allow set-attr! with a function value

[aiba]

I have found it convenient to write template functions like this:

(for [x (some-list)]
  [:a {:onclick #(do-something x)} "click me to operate on" x])

Rather than serialize data in/out of the node and have a separate way of handling events.

Thus, I submit this pull request which allows dommy to generate nodes that work this way.

A possible disadvantage to this style of coding is that it leaves a bunch of function pointers (and their scopes) in memory. These functions should be garbage-collected with the dom node itself.

The advantage of coding this way is it's more concise and lets you store the context data of an event-handling function in a scope rather than serialized in the dom node itself.

[jenanwise]

Have you looked into delegated event handlers? It usually ends up being cleaner to define delegated event handlers using selectors (e.g. see jQuery.on's delegated events) rather than adding handlers directly to generated elements.

[aiba]

@jenanwise can you elaborate on the pros and cons of using a jQuery event, and what makes it "cleaner"? Maybe I'm missing something, but as I see it:

The disadvantages of a delegated event are:

• The clickable element needs a unique selector (more code).
• You have to introduce a separate function into your code and give it a name (more code).
• If you're passing data to the function that is specific to the element, then you need to serialize that data and put it in an attribute that is read out from the click handler. (more code and slight runtime performance cost).
• Handler has to wait for event to bubble and check it against the selector (runtime performance cost).

The disadvantages of the direct function approach are:

• It could potentially take up more RAM because you have a bunch of scopes hanging around.
• Because of this, the CPU could become busy with more garbage collection if nodes are changing frequently.

Even if you put aside the performance considerations, I think it's often really useful to be able to express applications more concisely, which this pull request enables.