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Interactive Event Handling Techniques in Web Development

Learn how to implement event handlers for mouse clicks and drag-and-drop interactions in web applications. Explore global variables, cleaner implementation practices, and understanding which element receives events. Dive into practical examples and code snippets.

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Interactive Event Handling Techniques in Web Development

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  1. Access to Event Object • event variable (HTML): <div onclick="mouseClick(event);"> • Passed as argument to function (DOM/Firefox): element.onclick = mouseClick; function mouseClick(evt) { ... x = evt.clientX; ... } • Global variable (DOM/IE): element.onclick = mouseClick; function mouseClick() { ... x = window.event.clientX; ... } CS 142 Lecture Notes: Events

  2. Draggable Rectangle <style type="text/css"> #div1 { position: absolute; } </style> ... <div id="div1" onmousedown="mouseDown(event);" onmousemove="mouseMove(event);" onmouseup="mouseUp(event);">Drag Me!</div> CS 142 Lecture Notes: Events

  3. Dragging Application isMouseDown = false; function mouseDown(event) { prevX = event.clientX; prevY = event.clientY; isMouseDown = true; } function mouseMove(event) { if (!isMouseDown) { return; } element = document.getElementById("div1"); element.style.left = (element.offsetLeft + (event.clientX - prevX)) + "px"; element.style.top = (element.offsetTop + (event.clientY - prevY)) + "px"; prevX = event.clientX; prevY = event.clientY; } function mouseUp(event) { isMouseDown = false; } CS 142 Lecture Notes: Events

  4. Cleaner Implementation <body> <div id="div1">Drag Me!</div> <div id="div2">Drag Me Too!</div> <script type="text/javascript" src="dragger.js"> <script type="text/javascript"> //<![CDATA[ new Dragger("div1"); new Dragger("div2"); //]]> </script> </body>} CS 142 Lecture Notes: Events

  5. Dragger.js, part 1 function Dragger(id) { this.isMouseDown = false; this.element = document.getElementById(id); this.element.onmousedown = this.wrap(this, "mouseDown"); } Dragger.prototype.wrap = function(obj, method) { return function(event) { obj[method](event); } } Dragger.prototype.mouseDown = function(event) { this.oldMoveHandler = document.body.onmousemove; document.onmousemove = this.wrap(this, "mouseMove"); this.oldUpHandler = document.body.onmouseup; document.onmouseup = this.wrap(this, "mouseUp"); this.prevX = event.clientX; this.prevY = event.clientY; this.isMouseDown = true; } CS 142 Lecture Notes: Events

  6. Dragger.js, part 2 Dragger.prototype.mouseMove = function(event) { if (!this.isMouseDown) { return; } this.element.style.left = (this.element.offsetLeft + (event.clientX - this.prevX)) + "px"; this.element.style.top = (this.element.offsetTop + (event.clientY - this.prevY)) + "px"; this.prevX = event.clientX; this.prevY = event.clientY; } Dragger.prototype.mouseUp = function(event) { this.isMouseDown = false; document.onmousemove = this.oldMoveHandler; document.onmouseup = this.oldUpHandler; } CS 142 Lecture Notes: Events

  7. Which Element Gets Event? <body> <table> <tr> <td>xyz</td> </tr> </table> </body> Click on this CS 142 Lecture Notes: Events

  8. CS 142 Lecture Notes: Events

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