Declarative Dragging
declarative-dragging.github.io
declarative-dragging.github.io
At my very first job, we had a weekly meeting to assign tickets and estimates.
Our non-technical PM wrote "drag-and-drop" on a whiteboard, and then my name.
"Gavin, how long do you think this will take?"
"All week. At least. Maybe longer?"
PM laughed, then realized I wasn't making a joke.
"What do you mean 'all week'? That's ridiculous, do I need to give this to someone else?"
"Well, you could, but I wouldn't count on you getting it any sooner."
Appalled, he exclaims: "It's just drag-and-drop!"
After 10-15 minutes of a room full of engineers trying to explain the difficulty, he gave up with an unhappy "Fine."
The phrase "it's just drag-and-drop" became an inside joke for the rest of my time there...
Nowadays you would just "npm install drag-and-drop" but back then you often had to hand-roll solutions.
If anyone can dig up an ancient library on GitHub for drag-and-drop back in those days, I'd imagine it would be several thousand lines long, not to mention the absolute nightmare that testing something like this with a proper framework and accompanying testing library.
It's not insurmountable by any means, but it does have enormous inherent complexity compared to almost every other UI action there is.
> If anyone can dig up an ancient library on GitHub for drag-and-drop back in those days, I'd imagine it would be several thousand lines long,
I bet, the JS devs who do write code overcompensate for the ones who can only stitch together libraries. I suppose because the expectation is that the library consumer is incapable of writing their own code extending it, so the library itself must support every possible combination of use cases for a million different users, even though the code any individual consumer actually needs is more like 100 loc.
> What the hell do you need a library for?
Think of a grid of cards, and all the possible drag, hover, and drop state transitions it can be in with any other card, and the animations required to make it look nice
Now also consider that you need to persist the final state on the backend, and handle things like rejection, cancellation, failed updates/errors.
It becomes a massive state chart and web of complexity
If you are dragging within the page, then the draggable “ghost” is your problem too (it’s not automatic). As is deciding whether the ghost should affirmatively snap out of and into the source and target containers (that is, whether the destination container should appear to “make room” for the ghost at some transitional point), whether there’s a transition on that, etc.
Drag and drop in desktop non-web GUIs have all sorts of visual affordances that users expect for consistency that are your job in HTML.
I was once quite up-to-date with raw frontend work and I tried to implement this; it’s actually not trivial.
Okay, you've added a requirement. Poll the current cursor position after the first hit-test sets the dragging state and call a function to render your object with X% opacity at a higher Z level than anything else. Your 30 minute job has become a 60 minute job.
> As is deciding whether the ghost should affirmatively snap out of and into the source and target containers (that is, whether the destination container should appear to “make room” for the ghost at some point), whether there’s a transition on that,
Most applications don't go this far, including your operating system like Windows that sets baseline UX expectations, so you're well beyond the default expectation for drag-and-drop. If someone asks you to implement drag-and-drop without specifying they want all the bells and whistles, and you respond it's going to take you a week because you're planning to go all-out beyond necessary business requirements, they'd have a valid reason to be like "WTF mate?". Also this is still pretty paltry and not a week's worth of work.
Do you remember what browser testing was like?
Remember the quirks of browsers’ different understanding of transparency, of z-index, of events, of DOM compliance?
I think the suggestion that getting it right could take a week in the context of a particular application is actually not particularly hyperbolic.
Mind you, I was doing cross-browser testing long enough ago that I am one of those people who can say they independently invented AJAX in-house.
The problem is that even without a framework, you didn’t have a blank slate (which would have been better). You had a moving target of blank slates, you had platform differences, you had drag vs touch-drag, entrenched older browsers and the simple frustration of testing on an array of real devices, all sorts.
Now, if you were talking about 2015, then it starts to get a bit flatter.
serious question, why, when people from a field you are unfamiliar with tell you that something has particular constraints in that field, do you confidently assert that you know better?
It's no accident that after certain complexity people will just go for a blank canvas (afaik Google docs is also canvas-rendered).
