Is it possible to send a type parameter to a view template? I'd like to do something like:
#formErrors(productForm)
where the view formErrors might have been defined as [A](form:Form[A])... but I can't seem to get this right. The template engine does not seem to allow that type of definition. (Related Java question here.)
As noted in the linked Java question, it seems that an underscore can be sent, but under which circumstances, I haven't discovered yet. However in my case I can write:
#formErrors(form:Form[_])
#if( form.hasErrors ) { ... }
which was my intention, to be able to reuse the form error formatting/code. If anyone can shed any further light on the use of type parameters in templates, or the intention to support them in the future, that would be interesting.
Related
I'm using GWT 2.9 with elemental2-1.0.0-RC1.
The following code throws a ClassCastException at runtime:
DocumentRange documentRange = Js.cast(DomGlobal.document); // Fails
Range range = documentRange.createRange(); // Never reaches here
When I change to use an Js.uncheckedCast() instead, it succeeds:
DocumentRange documentRange = Js.uncheckedCast(DomGlobal.document);
Range range = documentRange.createRange(); // Works
The documentation for Js.uncheckedCast() says:
"You should always prefer regular casting over this (unless you know what you are doing!)."
I don't know why I'm having to use it, so I'm feeling nervous. Can someone explain how Js.cast() performs its type-checking and why I need to use an Js.uncheckedCast() in this instance?
Js.cast() is a way to cheat a bit, and do something that the Java language will not permit, but might actually be legal. Ignoring "how it actually works", the idea is that you can now get past issues where Java would complain, even if it turns out to be legit.
An example could be where you take a java.lang.Double or double and want to treat it as a JsNumber so you can call toPrecision(2) on it. Since java.lang.Double is final, it isn't legal to cast to an unrelated type, but Java doesn't know that in GWT, Double is really just a js Number. So, instead you can perform the cast with Js.cast(). The compiler will insert a runtime type check in there, verifying at runtime that your number is in fact a JS Number instance.
Another example could be trying to extend some native type that elemental2 provides, either to implement a workaround for a missing feature, or to do something browser-specific. Your new class may not extend the existing class - from JS's perspective this is okay, you are just describing the API that you know will exist at runtime. As such, we need to avoid the Java language check of "does this cast even make sense?", and just tell the compiler to try it.
On the other hand, you can "lie" to the compiler with Js.uncheckedCast(). This is used in cases where you are even asking the runtime to skip the check, and just pretend that it will work. This can let you do weird things, like treating Strings as if they were arrays, or solve cross-frame problems. No runtime check will be emitted, so instead you might just get a TypeError if a method/property is missing, instead of a proper ClassCastException.
In elemental2-dom 1.0.0-RC1, there is a class called DocumentRange, but it doesnt really make any sense - it is declared as a class, which means it can be type checked in JS, but the browser spec says that it should be an "interface" (which in JS-land means that it just is a description of a type, rather than something you can typecheck). https://www.w3.org/TR/DOM-Level-2-Traversal-Range/ranges.html#Level2-DocumentRange-method-createRange
This bug is inherited from closure-compiler, which claims that this has a constructor: https://github.com/google/closure-compiler/blob/6a418aa/externs/browser/w3c_range.js#L241-L251
The fix is for closure-compiler to refer to this as an interface, and for a new release of elemental2 to be made so you can use this.
There are two workarounds you can make here. The first is to cheat with Js.uncheckedCast(DomGlobal.document) and say "yes, I know that the Document is not instanceof DocumentRange, but that's because there is no such class as DocumentRange, so just pretend it worked so I can call createRange() on it". This is what you are doing already - it hides the fact there is a bug, but at the end of the day it works.
The "correct" answer is to declare your own DocumentRange, and do a Js.cast() to that instead. This is still gross - you have to keep your new interface around until closure gets fixed, and then elemental2 gets released, and then you have to remember to clean it up.
In this case, I would suggest lying to GWT and using Js.uncheckedCast() - there is only a single method on here, and it is unlikely to change in a meaningful way.
What we want is basically this:
/foo/* controllers.FooController.foo
However this doesn't work.
