Support async #with contexts

[andreas-gruenbacher]

In {{#with context}}body{{/with}}, when context is a function, the
functions's return value is used as the context in body. This allows
for computed context, but not when that context is provided
asynchronously, such as when querying a database.

To support asynchronous contexts as well, check if context is a function
that takes arguments. If it is, call it with the callback to invoke
when done as the first, and #with's hash arguments as the second
argument. For example, with the following context function:

 person: function(fn) {
    fn({first: 'Alan', last: 'Johnson'});
  }

The template:

  {{#with person}}{{first}} {{last}}{{/with}}

will render to:

  Alan Johnson

[andreas-gruenbacher]

A similar approach would work for #each as well, of course.

[andreas-gruenbacher]

This is unfortunately implemented wrong so far; need to figure out how to fix it.

[andreas-gruenbacher]

I see that asynchronous helpers have been discussed before (#141), and that people were not very interested. So sorry for the noise.

[nknapp]

Have a look at the package 'promised-handlebars'. It may be what you need. I don't think that your PR really solves your problem, because the compiled template returns a string in the same cycle of the event loop. If you were calling the callback from a setTimeout-callback, your test would probably fail. Am 5. Januar 2017 07:37:27 MEZ schrieb Andreas Gruenbacher <notifications@github.com>:

…

I see that asynchronous helpers have been discussed before (#141), and that people were not very interested. So sorry for the noise. -- You are receiving this because you are subscribed to this thread. Reply to this email directly or view it on GitHub: #1293 (comment)

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[andreas-gruenbacher]

@nknapp thanks. I've had a quick look at 'promised-handlebars' and the placeholder approach seems so awfully wrong that I'd much rather switch to a different template engine that supports async helpers in the long run. For now, I think I'll just resort to doing all possible database queries ahead of the rendering.