Explainer · what is an action in a dynamic block
What is an action in a dynamic block, explained
By Sumana Kumar · Published 22 Apr 2023 · Updated 15 Jan 2024
If a parameter is the steering wheel of a dynamic block, an action is the linkage that actually turns the wheels. An action ties a parameter to a set of objects and says how those objects respond when the parameter changes: stretch them, move them, scale them, array them, rotate them, mirror them. Without an action, a parameter just sits there with a grip that moves but no geometry following.
This explainer walks through the action types on the Block Editor's Actions panel, how each one pairs with the right parameter, the all-important stretch frame that decides which objects move, and the common reasons an action misbehaves. Understanding actions is what lets you read a dynamic block someone else built and predict exactly how it will flex.
An action links a parameter to geometry
Every action connects three things: a parameter, a selection of objects, and a behaviour. When you add a Stretch action you first associate it with a Linear parameter, then select the objects (and the stretch frame) that should respond, then the action is complete. From then on, dragging that parameter's grip stretches exactly those objects.
Because the action owns the object selection, the same parameter can drive several actions on different objects. A door's Width parameter might drive one Stretch action on the frame and another on the leaf, so a single grip widens both together. That separation of 'what controls' (parameter) from 'what responds' (action) is the core of the dynamic-block model.
The action types and their natural parameters
Each action expects a particular parameter. Move shifts objects rigidly and pairs with Point, Linear, Polar or XY. Stretch lengthens or shortens objects within a frame and pairs with Linear, Polar or XY. Polar Stretch stretches and rotates together, pairing with a Polar parameter. Scale resizes objects and pairs with Linear, Polar or XY. Rotate spins objects about a base and pairs with a Rotation parameter. Flip mirrors objects and pairs with a Flip parameter. Array repeats objects at an interval and pairs with Linear, Polar or XY. Lookup is special — it reads a Lookup parameter to swap values rather than transform geometry.
Matching the action to the parameter is half of authoring. A Stretch action on a Rotation parameter makes no sense; AutoCAD only offers compatible pairings, which is a useful guardrail when you are learning.
The stretch frame decides who moves
The Stretch action's most important and most misunderstood step is the stretch frame. After picking the parameter, you draw a crossing window — the stretch frame. Objects fully inside the frame move bodily; objects that cross the frame boundary get stretched; objects entirely outside the frame do not move at all. This is exactly how the ordinary STRETCH command works, applied automatically when the grip is dragged.
Get the frame right and a door leaf inside it slides while the frame edges that cross the boundary lengthen cleanly. Get it wrong — frame too small, or catching the wrong endpoints — and the block tears or leaves geometry behind. When a stretchy block misbehaves, the stretch frame is the first thing to check.
Action overrides: fine-tuning the response
Actions carry override properties that tune how they react. Distance Multiplier scales the response — set it to 0.5 and the objects move half as far as the parameter changes, useful for symmetric growth from a centre. Angle Offset rotates the direction of a move. For Array actions, the column spacing controls how tightly the repeated objects pack.
These overrides are what let one parameter drive asymmetric or proportional behaviour. A classic trick is a centre-justified stretch: two Stretch actions on the same Linear parameter, each with a 0.5 distance multiplier in opposite directions, so the block grows evenly from its middle rather than from one edge.
Action versus parameter, restated
It is worth nailing this because it is the source of most confusion. The parameter defines what can change and gives the user a control; the action defines what geometry changes and how. You need both. A parameter with no action gives a grip that moves but no response. An action needs a parameter to attach to and cannot exist alone.
A few parameters break the pattern: Visibility, Alignment and Base Point do their jobs without any action. But for everything that transforms geometry — stretching, moving, scaling, arraying, rotating, flipping — the parameter-plus-action pair is mandatory, and the action is the half that actually does the work.
Adding and testing an action
In the Block Editor, add the parameter first, then open the Actions panel and pick the action type. AutoCAD prompts you to select the parameter, then (for transform actions) a parameter point to associate the action with, then the objects to affect, then any frame. A small action icon appears near the parameter; you can move it for clarity since its position is cosmetic.
Always test by closing the Block Editor and dragging the real grip. Watch for objects that should move but don't (left out of the selection) and objects that move but shouldn't (caught in a stretch frame by accident). The BACTIONTOOL workflow and the BACTIONBARMODE display help you see which objects each action owns, which is invaluable when a complex block has several actions sharing one parameter.
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Questions
Frequently asked
What is an action in a dynamic block?+
An action is the behaviour that makes geometry respond when a parameter changes — stretch, move, scale, array, rotate or flip. It links a parameter to a selection of objects and defines how they transform.
Why does dragging a grip not change anything?+
Usually because the parameter has no action attached, or the action's object selection or stretch frame missed the geometry you expected to move. Re-open the block and check the action's selection set.
What is the stretch frame for?+
It is a crossing window that decides which objects a Stretch action affects: objects inside move, objects crossing the boundary stretch, and objects outside stay put — exactly like the standard STRETCH command.
How do I make a block grow from its centre?+
Add two Stretch actions to the same Linear parameter pointing in opposite directions, and set each action's distance multiplier to 0.5 so the block expands evenly from its middle.
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