On Mon, May 8, 2017 at 4:02 AM, Steve Litt <slitt@...2357...> wrote:

So to ease the path of those who follow me, I put up a map of the
landmines one's likely to run into while learning/using SVG clickmaps.
Once a person knows these landmines exist, he/she can go about his/her
business, and if something goes wrong, a simple reference to the
landmines page substitutes for hours of web search and experimentation.
Here's the URL of the SVG clickmaps landmines page:

I have a few suggestions with regard to some of your "landmines" that might make things a little easier...

1) "Use alert() commands"
Actually don't do this. Not for general logging of position or values in the script, at least. Alerts are intrusive and can only show you a text string. Get familiar with the developer console in all modern browsers, and especially the console.log() function. It's non-modal, and you can log out rich objects and data structures, then interrogate their properties, attributes and values interactively in the console. It's a much better experience for most cases when you might want to log via alert().

2) getElementsByTagName(), getElementsByClassName(), getElementById()
These work, but are somewhat out of fashion now. document.querySelector() and document.querySelectorAll() are the preferred approach these days. They take a CSS selector as a parameter, which allows for much richer ways to select objects:

document.querySelectorAll(".myclass");   // Gets all the elements with a class of "myclass"
document.querySelector(".myclass");       // The same, but only gets the first one

document.querySelector("#myID");          // Select by ID

document.querySelectorAll("circle");        // All the circles

document.querySelector("circle.myclass");  // The first circle with a class of "myclass"

document.querySelectorAll("g.myclass > circle"); // All the circles that are immediate children of a group with a class of "myclass"

CSS selectors are powerful things these days (though still with some limitations), and a well crafted querySelector() can save a lot of DOM traversal in code.

3) "Group Clicking Gotchas"
"Let me start with this piece of advice: You'll probably never want to put events on the group itself."

I disagree with this advice. There's nothing wrong with putting event handlers on the group, or on the items within it, or on both. You just need to understand how events propagate within browsers; it's a slightly odd process, borne of a need to combine the old Internet Explorer way of handling them with the W3C standard way. In short, the things you need to know are these:

* "evt" will be the name of the event in the handler.
* "" will be the object that fired the event (i.e. the object that was clicked on), not necessarily the one the handler is on.
* "this" will be the name of the object the handler is attached to.
* The handler can call another function, passing the event, but also passing other parameters.
* If a child object wants to consume an event and not pass it up to its ancestors, you must call evt.stopPropagation()

I've put an example file up on my site which shows some of this at work:

The whole file is a single group, but there are four onclick handlers (one on the group, one on each circle). Clicking a circle will change its colour randomly; clicking on the group (i.e. either lozenge shape) will change all three circles. There are also onmouseover and onmouseout handlers on the circles to thicken their borders when the mouse moves over them.

By putting a click handler on the group, and passing both "evt" and "this" you can get a reference to the object that was clicked ("") and the group itself ("this").



P.S. Did you see the message I posted a couple of weeks ago about making AJAX calls from SVG? I saw no feedback on it, so I'm not sure if it made it to the mailing list or not.