feat: add theme link expression dialect (#2438)

#### What type of PR is this?
/kind feature
/milestone 2.0
/area core

#### What this PR does / why we need it:
允许主题模板在 HTML 或 JavaScript 片段中使用表达式对象获得链接：
- `${#theme.assets('/js/main.js'))}`
- `${#theme.route('/categories')}`

#### Which issue(s) this PR fixes:

Fixes #2435

#### Special notes for your reviewer:
/cc @halo-dev/sig-halo 
#### Does this PR introduce a user-facing change?

```release-note
None
```

src/main/java/run/halo/app/theme/ThemeConfiguration.java

@@ -14,6 +14,7 @@ import org.springframework.web.reactive.function.server.ServerResponse;
 import org.thymeleaf.extras.java8time.dialect.Java8TimeDialect;
 import run.halo.app.infra.properties.HaloProperties;
 import run.halo.app.infra.utils.FilePathUtils;
+import run.halo.app.theme.dialect.LinkExpressionObjectDialect;
 import run.halo.app.theme.dialect.ThemeJava8TimeDialect;
 
 /**
@@ -52,4 +53,9 @@ public class ThemeConfiguration {
     Java8TimeDialect java8TimeDialect() {
         return new ThemeJava8TimeDialect();
     }
+
+    @Bean
+    LinkExpressionObjectDialect linkExpressionObjectDialect() {
+        return new LinkExpressionObjectDialect();
+    }
 }

src/main/java/run/halo/app/theme/ThemeLinkBuilder.java

@@ -11,8 +11,8 @@ import run.halo.app.infra.utils.PathUtils;
  * @since 2.0.0
  */
 public class ThemeLinkBuilder extends StandardLinkBuilder {
-    private static final String THEME_ASSETS_PREFIX = "/assets";
-    private static final String THEME_PREVIEW_PREFIX = "/themes";
+    public static final String THEME_ASSETS_PREFIX = "/assets";
+    public static final String THEME_PREVIEW_PREFIX = "/themes";
 
     private final ThemeContext theme;
 

src/main/java/run/halo/app/theme/dialect/DefaultLinkExpressionFactory.java

@@ -0,0 +1,66 @@
+package run.halo.app.theme.dialect;
+
+import java.util.Set;
+import org.thymeleaf.context.IExpressionContext;
+import org.thymeleaf.exceptions.TemplateProcessingException;
+import org.thymeleaf.expression.IExpressionObjectFactory;
+import org.thymeleaf.linkbuilder.ILinkBuilder;
+import org.thymeleaf.util.Validate;
+import run.halo.app.theme.ThemeLinkBuilder;
+
+/**
+ * A default implementation of {@link IExpressionObjectFactory}.
+ *
+ * @author guqing
+ * @since 2.0.0
+ */
+public class DefaultLinkExpressionFactory implements IExpressionObjectFactory {
+    private static final String THEME_EVALUATION_VARIABLE_NAME = "theme";
+
+    @Override
+    public Set<String> getAllExpressionObjectNames() {
+        return Set.of(THEME_EVALUATION_VARIABLE_NAME);
+    }
+
+    @Override
+    public Object buildObject(IExpressionContext context, String expressionObjectName) {
+        if (THEME_EVALUATION_VARIABLE_NAME.equals(expressionObjectName)) {
+            return new ThemeLinkExpressObject(context);
+        }
+        return null;
+    }
+
+    @Override
+    public boolean isCacheable(String expressionObjectName) {
+        return THEME_EVALUATION_VARIABLE_NAME.equals(expressionObjectName);
+    }
+
+    public static class ThemeLinkExpressObject {
+        private final ILinkBuilder linkBuilder;
+        private final IExpressionContext context;
+
+        /**
+         * Construct an expression object that provides a set of methods to handle link in
+         * Javascript or HTML through {@link IExpressionContext}.
+         *
+         * @param context expression context
+         */
+        public ThemeLinkExpressObject(IExpressionContext context) {
+            Validate.notNull(context, "Context cannot be null");
+            this.context = context;
+            Set<ILinkBuilder> linkBuilders = context.getConfiguration().getLinkBuilders();
+            linkBuilder = linkBuilders.stream()
+                .findFirst()
+                .orElseThrow(() -> new TemplateProcessingException("Link builder not found"));
+        }
+
+        public String assets(String path) {
+            String assetsPath = ThemeLinkBuilder.THEME_ASSETS_PREFIX + path;
+            return linkBuilder.buildLink(context, assetsPath, null);
+        }
+
+        public String route(String path) {
+            return linkBuilder.buildLink(context, path, null);
+        }
+    }
+}

src/main/java/run/halo/app/theme/dialect/LinkExpressionObjectDialect.java

@@ -0,0 +1,27 @@
+package run.halo.app.theme.dialect;
+
+import org.thymeleaf.dialect.AbstractDialect;
+import org.thymeleaf.dialect.IExpressionObjectDialect;
+import org.thymeleaf.expression.IExpressionObjectFactory;
+
+/**
+ * An expression object dialect for theme link.
+ *
+ * @author guqing
+ * @since 2.0.0
+ */
+public class LinkExpressionObjectDialect extends AbstractDialect implements
+    IExpressionObjectDialect {
+
+    private static final IExpressionObjectFactory LINK_EXPRESSION_OBJECTS_FACTORY =
+        new DefaultLinkExpressionFactory();
+
+    public LinkExpressionObjectDialect() {
+        super("themeLink");
+    }
+
+    @Override
+    public IExpressionObjectFactory getExpressionObjectFactory() {
+        return LINK_EXPRESSION_OBJECTS_FACTORY;
+    }
+}

src/test/java/run/halo/app/theme/dialect/LinkExpressionObjectDialectTest.java

@@ -0,0 +1,25 @@
+package run.halo.app.theme.dialect;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests for {@link LinkExpressionObjectDialect}.
+ *
+ * @author guqing
+ * @since 2.0.0
+ */
+class LinkExpressionObjectDialectTest {
+
+    private final LinkExpressionObjectDialect linkExpressionObjectDialect =
+        new LinkExpressionObjectDialect();
+
+    @Test
+    void getExpressionObjectFactory() {
+        assertThat(linkExpressionObjectDialect.getName())
+            .isEqualTo("themeLink");
+        assertThat(linkExpressionObjectDialect.getExpressionObjectFactory())
+            .isInstanceOf(DefaultLinkExpressionFactory.class);
+    }
+}