Provide a way to identify item of FormArray other than index

I’m submitting a…

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/angular/angular/blob/master/CONTRIBUTING.md#question

Current behavior

Index of array seems to be only way to identify item of FormControl in FormArray. If a form has sort functionality, there is no way to identify sorted items.

ngOnInit() {
  this.service.fetchTodos.subscribe(todos => {
    // todos: [ { id: 1, content: 'foo', sortNo: 0 }, { id: 2, content: 'bar', sortNo: 1 } ]
    this.form = this.fb.group({
      // user can edit content and sortNo by drag&drop
      todos: this.fb.array(todos.map(t => t.content)),
    });
  });
}

onSubmit() {
  const value = this.form.value;
  // { todos: [ 'foo yay', 'barrr' ] }
  // if todos are sortable, { id: 1, content: 'foo yay', sortNo: 0 } or { id: 2, content: 'foo yay', sortNo: 0 } is indeterminable
  ...
}

Expected behavior

FormControl or AbstractControl has a way to identify themselves, ex: FormControl#id like

ngOnInit() {
  this.service.fetchTodos.subscribe(todos => {
    // todos: [ { id: 1, content: 'foo', sortNo: 0 }, { id: 2, content: 'bar', sortNo: 1 } ]
    this.form = this.fb.group({
      todos: this.fb.array(todos.map(t => { value: t.content, id: t.id })),
    });
  });
}

onSubmit() {
  const value = this.form.value;
  // { todos: [ 'foo yay', 'barrr' ] }

  const controls = this.form.get('todos').controls.map((c, i) => { id: c.id, value: c.value, sortNo: i });
  // [ { id: 2, value: 'foo yay', sortNo: 0 }, { id: 1, value: 'barrr', sortNo: 1 } ]
}

Minimal reproduction of the problem with instructions

https://stackblitz.com/edit/angular-zcvvg4

What is the motivation / use case for changing the behavior?

I want to sort items of FormArray

Environment

Angular version: 4.3.6


Browser:
- [x] Chrome (desktop) version 61
- [ ] Chrome (Android) version XX
- [ ] Chrome (iOS) version XX
- [ ] Firefox version XX
- [ ] Safari (desktop) version XX
- [ ] Safari (iOS) version XX
- [ ] IE version XX
- [ ] Edge version XX
 
For Tooling issues:
- Node version: 8.2.1  
- Platform:  Mac 

Others:

[annuranjan]

Never mind, I found the solution too. I used “findIndex” method of the abstract control: index = (<FormArray>this.myForm.get('myFormArray')).controls.findIndex(x => x.value.id === myConditionId);

[adwd]

I handled this by another way, so closing this issue.