How to Validate FormArray length in angular2

Validators.required does the magic for you:

// using required 

this.formGroup = this.formBuilder.group({
    taskTreeId: [Common.UID()],
    executionProgrammedTime: ["", [Validators.required]],
    description: [""],
    tasks: this.formBuilder.array([], Validators.required)
});  

// using custom validation

export const minLengthArray = (min: number) => {
  return (c: AbstractControl): {[key: string]: any} => {
    if (c.value.length >= min)
      return null;

    return { MinLengthArray: true};
  }
}

 this.formGroup = this.formBuilder.group({
        taskTreeId: [Common.UID()],
        executionProgrammedTime: ["", [Validators.required]],
        description: [""],
        tasks: this.formBuilder.array([], minLengthArray(2))
    });

<button type="button" class="btn btn-success btn-rounded" 
    [disabled]="!formGroup.valid">SAVE</button>