Using Theme Colors in Custom Externals?

WOW! What a journey here! Was it worth it? I guess if we count the learnings as valuable, then yes. I don't know the actual value of style-aware UI Objects in Min is worth the hours I put into this. But they exist now. So here is my solution in case it helps anyone else.

They main reason I wanted to do this is so my UI object would look ok by default regradless of what version of Max it's running. The Max 8 and Max 9 themes are really different. And the first time I opened my object in Max 9 I realized I had a a problem.

Here is the solution, including a summary of all of the above minus the red herrings and false leads:

Step 1:
Associate your attribute with a style. This is done in the Maxclass Setup message because that is the only place you have access to the t_class for your object, which is required for stylemap function.

message<> maxclass_setup{ this, "maxclass_setup",
    MIN_FUNCTION {
        c74::max::t_class * c = args[0];
        c74::max::class_attr_stylemap(c, "myattr", "bgcolor");    
        return {};
    }
};

Step 2:
Get the value of the attribute while it is still attached to the style value, before it gets overridden by the attribute defaults. This must happen in the object constructor. The attribute defaults are applied after the object executes... overwriting what we just inherited from the style.

There's two parts to this. First, outside of the constructor, you need to declare a t_jrgba variable to store the styleColor. Then in the construct, use object_attr_getjrgba() to save the current value of your attribute to that variable.

c74::max::t_jrgba syleColor

//Constructor
myExternal(const atoms& args = {}) : ui_operator::ui_operator{ this, args } {
          c74::max::object_attr_getjrgba(maxobj(), c74::max::gensym("myattr"), &styleColor);
}

Step 3:
Reapply the styleColor after it has been overriden by the defaults you provided in the attribute declaration. Again, the attribute defaults are applied after the constructor finishes. So we need to find a way to continue our object setup after the constructor is finished.

I found two ways of doing this. One is gross. The other requires a modification of c74_min_object_wrapper.h.

The gross way. (I'll just give the psuedocode here. Although I have tested this and it does work.)
Outside of your constructor declare a bool "constructorFinished" and initialize it to false.
At the end of your contructor launch a thread and then detach it.
Then set "constructorFinished = true"
Inside your thread create a bool called "waiting" and set it to true.
Create a while loop.
While waiting = true, check if constructorFinished is true. If not, keep waiting. If so, If constructor finished is true, then proceed with your your post constructor setup, and then set "waiting=false" to break the loop and complete the thread.

A better way:
Update c74_min_object_wrapper.h
add the following line after line 56 and again after line 63

self->m_min_object.try_call("post_attribute_setup");

Now you've defined a new min message that gets called after the attributes have received their default values or any values assigned by the user when they instantiate your object in Max.

Then you can set the color of myAttr to the value you stored in styleColor with something like this:

message<> post_attribute_setup{this, "post_attribute_setup", MIN_FUNCTION {  
    myAttr= atoms{ styleColor.red, styleColor.green, styleColor.blue, styleColor.alpha };
    return {};  
}};

This last solution is courtesy of JBG in this thread. I have added the change to a pull request here.

Now your attribute will initialize to the value it inherited from the theme.

One odd thing bout this solution is that the Format Palette, doesn't seem to pick up the color you've applied to the attribute this way. If you select the object and then view the attribute in the Theme Palette, the palette editor will show the color as {0 0 0 0}. But the inspector certainly shows the proper color and object is responsive to changes to the style color. So I'll take it!

Thanks to everyone from C74 who helped with this. And thanks for making such powerful and flexible software that inspires an obsessive there-must-be-a-way mentality. Every single time I've gone down one of these rabbit holes Min, Max, and C74 always delivers the goods in the end.

I'm very excited to keep this project moving. Much appreciation!