What a drag: cancelling with onStart

So in this awesome new web application that I am writing I’ve got this totally sweet window/widget/bazfoo system where users can drag stuff around and it will remember the positions. This is all with Ruby on Rails and Prototype and Scriptaculous, and although the specifics are pretty Scriptaculous specific the generalities can apply to you favorite framework, unless it is … well they could probably apply.

Generally the UI doesn’t care what users want to drag when, but sometimes it does care. Say for example players can move stuff around in other players’ houses (to make it look like a ghost was there or something). This is very cool. Problem: what about not dragging things when not in ‘move-stuff-around-like-a-ghost’ mode? Perhaps there should only be one mode and things can always be moved around like a ghost, but even then, based on context, it seems that you might just not want the player to move stuff sometimes. I didn’t. Even though ‘rampant-ghost’ was the only mode I had so far I could envision wanting to drag an area-of-effect-spell-deployment or power meter or anything else without worrying about dragging the furniture.

So I had to sometimes cancel dragging like some companies sometimes cancel bonuses, but unlike those companies I decided to cancel during the onStart callback, not the somewhereInTheMiddleGodKnowsWhy callback. Scriptaculous provides a convenient hook to onStart (but surprisingly lacks one to halfwayThereButSimultaneoslyCutYourPay. They don’t have it? I know, it’s silly!). So lets just throw some code down to keep it real:

function drag_start(draggable, event) {
  draggable.element.should_revert = true;

  if($current_action == null || $current_action.id != "ghost_party_action") {
    draggable.finishDrag(event, false);

This is a pretty simple function, really standard, we have all our draggables sign it. First tell the element that it should revert unless it hears otherwise. Then check the $current_action (the $ lets me know I’m using it as a global, just like Ruby). If the current action doesn’t exist or it’s not the one where the ghosts party, then finish it off like Houchen. Great. Except, it still kind of drags and gets all weird. I could have sworn I was using the onStart callback, not the getWeirdAnyway callback… too bad.

So time to dive into Scriptaculous, source code that is! The file is dragdrop.js, the year 2008, film Noir has lost popularity in recent years but is still present in the minds of… The functions of interest are:

  updateDrag: function(event, pointer) {
    if(!this.dragging) this.startDrag(event);
    // ... Lots more omitted

  startDrag: function(event) {
    this.dragging = true;
    // ... Lots of setup, initialization ...

    // Bingo!
    Draggables.notify('onStart', this, event);

    if(this.options.starteffect) this.options.starteffect(this.element);

updateDrag gets called first when the user wiggles that mouse over the element. updateDrag then calls startDrag, which then calls your callback, which then ends the drag, but updateDrag is still in the dark, so let’s enlighten it:

  updateDrag: function(event, pointer) {
    if(!this.dragging) this.startDrag(event);
    // Added part
    if(!this.dragging) {

    // Lots more stuff stays the same

See what happened there? updateDrag wasn’t expecting the drag to end so soon after it got started! Now it knows, let that be a lesson. This keeps it from going all loosey goosey everywhere.

Now that is how you cancel a drag with onStart. Leave a comment, it will probably be at least as cool as the UK lottery comment. Also subscribe to my RSS and tell a friend, it’s like twice a month that I publish anything and then you can just scroll past it in Google Reader to get back to your 300 unread TechCrunch posts. Peace!

Author: Daniel X

Heretic priest of the Machine God. I enjoy crawling around in Jeff Bezo's spaceship, bringing technology to the people, and long walks outside of time and space.

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