style changes (contd.) when a user event happens in the context of some element, we may wish several...

Style changes (contd.)• When a user event happens in the context of

some element, we may wish several aspect of the style to change

• For example, we may wish to change both the font size and the color

• To do this, we can have several style-property settings in one event handler:

onmouseover=‘this.style.color=“red”;

this.style.fontSize=“50” ‘

onmouseout= ‘this.style.color=“blue”;


Analysis of this:

onmouseover=‘ this.style.color=“red”;


onmouseout= ‘ this.style.color=“blue”;


• the overall event-handler is enclosed by apostrophes

• individual style settings are separated by semi-colons

• layout should be chosen to support human readability of the HTML specification

Style Property Names: CSS versus Javascript

• CSS property names can be written in upper or lower case -- although I encourage you to use lower-case, as in font-size

• Javascript property names do not contain hyphens and must be written in lower-case, except for capitals at word boundaries, as infontSize, which corresponds to font-size

borderLeftWidth, which corresponds to border-left-width

Example

• Consider the next three slides– the first one shows a document before the

mouse is placed over the heading– the second one shows what the document looks

like when the mouse is over the heading– the third one shows what the document looks

like when the mouse is moved away again

Mouse not on heading

Mouse is on heading

Mouse moves off heading again

Event-handlers to achieve this

<h1

onmouseover=

'this.style.borderStyle="solid";

this.style.borderColor="blue";

this.style.borderLeftWidth="50";

this.style.backgroundColor="black";

this.style.color="white"'

onmouseout=

'this.style.borderStyle="none";

this.style.backgroundColor="white";

this.style.color="black"'

> Some Subject or Other </h1>

<p>The heading above gets ...</p>

Multiple similar event handlers:

• Handling these two events for one heading required a lot of typing

• It would be worse if we wanted to do the same thing for many headings

• We need some way of abbreviating event handler specification

First, a motivating example:

• Consider the following document with several headings

• Consider what happens when we move the mouse over/out with respect to these headings

Before we move mouse over anything

When mouse is over 1st heading

After mouse is moved away

When mouse is over 2nd heading

After mouse has moved out

When mouse is on 3rd heading

How this can be done economically:

• We must define some new JavaScript functions

Using the new functions

<body>

<h1 onmouseover=’highlight(this)’ onmouseout=’restore(this)'>

Some Subject or Other </h1>

<p> Blah blah blah. </p>

<h1 onmouseover=’highlight(this)’ onmouseout=’restore(this)'>

Another Subject </h1>


<h1 onmouseover=’highlight(this)' onmouseout=’restore(this)'>

A Third Subject </h1>


</body>

The new functions

• We have used two new functions:– highlight– restore

• Each time one of these functions is used, it is given one argument: the word this, which refers to the element in question

<h1 onmouseover=’highlight(this)' onmouseout=’restore(this)'>

Function definition• General form:

function <name>(<formal-argument-list>)

{ <statement-list> }

• Example:

function changeStyleOf(someThing)

{ someThing.style.color=“red”;

someThing.style.fontSize=“20” }

• I require you to have verbs in the names of function which “do” something

Formal versus Actual arguments• In the function definitionfunction changeStyleOf(someThing)

{ someThing.style.color=“red”;

someThing.style.fontSize=“20” }

the word someThing is a formal argument

• In the event-handleronmouseover=‘changeStyleOf(this)’

the word this is an actual argument

Where to put function definitions• They are placed in an element called a

script element

• Such an element is delimited by two tags: <script> and </script>

• We can have lots of script elements in a document

• However, a script element which contains function definitions should be placed in the head of a HTML specification

Defining the functions used earlier<script language=“JavaScript”>

function highlight(someThing)

{someThing.style.borderStyle="solid";

someThing.style.borderColor="blue";

someThing.style.borderLeftWidth="50";

someThing.style.backgroundColor="black";

someThing.style.color="white" }

function restore(someThing)

{someThing.style.borderStyle="none";

someThing.style.backgroundColor="white";

someThing.style.color="black" }

</script>

Overall Document Spec. (Part I)<html>

<head>

<title> Simple Mouse Event </title>

<script language=“JavaScript”>

function highlight(someThing)

{someThing.style.borderStyle="solid";

someThing.style.borderColor="blue";

someThing.style.borderLeftWidth="50";

someThing.style.backgroundColor="black";

someThing.style.color="white" }

function restore(someThing)

{someThing.style.borderStyle="none";

someThing.style.backgroundColor="white";

someThing.style.color="black" }

</script>

</head>

Overall Document Spec. (Part II)<body>

<h1 onmouseover='highlight(this)' onmouseout='restore(this)'>

Some Subject or Other </h1>








</body>

</html>

Event-handlers can have widespread effect

• An event-handler attached to one element of a document can affect any part of a document

• In the next example, there are event-handlers attached to three headings but they all affect only the display of the first paragraph

Appearance when mouse is not on any heading

Appearance when mouse is on first heading

Appearance when mouse is on second heading

Appearance when mouse is on third heading

How this is done:

• Only two changes from the earlier HTML specification:– the first paragraph is given an ID, which

happens to be “blah1”– all the event-handlers specify that blah1 should

be highlighted or restored, as the case may be– in other words, in every event-handler, blah1 is

the actual argument passed to the functions

The body of this revised document:

<body>

<h1 onmouseover='highlight(blah1)' onmouseout='restore(blah1)'>

Some Subject or Other</h1>

<p id="blah1"> Blah blah blah. </p>







</body>

A better way of using id attributes

• The usage of the id attribute that we have just seen was introduced in the late 1990s by Microsoft

• It still works in many browsers

• But, to future-proof your programs, you should conform to the W3C standard,– which is a little verbose

The body of this W3C-compatible document:

<body><h1 onmouseover=‘highlight(“blah1”)' onmouseout='restore(“blah1”)'> Some Subject or Other</h1><p id="blah1"> Blah blah blah. </p>

<h1 onmouseover='highlight(“blah1”)' onmouseout='restore(“blah1”)'> Another Subject </h1><p> Blah blah blah. </p>

<h1 onmouseover='highlight(“blah1”)' onmouseout='restore(“blah1”)'> A Third Subject </h1><p> Blah blah blah. </p></body>

The head of this W3C-compatible document:<html><head><title> Simple Mouse Event </title><script language=“JavaScript”>function highlight(someString) {var someThing=document.getElementById(someString); someThing.style.borderStyle="solid"; someThing.style.borderColor="blue"; someThing.style.borderLeftWidth="50"; someThing.style.backgroundColor="black"; someThing.style.color="white" }function restore(someString) {var someThing=document.getElementById(someString); someThing.style.borderStyle="none"; someThing.style.backgroundColor="white"; someThing.style.color="black" }</script></head>

Another example: events on headings affecting their related

paragraphs<body>

<h1 onmouseover='highlight(“blah1”)' onmouseout='restore(“blah1”)'>

Some Subject or Other</h1>

<p ID="blah1"> Blah blah blah. </p>



<p ID=“blah2”> Blah blah blah. </p>



<p ID=“blah3”> Blah blah blah. </p>

</body>

Mouse on first heading

Mouse on second heading

Mouse on third heading

style changes (contd.) when a user event happens in the context of some element, we may wish several...

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