We have found the following workaround:
/foo/*ignore controllers.FooController.foo(ignore)
But this makes the code of the method controllers.FooController.foo slightly ugly. Is there a better way to do this?
Looking at the code over here, the router isn't able to deal with the "slug" part without specifying an identifier... the parser combinator doesn't declare it as optional, and the map (^^) is clearly using it as is.
It could be a good feature request if it wouldn't induce other problems where a pattern will hide all other routes because it's defined higher in the file (or even worst, included).
And it looks like it has been done on purpose if we look here, we can figure out that dynamic parameter cannot be assigned a default value -- indeed, in this case we'll fall in the case I've just mentioned :-/.
My first advice would be to tell you to use ignore as an Option[String] and the action definition to set it as None (rather than an empty String because it's more expressive).
My second would be to incite you to wonder if such case is really relevant, because it's error prone and could hide further problems
I'm trying to set up a route in the Playframework 2.0 (Scala) that includes optional parameters in the query string, following the examples in the documentation:
GET /my/path controllers.foo.Bar.list(offset: Int ?= 0, limit: Int ?= 20)
However when compiling, I get the following error message:
object controllers.foo.Bar does not take parameters
I made sure that the controllers.foo.Bar.list method does in fact take two Ints as parameters. One key observation (I hope) may be that this used to work previously, when I had the Controller class directly in the controllers package, i.e.
controllers.Bar.list
But it ceased working as soon as I introduced a "foo" subpackage in Controllers.
Any input on what I'm doing wrong highly appreciated!
UPDATE: Sorry - I did some more experimenting and it seems the reason is something entirely different (d'oh). In my concrete case, my controller class was
controllers.foo.List.list
and that seemed to cause a name clash. Renaming to something else ("FooList") solved the issue.
For anyone discovering this question, it seems very likely that this was due to a bug in the Play Framework.
You can follow its progress on the issue tracker ticket.
I've read some documentation and downloaded samples but I still got a question.
In most sample, the GIN module uses something like :
bind(MainActivityMapper.class);
bind(VerticalMasterActivityMapper.class);
I don't understand what it does? If I remove it from the sample code, everything works perfectly.
Thanks for any answer.
Someone pointed me to the GUICE page : http://google-guice.googlecode.com/git/javadoc/com/google/inject/Binder.html
There is no reason for it to be different in Gin.
Here is what it says for this particular case :
This statement does essentially nothing; it "binds the
class to itself" and does not change Guice's default behavior. You may
still want to use this if you prefer your Module class to serve as an
explicit manifest for the services it provides. Also, in rare cases,
Guice may be unable to validate a binding at injector creation time
unless it is given explicitly.
Note: I'm not sure wether or not I should Accept my own answer, so I'll leave it as is.
Sorry if this sounds stupid but I'm really new to LiftWeb and just struggling with the basic stuff:)
So I have a parametrized site map entry in Lift's bootstrap. This should be for the view page of an object of type MyItem. The URL would be like: "/myitems/UUID".
Menu.param [UUID]("MyItemView", "MyItemView", p=>Full(UUID.fromString(p)), p=>p.toString) / "myitems"
This adds the sitemap entry correctly. If I go to "/myitems/NOT_AN_UUID", it will throw the "Invalid UUID" exception as expected. But if I go to "/myitems/UUID" I get 404.
I know that I need a view and a snippet class that takes UUID as parameter in order for this to work but I have no idea how to name these and where to place them.
Btw, how would one new to Lift learn something like this? From the hundreds of articles and samples out there I found many to mention more complex stuff but haven't seen any to mention a basic thing like this. Do you know any secret start-up documentation for human beings?
Update: To summarize in case all you see above is jibber-jabber :) HOW DOES LIFT LOCATE TEMPLATES/VIEWS/SNIPPETS FOR PARAMETRIZED MENU ENTRIES?
It was actually the obvious answer. The template name is obtained from the path and the snippet can be whatever you want as long as you call it from the template xml.
I need to get used to all this convention over configuration :) However it would be nice for someone to tell you what is the convention.