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Lesson 2 - DHTML in NS 4- The <layer> tag
This could either be a good thing, or a bad one, depending on
your view. DHTML in NS 4 is quite simple, and comes down to essentially one word- Layer. I
was pretty astonished myself, but its true- NS 4 relies completely on a new tag, called
the <layer> tag, to spin up its DHTML magic. This new tag is dynamic in that it can
be positioned anywhere on a web page (without relation to other content), moved around,
its content inside updated on demand, and more.
Basic syntax
The basic syntax of the <layer> tag can't be simpler (as if
any tag in HTML is complicated!):
<layer>Text inside layer</layer>
The <layer> tag is a content tag, which means you can add
into it content (like <table>). Go ahead, try inserting the above code onto your
page...you'll notice that the text inside the layer floats above other
text, and overlaps them. Imagine a layer as a sheet of paper that resides on top of the
rest of the page, and does not take up space within the flow of the document.
Layer attributes
A layer by itself can't be more boring, not to mention useless.
Fortunately, there's more to it. Layers support attributes that allow you to position it
using the x,y coordinates-system, give it a background, clip it (make only certain area of
the layer visible), hide it from view, and so on. I've listed the most important layer
attributes below:
Layer attributes
| id |
The name of the layer, used to identify it in your script |
|
left |
The position of the layer in relationship to the x coordinates |
|
top |
The position of the layer in relationship to the y coordinates |
|
width |
The width of the layer, in px or % |
|
height |
The height of the layer, in px or % |
|
bgColor |
The background color of the layer |
|
background |
The background image of the layer |
|
src |
The external html document contained inside the layer |
Mix and match
different attributes any way you like. Here's a sample layers that uses some of the above
attributes:
<layer
id="mylayer" width=100px height=70px bgColor="yellow"><h3>A
layer</h3></layer>
A layer
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Notice I didn't
specify the left and top attributes. When you don't, the layer is positioned where you
defined it.
Scripting layers
Here's one of the most important thing to learn about layers- how
to script them. After all, its the scripts that make layers come alive. To access a layer,
you need to use the following syntax:
document.layername
Accessing the layer is just the first step. Once you've accessed
a layer, you can then go on and manipulate one of the layer's attributes to produce
dynamic effects. I'll show a simple example where a layer's background color interchanges
between red and green:
Here's the source
code I used:
<layer id="test" width=80px
height=80px></layer>
<script
language="JavaScript1.2">
//variable that helps alternate between red and blue
var thecolor=true
//Apply a bgColor of blue as the initial layer color
document.test1.bgColor="blue"
function changecol(){
//if thecolor=true
if (thecolor)
document.test.bgColor="blue"
else
document.test.bgColor="red"
//set thecolor to the opposite of its current state
thecolor=!thecolor
setTimeout("changecol()",1000)
}
changecol()
</script>
All of the layers' attributes are read/write, so be sure to
experiment with each of them!
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