引入Post，新建两个方法：findAll，create

src/posts/providers/demo/demo.service.ts

@@ -1,4 +1,15 @@
 import { Injectable } from '@nestjs/common';
+import { Post } from 'src/posts/interfaces/post.interface';
 
 @Injectable()
-export class DemoService {}
+export class DemoService {
+    private readonly posts: Post[] = [];
+
+    findAll(): Post[] {
+        return this.posts;
+    }
+
+    create(post: Post) {
+        this.posts.push(post);
+    }
+}