如果在Angular中使用stepper表单时，程序化设置值后无效，可以尝试以下解决方法：

1. 使用ngAfterViewInit钩子函数确保在视图加载完毕后再进行设置值操作。

import { Component, AfterViewInit, ViewChild } from '@angular/core';
import { MatStepper } from '@angular/material/stepper';

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.css']
})
export class MyComponent implements AfterViewInit {
  @ViewChild(MatStepper) stepper: MatStepper;

  ngAfterViewInit() {
    this.stepper.selectedIndex = 1; // 设置要激活的步骤索引
  }
}

2. 使用ngZone.run()方法确保在Angular的变化检测周期内执行设置值操作。

import { Component, NgZone, ViewChild } from '@angular/core';
import { MatStepper } from '@angular/material/stepper';

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.css']
})
export class MyComponent {
  @ViewChild(MatStepper) stepper: MatStepper;

  constructor(private ngZone: NgZone) {}

  setValue() {
    this.ngZone.run(() => {
      this.stepper.selectedIndex = 1; // 设置要激活的步骤索引
    });
  }
}

3. 使用setTimeout()函数将设置值操作延迟到下一个变化检测周期。

import { Component, ViewChild } from '@angular/core';
import { MatStepper } from '@angular/material/stepper';

@Component({
  selector: 'app-my-component',
  templateUrl: './my-component.component.html',
  styleUrls: ['./my-component.component.css']
})
export class MyComponent {
  @ViewChild(MatStepper) stepper: MatStepper;

  setValue() {
    setTimeout(() => {
      this.stepper.selectedIndex = 1; // 设置要激活的步骤索引
    }, 0);
  }
}

希望以上解决方法能够帮助您解决Angular stepper表单在程序化设置值后无效的问题。