After Effects Position Expression Help

Um I kinda found the solution through some other videos. if I can be honest, your solutions were more complex than what I needed but I understand that it's because I'm using an old method. Are there any websites where I can basically learn these expressions through tutorials or such, maybe like Codecademy?

Solution:

Add to the position of the caption...

title = thisComp.layer("Rectangle").sourceRectAtTime();

value + [title.width, 0]

If you have a new enough version of After Effects you can use the sourceRecAtTime() method. It is fairly well explained here:

Evan Abrams | SourceRectAtTime() Expression- Adobe After Effects

I use a very similar technique but I add a bit to height and width to clean up the design and I'll also use a time offset to make the text box expand. I add a text animator to the text layer to make it fade on. Then I add a shape layer to the timeline above the text layer by double-clicking the tool then add two sliders, name them Padding and Offset, then I'll add this expression to the size of the rectangle:

pad = effect("Padding")("Slider");

ofst = effect("Offset")("Slider") / 100;

src =thisComp.layer(index + 1);

x= src.sourceRectAtTime(time-src.inPoint,true).width + pad;

y= src.sourceRectAtTime(time-src.inPoint,true).height + pad;

adj = src.text.animator("Animator 1").selector("Range Selector 1").start.valueAtTime(time + ofst)/100;

[x * adj, y]

You also add an expression to the Transform Rectangle 1>Anchor point to make the rectangle expand from the left. It looks like this:

x = content("Rectangle 1").content("Rectangle Path 1").size[0]/2;

[- x, value[1]]

The next step is to let the animation complete by going to the last Animator keyframe, position the text box over the text layer, adjust the Padding and then move back in time a bit and adjust the Offset to line the timing of the expanding text box up with the words as they animate on.

I hope that makes sense. Here's the animation preset that I have made to speed up the process: Dropbox - Auto Text background.ffx

All you have to do is download the preset, animate a text layer with left justified text, deselect the text layer and browse to the ahimation preset and run it. You will get a text layer that you can position over your text and then parent to the text layer. You'll get a comp that looks like this: