Use gwt ui-binding in conjunction with google-gin

Question

is there a way to use dependency injection in a widget, that is created via ui binding?

Suppose I have a simple widget (in package com.example.client.ui.widget):

public class Foo extends Composite {

    private final EventBus eventBus;

    @Inject
    public Foo(final EventBus eventBus) {
        this.eventBus = eventBus;
        // create ui
    }
}

and then I have a view that is defined via ui binding which uses this widget. eg:

<ui:UiBinder xmlns:ui='urn:ui:com.google.gwt.uibinder'
    xmlns:g='urn:import:com.google.gwt.user.client.ui'
    xmlns:app='urn:import:com.example.client.ui.widget'>

    <g:HorizontalPanel>
        <app:Foo/>
    </g:HorizontalPanel>
</ui:UiBinder>

Doing it like this does not work, because of the no-arg constructor constraint for widgets that are used in ui binding. But wouldn't it be nice if the GWT compiler just uses GIN if there is an @Inject annotation? Or can this be done in any other way? Maybe I totally miss the concept of GIN and GWT, if so, any hints are much appreciated.

Thanks in advance, Markus

It is clear, that the view, that is declared via ui-binding itself is binded in the gin config. — schub, Jan 04 '12 at 18:57
See http://code.google.com/p/google-web-toolkit/issues/detail?id=6151 — Thomas Broyer, Jan 05 '12 at 03:50

score 3 · Accepted Answer · answered Jan 04 '12 at 19:06

3

if you need dependency injection for widgets you are doing something wrong.

BUT

It is possible to use GIN to for injection of some stuff into your widgets.

Think about the following: GIN is a standalone dependency injection framework, you just ask what you need, GIN will create a bean with dependencies resolved. Injection will be performed only if object was obtained from Ginjector instance. UI binder is a framework for declarative UI, it creates widgets based on your XML (and does a bunch of other stuff, but we can ignore it for now).

So if we want to use GIN to inject dependecies into Widget, it means widget should be created by GIN, not UiBinder. UiBinder will have to use an instace supplied by GIN. Is it possible? Yes, it UiBinder allows it by:

using annotation @UiField(provided=true) on widget field

or

using @UiFactory

So what you need to do is to create binding for widget in your GinModule, obtain it's instance via Ginjector or inject provider for widget, than you can give instance of widget to UiBinder via @UiField(provided=true) or @UiFactory.

answered Jan 04 '12 at 19:06

jusio

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Hi jusio, thank's for your answer. UiFactory was exactly what I was looking for :) The view, now injects a Provider. Then I annotated a method with UiFactory, that provides an instance of Foo using the this provider. But according to your first sentence, why is this bad design? What could be done better in order to achieve decoupling of the widgets but still have an EventBus within these widgets to react on user interaction? – schub Jan 04 '12 at 22:38
Widget is usually low-level, very universal thing and usually in order to create a widget, you need only to configure only really necessarily stuff. I can't really imagine why a button or CellList would need a dependency injection or an EventBus for example. They need to be very reusable. And all their dependencies should be managed not by dependency injection, but for example by View. Basically dependency injection fwk shouldn't be used for all your code, but for some high-level abstractions (like Views, Presenters, Services and etc).PS:that's my personal opinion, many people might disagree – jusio Jan 04 '12 at 23:55
I agree on the general widget point of view, in my case, although it is technically a widget (as it extends Composite), it should be seen as a reusable component within a specific application context (a rather complex widget instead of just a context independent ui control) with user interaction elements and therefore it needs an EventBus etc. But as you said, opinions might differ, thanks for the discussion :) – schub Jan 05 '12 at 10:13
I can't imagine my applications without ClientBundle facility - it handles all CSS for me in a very strict way and has the maximum level of bug-protection: fails at compile time. Custom widgets get their styles from related ClientBundle instances. Mixing GWT.create() and @Inject is a bad practice imo, therefore I prefer to inject everything, including my ClientBundle. Hence I create the corresponding singleton binding in the related gin configuration module. This simple logic leaves no space for doubts whether widgets are subjects of injections or not. They are. – vitrums Oct 26 '16 at 17:41

Use gwt ui-binding in conjunction with google-gin

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