angular 自定义表单验证问题

Question

表单中有建筑面积和出租面积想要验证出租面积必须小于等于建筑面积应怎么写啊小白求教  add = this.form.group({     buildingId: [null, [Validators.required]],     roomNo: [null, [Validators.required]],     roomResourceType: ['1'],     roomBuildAcreage: [null, [Validators.required]],     roomEquipments: [null],     roomDecorateState: ['1'],     roomRentedAcreage: [null, [Validators.required, this.validateRentedAcreage,rentedAcreageValidator(add)]],     roomResourceState: ['1', [Validators.required]],     isCharge: ['1', [Validators.required]],     heatingCost: [null, [Validators.required]],     roomDesc: [null],   });