CS 318: Lab 9 – Fixed Positioning
Dear students,
Today we begin our exploration of the several ways to break away from the normal left-to-right, top-to-bottom flow of a web page. We started with fixed positioning, because it introduces the ideas of anchoring outside the hierarchy. In fact, fixed position effectively removes an element entirely from its natural position in the hierachy and places it relatively to browser’s viewport.
Suppose you want a top navbar across your page. You might add this:
<header>WKRP - when the news hits the fan, we scoop it.</header>
With our randomly-generated main copy:
<main>
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</main>
Let’s style this to add some contrast:
* {
box-sizing: border-box;
}
body {
font-size: 18pt;
}
header {
background-color: black;
font-size: 24pt;
color: white;
text-align: center;
}
By default, these elements will flow, but when the font gets bigger and the user starts scrolling, we lose the top bar. Web developers are afraid that if viewers lose the ability to navigate a page quickly, they will abandon the site. Fixed positioning helps us keep an element—like navlinks—on screen at all times. Let’s add this CSS, which anchors the bar to the top and widens it to fill the window:
header {
...
position: fixed;
top: 0;
left: 0;
right: 0;
}
The downfall is that the main content appears under the top bar. How do we fix that? We’ll add a little margin.
Here’s your TODO list for next time:
- Read chapter 7 in your book. Scan real webpages that you regularly visit and find three elements that have been positioned in a non-
staticway, one for each of the valuesabsolute,fixed, andrelative. On a quarter sheet, identify the site and the element, and describe why you think it was positioned the way it was. - Be sure to get your WordPress posts made before March 1. You should have received an email with login information. Let me know if you didn’t.
See you then!
Sincerely,
Lab
Our goals for this lab include:
- Learning how to anchor elements to the browser window with
fixedpositioning. - Designing a click-driven slideshow display, powered by a wee bit of Javascript. This should feel a little bit like the scrolling image viewers found on many social media sites.
The exercises from last lab took a bit longer, so this lab only introduces one new exercise. Please complete the last lab’s exercises before diving into the quokka slideshow described below.
Slideshow
Your task is to create quokka slideshow that looks and behaves something like this:
The yellow rings serve only to highlight click events. They were added by the screen capture software and should not appear on your page.
Follow these steps to get your page up and smiling:
- Give the page a non-white background color.
- Create a
divwhoseidispanel. - Nest inside
panelanimgelement whoseidispicture. This will be the only child element inpanel. For itssrcattribute, supply the URL of an image of a quokka or something else that’s cute. - Give
panelfixed positioning. Anchor it from the browser window on all sides by 50 pixels. - Add a subtle shadow on all sides of
panel. Place the shadow on all slides by leaving the horizontal and vertical shifts at 0. (As a matter of style, web designers tend not to put units on 0. Use0, not0px.) But use a non-zero blur radius. - Force the image to completely fill its parent by setting its width and height—to what?
- When you set both the width and the height of an image, it’s likely to appear distorted. Look up the
object-fitproperty on MDN and set this property on your image to preserve its width:height ratio—its aspect ratio. Try its various values to learn their differences. - Add navigation icons via two
imgelements that are siblings topanel. I used this one and this one. Feel free to use them or others. - Position the navigation icons in a fixed manner. Vertically center them by anchoring them from the top by 50%. For properties that are common between the two icons, use a shared
class. It is bad style and a maintenance nightmare to define the same properties in separate CSS rules. For properties that are unique, use individualids. - Anchor the left icon to the left side of the window, and the right icon to the right. Set the images’ width and offsets such that there are 5 pixels of space to image’s left and right. (Recall from above how much space you’ve got in the “gutter” where the icons appear.)
- Verify that the anchoring behaves properly. Resize the window. Does the gap between the image and browser stay fixed in size?
- Make the navigation icons feel like links by turning the mouse into a pointer when it hovers over them. Use this CSS property:
cursor: pointer;
- Respond to mouse-clicks on the icons with Javascript. This sort of task is not the subject of this course. Take CS 319 to learn more, or find a book on Javascript. For the time being, create a file named
slideshow.jsand paste in this code:In the HTML, paste in this element after your navigation icons:// The list of image URLs var urls = [ 'http://www.traveller.com.au/content/dam/images/g/u/n/q/h/0/image.related.articleLeadwide.620x349.gunpvd.png/1488330286332.png', 'http://images.mentalfloss.com/sites/default/files/styles/mf_image_16x9/public/57159-istock-486456250.jpg?itok=pYToxS1l&resize=1100x619', ]; // The index (serial number) of the current image var i = 0; // Grab handles onto HTML elements. var leftButton = document.getElementById('left'); var rightButton = document.getElementById('right'); var picture = document.getElementById('picture'); // Retreat leftButton.onclick = function() { i = (i - 1 + urls.length) % urls.length; picture.src = urls[i]; } // Advance rightButton.onclick = function() { i = (i + 1) % urls.length; picture.src = urls[i]; }
Add a few other image URLs to the definition of<script src="slideshow.js"></script>
urls. As for the current two URLs, each line you add should have this form:'URL',
- Keen observers will notice that the icons are not centered perfectly on the vertical axis. This is more obvious if you shorten the window. If you care, call over your instructor or teaching assistant to fix this.
Publish and Validate
Commit and push your work. Verify that all pages have been uploaded by visiting https://USERNAME.github.io/cs318/lab09 in your browser.
Validate your pages with the W3C CSS Validator. Paste in the URLs to your pages, one at a time, and address all concerns raised by the validator. Fix your changes on the local machine, commit, and push until the validator reports no errors or warnings.