The problem is that for drag n drop you mostly want to support normal dom.
This is how I feel about global illumination. Game devs claim it's hard but they're just noobs who don't know how to write a lighting shader.
I'm not a game dev, but it can't be that hard.
If you want to splash up the "drop here" thing it's a frickin nightmare. The events don't fire when they should (e.g. mouse leave doesn't fire so it gets stuck, or you get duplicate events or missing events) so you get a bunch of flicker unless you do a bunch of debouncing and fail safes to cover all the browser quirkiness. And this is the easy drop-a-file scenario.
Doing all the stuff shown in https://www.youtube.com/watch?v=l7v-UwpsEbk I'm not sure I'd even know where to begin.
Paper here: https://declarative-dragging.github.io/D2.pdf
IMO one of the remaining gaps of agentic coding is that it's hard for even multimodal LLMs to check interactive motion effects for correctness and "feel." Like, sure, they can take screenshots of a few snapshotted states, but in terms of how things react as underlying nodes move and (potentially) reflow. How do you QA the feel of moving your wrist rapidly in the context of nested zones?
The answer is that you avoid the need to: you have a (nearly-provably) correct foundation that ensures that behavior is predictable regardless of state, and just render accordingly.
The paper's approach is incredible here: simultaneous optimistic rendering of multiple possible states, and a composable algebra (a small combinator language) over how drop zones and distance are derived - allowing for an extremely high-level interface in practice. Everything feels snappy and right, and the code you write is about as minimal as you can get - though the paper notes that state management is somewhat complex. But this seems solvable!
Once this is ported to HTML, it'll become an incredible part of the modern dev toolkit. I'm very excited for what this means.
[0]: https://hazel.org/
In huge "Stop Drawing Dead Fish" / "Stop Writing Dead Programs" ways. Which I'm such a fan of. Asked Astra to chase down references on this a couple weeks ago, quite enjoyed the result. https://news.ycombinator.com/item?id=5634662 https://news.ycombinator.com/item?id=33251799 https://news.ycombinator.com/item?id=33270235 https://chatgpt.com/share/6aea25b4-e294-83e9-b925-431bb13b00...
Thanks for the mention. I ran across Declarative Drawing & I really want excuses to use this. But I don't remember Hazel from the video I watched in it!
> Write your interface as a function of continuous variables, ui = ui(x1,x2,...) = ui(X). When dragging an element, compute the barycentric coordinates λ₁,λ₂,λ₃ of the cursor between the three nearest dropzones. Then render by interpolating the ui parameters between their values at those three dropzones, ui(λ₁X₁ + λ₂X₂ + λ₃X₃).
I fear this might lead to an exponential blowup.
(The examples shown are all tiny)
You have a list of sets and their subsets below them shown on screen. Now you take one element from one of the sets and add it to another set want things updated, this will trigger exponential computation. Since we (human) can't really visually look at all the exponential subsets anyway it will be not a very useful visulalization.
So exponential state space will be a problem only in the cases we don't care about it. Even then we could choose only few of the states by some criteria.
Also this approach seems to introduce new complicated concepts like "composables" and even a DSL. Probably writing custom code for Drag-and-drop would be easier that figuring out how this works. I read the page and did not understand much. For example, this phrase is difficult to understand: "Rather than programming all that visual behavior directly, you simply say, for each element you want to make draggable, which future states of your application dragging it should lead to.". Not "simply" at all. Writing new code would be easier especially with LLM.
Here the goal can be a render state and it is the solvers problem to figure out what should be there.
So you define these things very atomically, but the behavior automagically finds a valid render state (usually through converting things into a standard planner or solver language, haven’t dived into this repo yet)
Edit: nvm, answered by the Background section of the paper
The Hazel programming env is dope. I can already imagine using it to explain refactorings to new programmers.
The twisted tree diagrams is also really neat. Is that a new idea as well? I got the beginnings of an idea where it might be a good diagram to display tangled code paths.
Very nice work