我不是着色器专家但我已经成功地做到了这一点，我认为这是一个简单的方法。 （我可能会非常困惑）。

I am not a shader expert but I have done this successfully in what I think is an easy way. (I may be very confused).

在我的java代码中，我的Main函数中有一个变量，它包含alpha值（我只使用一个用于gamestate过渡）：

In my java code I have a variable in my Main function that holds alpha values (I only use one for gamestate transitions):

public static float newAlph = 1;

在我的* .vert文件中，我声明了一个统一变量，将alpha作为输入，然后设置它是变量变量的值，所以.frag文件得到它。

In my *.vert file I declared a uniform variable to take the alpha as an input and then set it's value to a varying variable so the .frag file gets it.

...

uniform float u_newAlpha;
varying float v_newAlpha;

...

void main() {
    ...
    v_newAlpha = u_newAlpha;
}

然后在.frag文件中声明相同的变量变量并简单地乘以alpha。

Then in the .frag file i declare the same varying variable and simply multiply the alpha.

...
varying float v_newAlpha;
...

void main() {
    ...
    gl_FragColor = vec4(v_color.rgb, alpha*v_newAlpha);
}

最后在调用着色器时我设置了alpha值（这样你就可以了你在draw方法中的标签类）

Finally when calling the shader i set the alpha value(this you would do in your label class in the draw method)

sb.setShader(fontShader);
fontShader.setUniformf("u_newAlpha", Main.newAlph);

所以即使渲染的其余部分的alpha值正在改变，这个着色器也会渲染你的任何东西正在使用它拥有的alpha进行渲染。

So even if the alpha of the rest of everything that is rendering is changing this shader will render whatever you are rendering with the alpha it has.

在我的使用中，我使用了主类中的公共变量，但是你的draw方法将alpha作为输入，因此将其设置为制服.vert文件中的变量应该可以工作。

In mine I use a public variable from the main class but your draw method takes the alpha as an input so setting that to the uniform variable inside the .vert file should work.

因此你会得到：

public void draw(Batch batch, float parentAlpha) {
        if(shaderActive) {
            batch.setShader(Assets.assetFont.fontShader);
        }
        Assets.assetFont.fontShader.setUniformf("u_newAlpha", parentAlpha);
        super.draw(batch, parentAlpha);
        batch.setShader(null);          
    }

如果有意义，请告诉我！

Let me know if that makes sense!