How to affect normals and viewDir DX11 hull/domain

Hi,
I made a DX11 shader with tessellation and displacement and I want the normals and the viewDir of the surface to be affected by the displacement.

so I calculate my viewDir inside the domain shader but it doesn't work, I get the viewDir as if the mesh had no displacement at all.

Domain Shader:

Code (CSharp):

1. [domain("tri")]
2. v2f domain (OutputPatchConstant tessFactors, const OutputPatch<TessVertex,3> vi, float3 bary : SV_DomainLocation) {
3.  
4. appdata v = (appdata)0;
5.  
6. displacement(v);
7.  
8. v.vertex = vi[0].vertex*bary.x + vi[1].vertex*bary.y + vi[2].vertex*bary.z;
9. v.normal = vi[0].normal*bary.x + vi[1].normal*bary.y + vi[2].normal*bary.z;
10. v.tangent = vi[0].tangent*bary.x + vi[1].tangent*bary.y + vi[2].tangent*bary.z;
11. v.texcoord = vi[0].texcoord*bary.x + vi[1].texcoord*bary.y + vi[2].texcoord*bary.z;
12.  
13. v2f o = vert(v);
14.  
15. TANGENT_SPACE_ROTATION;
16. o.viewDir = mul(rotation, ObjSpaceViewDir(v.vertex));
17. o.lightDir = mul(rotation, ObjSpaceLightDir(v.vertex));
18.  
19. return o;
20. }

Thanks!