feat: add TemplateFooterProcessor extension point for extending footer tag content in theme (#6191)

#### What type of PR is this?
/kind feature
/area core
/milestone 2.17.x

#### What this PR does / why we need it:
提供对模板中 halo footer 标签内容的扩展点以支持扩展页脚内容

#### Which issue(s) this PR fixes:
Fixes #6189 

#### Does this PR introduce a user-facing change?
```release-note
提供对模板中 halo footer 标签内容的扩展点以支持扩展页脚内容
```

api/src/main/java/run/halo/app/theme/dialect/TemplateFooterProcessor.java

@@ -0,0 +1,20 @@
+package run.halo.app.theme.dialect;
+
+import org.pf4j.ExtensionPoint;
+import org.thymeleaf.context.ITemplateContext;
+import org.thymeleaf.model.IModel;
+import org.thymeleaf.model.IProcessableElementTag;
+import org.thymeleaf.processor.element.IElementTagStructureHandler;
+import reactor.core.publisher.Mono;
+
+/**
+ * Theme template <code>footer</code> tag snippet injection processor.
+ *
+ * @author guqing
+ * @since 2.17.0
+ */
+public interface TemplateFooterProcessor extends ExtensionPoint {
+
+    Mono<Void> process(ITemplateContext context, IProcessableElementTag tag,
+        IElementTagStructureHandler structureHandler, IModel model);
+}

application/src/main/java/run/halo/app/theme/dialect/TemplateFooterElementTagProcessor.java

@@ -1,14 +1,19 @@
 package run.halo.app.theme.dialect;
 
+import static org.thymeleaf.spring6.context.SpringContextUtils.getApplicationContext;
+
 import org.springframework.context.ApplicationContext;
 import org.thymeleaf.context.ITemplateContext;
+import org.thymeleaf.model.IModel;
 import org.thymeleaf.model.IProcessableElementTag;
 import org.thymeleaf.processor.element.AbstractElementTagProcessor;
 import org.thymeleaf.processor.element.IElementTagStructureHandler;
 import org.thymeleaf.spring6.context.SpringContextUtils;
 import org.thymeleaf.templatemode.TemplateMode;
+import reactor.core.publisher.Flux;
 import run.halo.app.infra.SystemConfigurableEnvironmentFetcher;
 import run.halo.app.infra.SystemSetting;
+import run.halo.app.plugin.extensionpoint.ExtensionGetter;
 
 /**
  * <p>Footer element tag processor.</p>
@@ -42,12 +47,23 @@ public class TemplateFooterElementTagProcessor extends AbstractElementTagProcess
     @Override
     protected void doProcess(ITemplateContext context, IProcessableElementTag tag,
         IElementTagStructureHandler structureHandler) {
+
+        IModel modelToInsert = context.getModelFactory().createModel();
         /*
          * Obtain the Spring application context.
          */
         final ApplicationContext appCtx = SpringContextUtils.getApplicationContext(context);
+
         String globalFooterText = getGlobalFooterText(appCtx);
-        structureHandler.replaceWith(globalFooterText, false);
+        modelToInsert.add(context.getModelFactory().createText(globalFooterText));
+
+        getTemplateFooterProcessors(context)
+            .concatMap(processor -> processor.process(context, tag,
+                structureHandler, modelToInsert)
+            )
+            .then()
+            .block();
+        structureHandler.replaceWith(modelToInsert, false);
     }
 
     private String getGlobalFooterText(ApplicationContext appCtx) {
@@ -57,4 +73,13 @@ public class TemplateFooterElementTagProcessor extends AbstractElementTagProcess
             .map(SystemSetting.CodeInjection::getFooter)
             .block();
     }
+
+    private Flux<TemplateFooterProcessor> getTemplateFooterProcessors(ITemplateContext context) {
+        var extensionGetter = getApplicationContext(context).getBeanProvider(ExtensionGetter.class)
+            .getIfUnique();
+        if (extensionGetter == null) {
+            return Flux.empty();
+        }
+        return extensionGetter.getExtensions(TemplateFooterProcessor.class);
+    }
 }

application/src/main/resources/extensions/extensionpoint-definitions.yaml

@@ -76,3 +76,15 @@ spec:
   displayName: "搜索引擎"
   type: SINGLETON
   description: "扩展内容搜索引擎"
+
+---
+apiVersion: plugin.halo.run/v1alpha1
+kind: ExtensionPointDefinition
+metadata:
+  name: template-footer-processor
+spec:
+  className: run.halo.app.theme.dialect.TemplateFooterProcessor
+  displayName: 页脚标签内容处理器
+  type: MULTI_INSTANCE
+  description: "提供用于扩展 <halo:footer/> 标签内容的扩展方式。"
+  

application/src/test/java/run/halo/app/theme/dialect/HaloProcessorDialectTest.java

@@ -125,6 +125,9 @@ class HaloProcessorDialectTest {
         when(fetcher.fetch(eq(SystemSetting.Basic.GROUP),
             eq(SystemSetting.Basic.class))).thenReturn(Mono.just(basic));
 
+        when(extensionGetter.getExtensions(TemplateFooterProcessor.class))
+            .thenReturn(Flux.empty());
+
         Context context = getContext();
 
         String result = templateEngine.process("index", context);
@@ -172,6 +175,9 @@ class HaloProcessorDialectTest {
         when(fetcher.fetch(eq(SystemSetting.Basic.GROUP),
             eq(SystemSetting.Basic.class))).thenReturn(Mono.just(basic));
 
