要在Angular mat-tree的顶部插入新项目，可以使用以下步骤：

1. 在模板文件中，将mat-tree组件放置在一个mat-tree-node容器中：

  
    
    {{node.name}}
  
  
    
      
        {{treeControl.isExpanded(node) ? 'expand_more' : 'chevron_right'}}
      
    
    {{node.name}}
    
  

2. 在组件类中，定义树的数据源和控制器：

import {Component} from '@angular/core';
import {MatTreeFlatDataSource, MatTreeFlattener} from '@angular/material/tree';
import {FlatTreeControl} from '@angular/cdk/tree';

interface TreeNode {
  name: string;
  children?: TreeNode[];
}

interface FlatTreeNode {
  expandable: boolean;
  name: string;
  level: number;
}

@Component({
  selector: 'app-tree',
  templateUrl: 'tree.component.html',
  styleUrls: ['tree.component.css'],
})
export class TreeComponent {
  treeControl: FlatTreeControl;
  treeFlattener: MatTreeFlattener;
  dataSource: MatTreeFlatDataSource;

  constructor() {
    this.treeFlattener = new MatTreeFlattener(
      this.transformer,
      this.getLevel,
      this.isExpandable,
      this.getChildren
    );
    this.treeControl = new FlatTreeControl(
      this.getLevel,
      this.isExpandable
    );
    this.dataSource = new MatTreeFlatDataSource(
      this.treeControl,
      this.treeFlattener
    );

    const TREE_DATA: TreeNode[] = [
      {
        name: 'Root',
        children: [
          {name: 'Child 1'},
          {name: 'Child 2'},
        ]
      }
    ];

    this.dataSource.data = TREE_DATA;
  }

  transformer = (node: TreeNode, level: number) => {
    return {
      expandable: !!node.children && node.children.length > 0,
      name: node.name,
      level: level,
    };
  }

  getLevel = (node: FlatTreeNode) => {
    return node.level;
  }

  isExpandable = (node: FlatTreeNode) => {
    return node.expandable;
  }

  getChildren = (node: TreeNode): TreeNode[] => {
    return node.children;
  }
}

3. 要在顶部插入新项目，可以使用以下方法：

insertNewChild() {
  const TREE_DATA: TreeNode[] = this.dataSource.data;
  const newChild: TreeNode = {
    name: 'New Child'
  };
  TREE_DATA.unshift(newChild); // 使用unshift()方法将新项目插入到数组的开头
  this.dataSource.data = TREE_DATA;
}

4. 在模板文件中，添加一个按钮来触发插入新项目的方法：

Insert New Child

这样，当点击按钮时，新项目将插入到mat-tree的顶部。