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17.1.1.3 Mouse events

17.1.1.3 Mouse events


Mouse events are generated when the user moves or clicks the mouse over a document. These events are triggered on the most deeply nested element that the mouse pointer is over, but they bubble up through the document. The event object passed to mouse event handlers has properties set that describe the position and button state of the mouse and also specify whether any modifier keys were held down when the event occurred. The clientX and clientY properties specify the position of the mouse in window coordinates. The button and which properties specify which mouse button (if any) was pressed. (See the Event reference page, however, because these properties are difficult to use portably.) The altKey, ctrlKey, metaKey, and shiftKey properties are set to true when the corresponding keyboard modifier keys are held down. And for click events, the detail property specifies whether this was a single, double, or triple click.

The mousemove event is triggered any time the user moves or drags the mouse. These events occur frequently, so mousemove handlers must not trigger computationally intensive tasks. The mousedown and mouseup events are triggered when the user presses and releases a mouse button. By registering a mousedown handler that registers a mousemove handler, you can detect and respond to mouse drags. Doing this properly involves being able to capture mouse events so that you continue to receive mousemove events even when the mouse has moved out of the element it started in. §17.5 includes an example of handling drags.

After a mousedown and mouseup event sequence, the browser also triggers a click event. The click event was described above as a device-independent form event, but it is actually triggered on any document element, not just form elements, and it is passed an event object with all of the extra mouse-related fields described above. If the user clicks a mouse button twice in a row (within a sufficiently short amount of time), the second click event will be followed by a dblclick event. Browsers often display a context menu when the right mouse button is clicked. They generally fire a contextmenu event

17.1 Types of Events | 451

before displaying the menu, and if you cancel the event, you can prevent the display of the menu. This is also an easy way to be notified of right mouse button clicks.

When the user moves the mouse so that it goes over a new element, the browser fires a mouseover event on that element. When the mouse moves so that it is no longer over an element, the browser fires a mouseout event on that element. For these events, the event object will have a relatedTargetproperty that specifies the other element involved in the transition. (See the Event reference page for the IE equivalent of the relatedTarget property.) mouseover and mouseout events bubble like all of the mouse events described here. This is often inconvenient, because when a mouseout handler is triggered, you have to check whether the mouse actually left the element you are interested in or if it merely transitioned from one child of the element to another. Because of this, IE supports nonbubbling versions of these events known as mouseenter and mouseleave. jQuery emulates support for these events in browsers other than IE (see Chapter 19 ), and the DOM Level 3 Events specification standardizes them.

When the user rotates the mouse wheel, browsers trigger a mousewheel event (or, in Firefox, a DOMMouseScroll event). The event object passed with these events includes properties that specify how much, and in which direction, the wheel was rotated. The DOM Level 3 Events specification is standardizing a more general multidimensional wheel event that, if implemented, will supersede both mousewheel and DOMMouseScroll. §17.6 includes a mousewheel event example.

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