为什么GWT编辑器框架中的子编辑器必须是程序包私有的？

Question

I've just tried out GWT's editor framework and got it working for a small example. But I was wondering why sub-editors need to be package-private?

There is a comment in an example in the linked page that specifically mentions package-protected fields, but I cannot find the reason why.

// Sub-editors are retrieved from package-protected fields, usually initialized with UiBinder.
// Many Editors have no interesting logic in them
public class PersonEditor extends Dialog implements Editor<Person> {
    Label nameEditor;
    AddressEditor addressEditor;
    ...
}

In the example I tried myself, I only got it working if my sub-editors are package-private, if i make them private, the binding does no longer work.

Can anyone explain to me why this restriction exists? It makes my coding style appear a bit inconsistently. Thanks!

Answer 1

Similar to UiBinder, the generator for an EditorDriver generates classes along-side Editor classes. These classes need access to the editors to be able to work with them.

Put differently, the editor framework won't modify you classes ( there's no magic ), so you have to somehow expose your sub-editors: package-private is enough, but public would of course work too.
The best way to understand what's going on is to pass the -gen option (followed by a directory path) to the GWT compiler of dev mode, so that it outputs all the generated classes to disk. Be warned though: the editor framework is really hard to comprehend!

Also, if your editor extends some other class in another package that contains a sub-editor (that you want to inherit), that subeditor must be visible from the package of the child class, so it has to be public in the parent class, or be explicitly exposed by the child class (using an accessor method that's not private ).

Answer 2

GWT generates an editor delegate (and a context) for each editor that will be used by the driver. This delegate (and context) require access to the subeditors they will manipulate as part of passing data in and out of the editors. As these are written in Java, and not using JSNI to access private fields, your editors must be accessible to other classes in the same package.

That said, there are several options for you. The first is to make them explicitly public , or protected , which may or may not fit your code style. The second is to expose them via methods, with the method named the same as the property to be edited, optionally suffixed with 'Editor'. See http://code.google.com/webtoolkit/doc/latest/DevGuideUiEditors.html#Editor_contract for more details on this.

This restriction is the same as the uibinder restriction - the field must be accessible to read/write it from generated classes. If you just have a private Label nameEditor; , then your IDE will likely complain at you that this is unused, as it can't see the code which hasnt been generated which could access this fields. Making it more public than private makes it clear that it will be used outside that single class.