What is the expected behavior?

ngOnInit should be called before writeValue

What is the current behavior?

writeValue gets called before ngOnInit when the value accessor is set directly as described in the docs under ngControl, i.e. by using

if (this.ngControl != null) {
    this.ngControl.valueAccessor = this;
}

in the constructor to avoid cyclic dependency error when providing NG_VALUE_ACCESSOR.

What are the steps to reproduce?

Stackblitz

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular/Material: 7.2.8

Is there anything else we should know?