Slidez | , an application | iOS library

I did not understand exactly the difference between them. Could anyone help me ?

The technique used in the code snippet of the question is Gouraud Shading.

In common Phong shading means the technique, which does the light calculations per fragment, in compare at Gouraud Shading, the light calculations ar done per vertex.

This means that at Phong shading the light calcualtions ar done in the fragment shader (Not to be confused with Phong reflection model).

Phong shading:

At Gouraud Shading the light calculations are done in the vertex shader, for the corners of the primitives (corners of the triangles). The calculated light is (either in a perspective correct manner or linearly) interpolated for all the fragments covered by the primitive, according to the Barycentric coordinate. This increases the performance, but gives a big loss of quality, especially on large primitives and strong specular highlights.

Gouraud Shading:

Note, the light is not linear distributed on a surface. If the light is only calculated for some samples and interpolated in between them, this causes flat stains.

See the example, which compares the 2 techniques:

See? unrecognized selector sent to instance xxx which means you have a method that does not implement at a certain controller.

And reason: '-[App.ViewController aButton:]: unrecognized selector sent to instance 0x7fe464c131c0' which means the method of aButton: in App.ViewController does not implement or you call App.ViewController does not contain this method