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Beta documentation · eXLib v0.145.4. Public release is pending.

Write & Apply an Expression

The starting job: drive one property with an expression, confirm it works, and commit it.

The task

You have a property — say a layer's Position — and you want an expression on it. Three things have to happen, in this order:

  1. Get an editing surface for that property's expression.
  2. Write the code, including any references to other properties.
  3. Find out whether After Effects accepts it, and commit it if so.

How it goes in plain After Effects

  1. Option/Alt-click the property's stopwatch in the Timeline. The expression is created empty and the caret lands in the inline expression field beside the property.
  2. Type into that field. It is a plain text field — one line tall until you drag it taller — with no syntax colouring, no completion, and no signature help.
  3. Reference another property by dragging the expression pickwhip (the spiral beside the field) onto that property in the Timeline. It writes the reference for you, inside the composition you're in.
  4. Commit with Enter on the numeric keypad, or by clicking anywhere outside the field.
i

Committing is the change. There is no way to try the expression and get the property back — if you don't like the result, you undo.

  1. If After Effects rejects it, it says so and switches the expression off, leaving a warning marker on the property. Re-open the field to read the message, fix, commit again. That loop is the whole feedback mechanism: nothing tells you about a problem until you commit.

What changes with eXLib

  • You write in a real editor, against a language that knows the AE dialect.
  • Problems surface while you type, not after you commit.
  • Audition asks After Effects itself for a verdict and puts the property back, so the "does this work?" question is answered before anything is committed.
  • Apply commits the expression and any controls it declares as one undo group.
✓

The status bar names the property Apply and Audition will use, with its dimension chip — 2D for Position. Read it before you click anything: it is the same live selection those two verbs re-read at the moment you press them.

Step by step

Step In plain After Effects With eXLib
1. Choose the property Select it in the Timeline Select it in the Timeline — the panel's status bar names it and shows its dimension
2. Get an editing surface Option/Alt-click the stopwatch; type in the inline field Click into the editor — a full Monaco surface, in its own tab
3. Write the code Plain text: no colouring, no completion Completions on ., ", ' or Ctrl+Space, merging AE's API, your own symbols, and the real layer and effect names in your comp
4. Reference another property Drag AE's pickwhip onto the property, in this comp ⌃⌘R opens the picker: click a leaf to insert it, and browse any comp in the project without moving AE
5. Find mistakes Nothing until you commit Live diagnostics as you type; hover an underline for the message; F8 / Alt+F8 walk them
6. Try it without committing Not available — committing is the change IDE → Audition, or ⌃⌘E: AE's own verdict plus the sampled value, property restored and the restore verified
7. Commit Enter on the keypad, or click away Apply on the toolbar
8. Read the verdict AE disables the expression and marks the property if it failed A toast naming the property written to — and, on failure, AE's own error text
9. Undo Cmd+Z in After Effects Cmd+Z once in After Effects — expression and any created controls together