to append a return false statement to the script statement assigned to the event handler: As an alternative, you can let the function invoked by the event handler supply the false part of the return falsestatement, as shown in the following sequence: function yourFunction() { [statements that do something here] return false } … Either methdology is acceptable. A third option is to not use the onClick event handler at all, but assign a javascript: pseudo-URL to the HREF attribute (see the link object in Chapter 21). elementObject.onClick
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195Chapter 15 .Generic HTML Element Objects onBlur event handler starting with NN3 and IE4. IE4 also extended the event handler to more form elements, predominantly on the Windows operating system because that OS has a user interface clue (the dotted rectangle) when items such as buttons and links receive focus (so that you may act upon them by pressing the keyboard s spacebar). For IE5, the onBlur event handler is available to virtually every HTML element. For most of those elements, however, blur and focus are not possible unless you assign a value to the TABINDEXattribute of the element s tag. For example, if you assign TABINDEX=1 inside a

tag, the user can bring focus to that paragraph (highlighted with the dotted rectangle in Windows) by clicking the paragraph or pressing the Tab key until that item receives focus in sequence. If you plan to use the onBlur event handler on window or text-oriented input elements, be aware that there might be some unexpected and undesirable consequences of scripting for the event. For example, in IE, a windowobject that has focus loses focus (and triggers the onBlur event) if the user brings focus to any element on the page (or even clicks a blank area on the page). Similarly, the interaction between onBlur, onFocus, and the alert() dialog box can be problematic with text input elements. This is why I generally recommend using the onChange event handler to trigger form validation routines. If you should employ both the onBlur and onChangeevent handler for the same element, the onChange event fires before onBlur. For more details about using this event handler for data validation, see Chapter 43. IE5.5 adds the onDeactivate event handler, which fires immediately before the onBlur event handler. Both the onBlur and onDeactivateevents can be blocked if the onBeforeDeactivate event handler function sets event.returnValueto false. Example (with Listing 15-34) on the CD-ROM Related Items: blur(), focus() methods; onDeactivate, onBeforeDeactivate, onFocus, onActivate event handlers. onClick NN2 NN3 NN4 NN6 IE3/J1 IE3/J2 IE4 IE5 IE5.5 Compatibility The onClick event fires when a user presses down (with the primary mouse button) and releases the button with the pointer atop the element (both the down and up strokes must be within the rectangle of the same element). The event also fires with non-mouse click equivalents in operating systems such as Windows 95 and later. For example, you can use the keyboard to give focus to a clickable object and then press the spacebar or Enter key to perform the same action as clicking the element. In IE, if the element object supports the click() method, the onClick event fires with the invocation of that method (notice that this does not apply to Navigator). On the CD-ROM elementObject.onClick
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194 Part III .Document Objects Reference Example on the CD-ROM Related Items: document.designMode, contentEditable, isContentEditable properties. onBeforePaste NN2 NN3 NN4 NN6 IE3/J1 IE3/J2 IE4 IE5 IE5.5 Compatibility Like onBeforeCopy and onBeforeCut, the onBeforePasteevent (not implemented in IE5/Mac) occurs just prior to the display of either the context or menu bar Edit menu when the current object is selected (or has a selection within it). The primary value of this event comes when you use scripts to control the copy and paste process of a complex object. Such an object may have multiple kinds of data associated with it, but your script captures only one of the data types. Or, you may want to put some related data about the copied item (for example, the idproperty of the element) into the clipboard. By using the onBeforePasteevent handler to set the event.returnValue property to false, you guarantee that the pasted item is enabled in the context or Edit menu (provided the clipboard is holding some content). A handler invoked by onPasteshould then apply the specific data subset from the clipboard to the currently selected item. Example on the CD-ROM Related Items: onCopy, onCut, onPaste event handlers. onBlur NN2 NN3 NN4 NN6 IE3/J1 IE3/J2 IE4 IE5 IE5.5 Compatibility The onBlur event fires when an element that has focus is about to lose focus because some other element is about to receive focus. For example, a text input element fires the onBlurevent when a user tabs from that element to the next one inside a form. The onBlur event of the first element fires before the onFocus event of the next element. The availability of the onBlur event has expanded with succeeding generations of script-capable browsers. In the earlier versions, blur and focus were largely confined to text-oriented input elements (including the SELECT element). These are safe to use with all scriptable browser versions. The window object received the On the CD-ROM On the CD-ROM elementObject.onBlur
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