publish a paramater at a specific keyframe timing

hello
I know my title is not clear at all, but the explanation should be.

I'm creating a little title with a white-colored text, which background is a black rectangle. This rectangle appears sooner than the text, and with a growing length (or width) from left to right. This length has to fit the length of the text, which changes each time I will be including this title onto various rushes. Once the rectangle has reached the specific length, text fades in. Ok ?

Onset for this rectangle is at frame 10, width 0 and shall grow to a set width at frame 40 (30 frames duration for this rectangle to get in place before text fades in).

I'd like to know what to publish so that, once in FCP, automatically width is 0 at frame 10 and so that I only have to set the width, because this latter depends on the text length, knowing that this target length has to occur at frame 40.

So far, in FCP, I still have to head up to frame 10 (step 1), input value 0 (step 2), then up to frame 40 (step 3) and set target value (step 4).

How can I create and publish this title so that step 1, 2 and 3 are already done ?

Many thanks in advance

You can automatically have the width accommodate any length text. Add a link to Object > Shape > Size > Width and apply the Text object to the Source Parameter and select Object Attributes > Size > Width. You can apply "padding" by Scaling the Link to slightly above 1.0. This will work with any text length and multiline texts too*.

You can animate the Link > Scale from 0 to 1.0 (plus padding) over 30 frames to apply the "grow" effect.

*you can similarly apply a Link to the Rectangle Height parameter and use the Object Attributes > Size > Height to automatically size the rectangle to the text object's height.

You can *align* the rectangle to the text by applying a Behaviors > Basic Motion > Align To behavior to the rectangle and apply the Text to the Source Object. No matter where you drag your text, the rectangle will follow. Set both Align parameters to Left. If you applied padding, you can use the Offset X parameter to "recenter" the rectangle w/r/t the text.

By using the Align To set at Left, the rectangle will grow from left to right instead of center out.

How you fix the timing in FCP may depend on whether or not you want to use Build In or Build Out markers because timing in FCP will be influenced by how long you make your title.

HTH