HarmonyOS this和箭头函数相关问题

如下代码中click1的this为undefined，click2的this正常是什么原因？

import { hilog } from '@kit.PerformanceAnalysisKit';

class A {
  click1(event: ClickEvent) {
    hilog.info(0, 'TAG', `click1 ${this}`)
  }

  click2 = (event: ClickEvent) => {
    hilog.info(0, 'TAG', `click2 ${this}`)
  }
}

@Entry
@Component
struct Index {
  @State message: string = 'Hello World';
  a: A = new A();

  build() {
    RelativeContainer() {
      Row() {
        Text('click1').onClick(this.a.click1)
        Text('click2').onClick(this.a.click2)
      }
    }
    .height('100%')
    .width('100%')
  }
}