Sprite Kit Shader Uniforms Ignored - opengl-es

Question

The shader that I'm using relies upon the position of the tiles in my game. I haven't found anything on using attribute variables with SKShader objects, so I went with updating the uniform variables. But it would seem that the shader won't communicate with the variables, especially once their values have been updated and changed. I am trying to make a basic lighting effect, but I can't get anything out of the shader at all. Any help? My code for the shader and for the Objective C classes are below.
Shader
uniform float midX, midY;
uniform float posX;
uniform float posY;
void main()
{
vec4 temp = SKDefaultShading(); // get the default shading
float lightRad = 200.0; // Light radius
float dist = distance(vec2(posX, posY), vec2(midX, midY)); // location of the light on the screen
vec4 color = vec4(1.0, 0, 0.0, (float)(dist / lightRad)); // creates an alpha gradient for the light. (falloff)
if (dist < lightRad) // only applies the light color if the distance from the light to the tile is smaller than the radius of the light
{
gl_FragColor = temp * color; // applies the color
}
else // otherwise, do nothing
{
gl_FragColor = temp;
}
}
Code
- (void) loadShaders
{
SKUniform* posX = [SKUniform uniformWithName:#"posX" float: 0.0f]; // adds the x position (with a placeholder value)
SKUniform* posY = [SKUniform uniformWithName:#"posY" float: 0.0f]; // adds the y position (with a placeholder value)
[_shader addUniform:posX];
[_shader addUniform:posY];
}
-(void)update:(CFTimeInterval)currentTime
{
for (int i = 0; i < _array.count; i++) // Loop through all tiles
{
float x = ((i % 100) - 13.5f) * 15.0f; // Calculate x pos of the tile
float y = ((1 - (i / 100)) + 6.5f) * 15.0f; // Calculate y pos of the tile
SKUniform* uniX = [[_tMap getShader] uniformNamed:#"posX"]; // get the uniform with the name posX
uniX.floatValue = x; // set the value of that uniform
SKUniform* uniY = [[_tMap getShader] uniformNamed:#"posY"]; // get the uniform with the name posY
uniY.floatValue = y; // set the value of that uniform
}
}
I'm fairly new to sprite kit, and I'm also new to GLSL.