yxk-20260506-自定义流程表达式：

1、saveCurrentTaskAssigneeLocal保存当前节点办理人到子流程的局部变量中去
2、getSonProcessVariableString获取子流程变量，返回类型是String

jeecg-boot-platform/jeecg-boot-module-bpm-flowable/src/main/java/org/jeecg/modules/extbpm/process/common/expression/FlowNodeExpression.java

@@ -8,6 +8,7 @@ import org.apache.commons.lang.StringUtils;
 import org.flowable.engine.RuntimeService;
 import org.flowable.engine.delegate.DelegateExecution;
 import org.flowable.engine.runtime.Execution;
+import org.flowable.task.service.delegate.DelegateTask;
 import org.jeecg.common.constant.SymbolConstant;
 import org.jeecg.common.system.api.ISysBaseAPI;
 import org.springframework.beans.factory.annotation.Autowired;
@@ -152,6 +153,49 @@ public class FlowNodeExpression {
         return Collections.emptyList();
     }
 
+    /**
+     * Get first subprocess variable username.
+     * Example: ${flowNodeExpression.getSonProcessVariableString(execution, 'subApproveUser')}
+     *
+     * @param execution current execution
+     * @param variableName subprocess variable name
+     * @return username
+     */
+    public String getSonProcessVariableString(DelegateExecution execution, String variableName) {
+        List<String> usernames = getSonProcessVariable(execution, variableName);
+        if (CollectionUtils.isEmpty(usernames)) {
+            return null;
+        }
+        return usernames.get(0);
+    }
+
+    /**
+     * Save current task assignee as execution local variable.
+     * Example: ${flowNodeExpression.saveCurrentTaskAssigneeLocal(task, 'subApproveUser')}
+     *
+     * @param task current task
+     * @param variableName local variable name
+     */
+    public void saveCurrentTaskAssigneeLocal(DelegateTask task, String variableName) {
+        if (task == null || StringUtils.isBlank(variableName)) {
+            return;
+        }
+        String assignee = task.getAssignee();
+        if (StringUtils.isBlank(assignee)) {
+            log.warn("saveCurrentTaskAssigneeLocal skipped, taskId={}, taskDefinitionKey={}, assignee is blank",
+                    task.getId(), task.getTaskDefinitionKey());
+            return;
+        }
+        if (StringUtils.isBlank(task.getExecutionId())) {
+            log.warn("saveCurrentTaskAssigneeLocal skipped, taskId={}, taskDefinitionKey={}, executionId is blank",
+                    task.getId(), task.getTaskDefinitionKey());
+            return;
+        }
+        runtimeService.setVariableLocal(task.getExecutionId(), variableName, assignee);
+        log.info("saveCurrentTaskAssigneeLocal success, taskId={}, taskDefinitionKey={}, variableName={}, assignee={}",
+                task.getId(), task.getTaskDefinitionKey(), variableName, assignee);
+    }
+
     private List<String> splitUsernames(Object value) {
         if (value == null) {
             return Collections.emptyList();