RFR: 8267546: Add CSS themes as a first-class concept

Wed Nov 9 22:30:43 UTC 2022

This PR adds style themes as a first-class concept to OpenJFX. A style theme is a collection of stylesheets and the logic that governs them. Style themes can respond to OS notifications and update their stylesheets dynamically. This PR also re-implements Caspian and Modena as style themes.

### New APIs in `javafx.graphics`
The new theming-related APIs in `javafx.graphics` provide a basic framework to support application-wide style themes. Higher-level theming concepts (for example, "dark mode" detection or accent coloring) are not a part of this basic framework, because any API invented here might soon be out of date. Implementations can build on top of this framework to add useful higher-level features.
#### 1. StyleTheme
A style theme is an implementation of the `javafx.css.StyleTheme` interface:

/**
 * {@code StyleTheme} is a collection of stylesheets that specify the appearance of UI controls and other
 * nodes in the application. Like a user-agent stylesheet, a {@code StyleTheme} is implicitly used by all
 * JavaFX nodes in the scene graph.
 * <p>
 * The list of stylesheets that comprise a {@code StyleTheme} can be modified while the application is running,
 * enabling applications to create dynamic themes that respond to changing user preferences.
 * <p>
 * In the CSS subsystem, stylesheets that comprise a {@code StyleTheme} are classified as
 * {@link StyleOrigin#USER_AGENT} stylesheets, but have a higher precedence in the CSS cascade
 * than a stylesheet referenced by {@link Application#userAgentStylesheetProperty()}.
 */
public interface StyleTheme {
    /**
     * Gets the list of stylesheet URLs that comprise this {@code StyleTheme}.
     * <p>
     * If the list of stylesheets that comprise this {@code StyleTheme} is changed at runtime, this
     * method must return an {@link ObservableList} to allow the CSS subsystem to subscribe to list
     * change notifications.
     *
     * @implNote Implementations of this method that return an {@link ObservableList} are encouraged
     *           to minimize the number of subsequent list change notifications that are fired by the
     *           list, as each change notification causes the CSS subsystem to re-apply the referenced
     *           stylesheets.
     */
    List<String> getStylesheets();
}

A new `styleTheme` property is added to `javafx.application.Application`, and `userAgentStylesheet` is promoted to a JavaFX property (currently, this is just a getter/setter pair):

public class Application {
    ...
    /**
     * Specifies the user-agent stylesheet of the application.
     * <p>
     * A user-agent stylesheet is a global stylesheet that can be specified in addition to a
     * {@link StyleTheme} and that is implicitly used by all JavaFX nodes in the scene graph.
     * It can be used to provide default styling for UI controls and other nodes.
     * A user-agent stylesheets has the lowest precedence in the CSS cascade.
     * <p>
     * Before JavaFX 20, built-in themes were selectable using the special user-agent stylesheet constants
     * {@link #STYLESHEET_CASPIAN} and {@link #STYLESHEET_MODENA}. For backwards compatibility, the meaning
     * of these special constants is retained: setting the user-agent stylesheet to either {@code STYLESHEET_CASPIAN}
     * or {@code STYLESHEET_MODENA} will also set the value of the {@link #styleThemeProperty() styleTheme}
     * property to a new instance of the corresponding theme class.
     * <p>
     * Note: this property must only be modified on the JavaFX application thread.
     */
    public static StringProperty userAgentStylesheetProperty();
    public static String getUserAgentStylesheet();
    public static void setUserAgentStylesheet(String url);

    /**
     * Specifies the {@link StyleTheme} of the application.
     * <p>
     * {@code StyleTheme} is a collection of stylesheets that define the appearance of the application.
     * Like a user-agent stylesheet, a {@code StyleTheme} is implicitly used by all JavaFX nodes in the
     * scene graph.
     * <p>
     * Stylesheets that comprise a {@code StyleTheme} have a higher precedence in the CSS cascade than a
     * stylesheet referenced by the {@link #userAgentStylesheetProperty() userAgentStylesheet} property.
     * <p>
     * Note: this property must only be modified on the JavaFX application thread.
     */
    public static ObjectProperty<StyleTheme> styleThemeProperty();
    public static StyleTheme getStyleTheme();
    public static void setStyleTheme(StyleTheme theme);
    ...
}

`styleTheme` and `userAgentStylesheet` are correlated to preserve backwards compatibility: setting `userAgentStylesheet` to the magic values "CASPIAN" or "MODENA" will implicitly set `styleTheme` to a new instance of the `CaspianTheme` or `ModenaTheme` class. Aside from these magic values, `userAgentStylesheet` can be set independently from `styleTheme`. In the CSS cascade, `userAgentStylesheet` has a lower precedence than `styleTheme`.

#### 2. PlatformPreferences
`javafx.application.PlatformPreferences` can be used to query UI-related information about the current platform to allow theme implementations to adapt to the operating system. The interface extends `Map` and adds several useful methods, as well as the option to register a listener for change notifications:

/**
 * Contains UI preferences of the current platform.
 * <p>
 * {@code PlatformPreferences} implements {@link Map} to expose platform preferences as key-value pairs.
 * For convenience, {@link #getString}, {@link #getBoolean} and {@link #getColor} are provided as typed
 * alternatives to the untyped {@link #get} method.
 * <p>
 * The preferences that are reported by the platform may be dependent on the operating system version.
 * Applications should always test whether a preference is available, or use the {@link #getString(String, String)},
 * {@link #getBoolean(String, boolean)} or {@link #getColor(String, Color)} overloads that accept a fallback
 * value if the preference is not available.
 */
public interface PlatformPreferences extends Map<String, Object> {
    String getString(String key);
    String getString(String key, String fallbackValue);

    Boolean getBoolean(String key);
    boolean getBoolean(String key, boolean fallbackValue);

    Color getColor(String key);
    Color getColor(String key, Color fallbackValue);

    void addListener(PlatformPreferencesListener listener);
    void removeListener(PlatformPreferencesListener listener);
}

An instance of `PlatformPreferences` can be retrieved via `Platform.getPreferences()`.

### Usage
In its simplest form, a style theme is just a static collection of stylesheets:

Application.setStyleTheme(() -> List.of("stylesheet1.css", "stylesheet2.css");

A dynamic theme can be created by returning an instance of `ObservableList`:

public class MyCustomTheme implements StyleTheme {
    private final ObservableList<String> stylesheets =
        FXCollections.observableArrayList("colors-light.css", "controls.css");

    @Override
    public List<String> getStylesheets() {
        return stylesheets;
    }

    public void setDarkMode(boolean enabled) {
        stylesheets.set(0, enabled ? "colors-dark.css" : "colors-light.css");
    }
}

`CaspianTheme` and `ModenaTheme` can be extended by prepending or appending additional stylesheets:

Application.setStyleTheme(new ModenaTheme() {
    {
        addFirst("stylesheet1.css");
        addLast("stylesheet2.css");
    }
});

-------------

Commit messages:
 - Removed unrelated code from ThemeBase
 - StyleTheme implementation

Changes: https://git.openjdk.org/jfx/pull/511/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=511&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8267546
  Stats: 2971 lines in 40 files changed: 2596 ins; 227 del; 148 mod
  Patch: https://git.openjdk.org/jfx/pull/511.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/511/head:pull/511

PR: https://git.openjdk.org/jfx/pull/511