+        when(extensionGetter.getExtensions(TemplateFooterProcessor.class))
+            .thenReturn(Flux.empty());
+
         String result = templateEngine.process("post", context);
         assertThat(result).isEqualTo("""
             <!DOCTYPE html>

application/src/test/java/run/halo/app/theme/dialect/TemplateFooterElementTagProcessorTest.java

@@ -0,0 +1,136 @@
+package run.halo.app.theme.dialect;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.eq;
+import static org.mockito.Mockito.lenient;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+import org.springframework.beans.factory.ObjectProvider;
+import org.springframework.context.ApplicationContext;
+import org.thymeleaf.IEngineConfiguration;
+import org.thymeleaf.TemplateEngine;
+import org.thymeleaf.context.Context;
+import org.thymeleaf.context.ITemplateContext;
+import org.thymeleaf.model.IModel;
+import org.thymeleaf.model.IProcessableElementTag;
+import org.thymeleaf.processor.IProcessor;
+import org.thymeleaf.processor.element.IElementTagStructureHandler;
+import org.thymeleaf.spring6.dialect.SpringStandardDialect;
+import org.thymeleaf.spring6.expression.ThymeleafEvaluationContext;
+import org.thymeleaf.templateresolver.StringTemplateResolver;
+import org.thymeleaf.templateresource.ITemplateResource;
+import org.thymeleaf.templateresource.StringTemplateResource;
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+import run.halo.app.infra.SystemConfigurableEnvironmentFetcher;
+import run.halo.app.infra.SystemSetting;
+import run.halo.app.plugin.extensionpoint.ExtensionGetter;
+
+/**
+ * Tests for {@link TemplateFooterElementTagProcessor}.
+ *
+ * @author guqing
+ * @since 2.17.0
+ */
+@ExtendWith(MockitoExtension.class)
+class TemplateFooterElementTagProcessorTest {
+    @Mock
+    private ApplicationContext applicationContext;
+
+    @Mock
+    ExtensionGetter extensionGetter;
+
+    @Mock
+    private SystemConfigurableEnvironmentFetcher fetcher;
+
+    private TemplateEngine templateEngine;
+
+    @BeforeEach
+    void setUp() {
+        HaloProcessorDialect haloProcessorDialect = new MockHaloProcessorDialect();
+        templateEngine = new TemplateEngine();
+        templateEngine.setDialects(Set.of(haloProcessorDialect, new SpringStandardDialect()));
+        templateEngine.addTemplateResolver(new MockTemplateResolver());
+
+        SystemSetting.CodeInjection codeInjection = new SystemSetting.CodeInjection();
+        codeInjection.setFooter(
+            "<p>Powered by <a href=\"https://www.halo.run\" target=\"_blank\">Halo</a></p>");
+        lenient().when(fetcher.fetch(eq(SystemSetting.CodeInjection.GROUP),
+            eq(SystemSetting.CodeInjection.class))).thenReturn(Mono.just(codeInjection));
+
+        lenient().when(applicationContext.getBeanProvider(ExtensionGetter.class))
+            .thenAnswer(invocation -> {
+                var objectProvider = mock(ObjectProvider.class);
+                when(objectProvider.getIfUnique()).thenReturn(extensionGetter);
+                return objectProvider;
+            });
+        lenient().when(applicationContext.getBean(eq(SystemConfigurableEnvironmentFetcher.class)))
+            .thenReturn(fetcher);
+    }
+
+    @Test
+    void footerProcessorTest() {
+        when(extensionGetter.getExtensions(TemplateFooterProcessor.class))
+            .thenReturn(Flux.just(new FakeFooterCodeInjection()));
+
+        String result = templateEngine.process("fake-template", getContext());
+        // footer injected code is not processable
+        assertThat(result).isEqualToIgnoringWhitespace("""
+            <p>Powered by <a href="https://www.halo.run" target="_blank">Halo</a></p>
+            <div>© 2024 guqing's blog</div>
+            <div th:text="${footerText}"></div>
+            """);
+    }
+
+    private Context getContext() {
+        Context context = new Context();
+        context.setVariable(
+            ThymeleafEvaluationContext.THYMELEAF_EVALUATION_CONTEXT_CONTEXT_VARIABLE_NAME,
+            new ThymeleafEvaluationContext(applicationContext, null));
+        return context;
+    }
+
+    static class MockTemplateResolver extends StringTemplateResolver {
+        @Override
+        protected ITemplateResource computeTemplateResource(IEngineConfiguration configuration,
+            String ownerTemplate, String template,
+            Map<String, Object> templateResolutionAttributes) {
+            return new StringTemplateResource("""
+                <halo:footer />
+                """);
+        }
+    }
+
+    static class MockHaloProcessorDialect extends HaloProcessorDialect {
+        @Override
+        public Set<IProcessor> getProcessors(String dialectPrefix) {
+            var processors = new HashSet<IProcessor>();
+            processors.add(new TemplateFooterElementTagProcessor(dialectPrefix));
+            return processors;
+        }
+    }
+
+    static class FakeFooterCodeInjection implements TemplateFooterProcessor {
+
+        @Override
+        public Mono<Void> process(ITemplateContext context, IProcessableElementTag tag,
+            IElementTagStructureHandler structureHandler, IModel model) {
+            var factory = context.getModelFactory();
+            // regular footer text
+            var copyRight = factory.createText("<div>© 2024 guqing's blog</div>");
+            model.add(copyRight);
+            // variable footer text
+            model.add(factory.createText("<div th:text=\"${footerText}\"></div>"));
+            return Mono.empty();
+        }
+    }
+}