Vaadin Util - CSS and Theme setters

[javier-godoy]

• If a component implements HasTheme, one can call addThemeName(themeName), but if it doesn't one has to use getElement().getThemeList().add(themeName)
• Both theme names and class names are strings, but they cannot be used interchangeably (if it is a theme-string, you need to call the theme setter, if it is a class-string you need to call the class setter)

Consider the following:

@RequiredArgsConstructor
public static class CssTheme {
    private final String themeName;

    public <E extends HasElement> E add(E e) {
        e.getElement().getThemeList().add(themeName);
        return e;
    }

    public <E extends HasElement> E set(E e, boolean set) {
        e.getElement().getThemeList().set(themeName, set);
        return e;
    }

    public <E extends HasElement> E remove(E e) {
        e.getElement().getThemeList().remove(themeName);
        return e;
    }
}

It allows to define semantic-aware constants CssTheme CUSTOM= new CssTheme("custom"); so that one can invoke CUSTOM.set(div); instead of div.getElement().getThemeList().add(CUSTOM), and it works the same regardless of whether CUSTOM is a theme name or a class name. It also allows extending CssTheme so that it does something different (such as setting two themes, or different themes depending on the component, or a theme plus a class name, or...)

It also plays nicely with the helper described in #3

add(__(new Div(), CUSTOM::add));

Improvement ideas:

• Perhaps both should extend a common interface (such as StyleSetter)
• Perhaps there should be method that receive an Element as parameter.
• (Not so sure on this) perhaps the type parameter should be moved to the class (instead of being present on every method), so one could have CssTheme<Button> (which avoid applying button-specific styles to a text area).

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