Create Your Own DHTML Slideshow With a Fading Effect - The Slideshow Script (contd.)

(Page 4 of 5 )

When all browser and DHTML compatibility calculations have been made, we actually apply the settings to the layer. In NN 4 it’s through the clip.top/right/bottom/left properties and we also set it's visibility to show (Netscape's proprietary value).

After that, we have finished with the NN 4 code and we check for DOM 1 or IE 4 support. The task is easier here, because no matter what clipping is set, we can always get the image height/width. We then make the same test for fade in or fade out, and at the end of the test we apply the settings as a string to the clip property.

I made my first version of this slideshow with the units specified as pixels (px), but I didn't like it at all because I couldn't get it to work in NN 4 and Opera 5. I then found this layer scroll script and I got the feeling for the different browsers compatability and decided to make a version in which the fading can be switched off.

function fadeLayer(layername, amt, tim) {
if (!DHTML) return;
thelayer = new getObj( layername );
if (!thelayer) return;
amount = amt;
theTime = tim;
realFade();
}

function stopFade() {
if (time) clearTimeout(time);
}

The fadeLayer function is used to set the global variables to the parameters passed to it and then invoke the realFade function to do the hard work.

The only thing that the stopFade function does is check the timer ID and if the timer is running, it stops it.

function realFade() {
clipTop += amount;
clipBottom -= amount;
if (clipTop < 0 || clipBottom > lyrheight || clipTop > middle) {
if ( clipTop > middle ) thelayer.style.visibility = 'hidden';
if ( firstFade ) nextFade();
return;
}
if (document.getElementById || document.all) {
clipstring = 'rect('+clipTop+' '+clipWidth+' '+clipBottom+' 0)'
thelayer.style.clip = clipstring;
} else
if (document.layers) {
thelayer.style.clip.top = clipTop;
thelayer.style.clip.bottom = clipBottom;
}
time = setTimeout('realFade()',theTime);
}

var firstFade = true;
function nextFade() {
firstFade = false;
prepLyr(slides[curImg], false);
fadeLayer(slides[curImg], -10, 50);
}

The realFade function adds the amount to the clipTop and subtracts it from the clipBottom. We then check whether the clipping is off the image. If it is, then we check whether it is faded out. If so, then we hide the slide.

If the firstFade variable is true, then we execute the nextFade function, which fades in the next slide. In both case of fading we end the function, preventing the setTimeout invocation which would result in an infinite loop. The next thing to do is to actually apply the calculated clipping area to the layer. There are differences in the DOM 1 / IE 4 way and the NN 4 (described above in the prepLyr function explanation). After this, we set a timeout that executes the same function again.

The nextFade function then sets the fisrtFade variable to false, which prevents looping and invokes the prepLyr function with parameters for the next slide and false to initialize the image to fade in (clipping top and bottom to the middle). The realFade function is then invoked, passing it the current slide and a change amount of -10 pixels, which means that the clipping area should get bigger. We also pass a delay of 50 milliseconds to widen the area.

Next: Conclusion >>
 

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