Web design careers - 856 Part III . Document Objects Reference convenient
856 Part III . Document Objects Reference convenient to call a style sheet-positioned element (in other words, an element governed by a style sheet rule with the positionattribute) a layer as a generic term for any positioned element. In fact, NN4 even treated an element that was positioned through style sheets as if it were a genuine layer object (although with some minor differences). In the end, the layer term made good sense because no matter how it was achieved, a positioned element acted like a layer in front of the body content of a page. Perhaps you have seen how animated cartoons were created before computer animation changed the art. Layers of clear acetate sheets were assembled atop a static background. Each sheet contained one character or portion of a character. When all the sheets were carefully positioned atop each other, the view (as cap tured by a still camera) formed a composite frame of the cartoon. To create the next frame of the cartoon, the artist moved one of the layers a fraction of an inch along its intended path and then took another picture. If you can visualize how that operation works, you have a good starting point for understanding how layers work. Each layer contains some kind of HTML content that exists in its own plane above the main document that loads in a window. You can change or replace the content of an individual layer on the fly without affecting the other layers; you can also reposition, resize, or hide the entire layer under script control. One aspect of layers that goes beyond the cartoon analogy is that a layer can contain other layers. When that happens, any change that affects the primary layer such as moving the layer 10 pixels downward also affects the layers nested inside. It s as if the nested layers are passengers of the outer layer. When the outer layer goes somewhere, the passengers do, too. And yet, within the vehicle, the passengers may change seats by moving around without regard for what s going on outside. With this analogy in mind, many commercial DHTML development tools and con tent authors refer to positioned elements as layers, which you can move, resize, stack, and hide independently of the body background. Therefore, references throughout this book to layers may mean anything from the NN4 layer object to an element positioned by way of style sheets. NN4 Layer Object Properties Methods Event Handlers above captureEvents() onBlur background handleEvent() onFocus below load() onLoad bgcolor moveAbove() onMouseOut clip.bottom moveBelow() onMouseOver clip.left moveBy() clip.right moveTo() document.layerObject
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