Annotation Commands

This page introduces how annotation commands are implemented in cad-viewer, including both the command layer and the UI interaction layer.
The design closely follows AutoCAD-like concepts such as system variables, current layer, and entity draw styles.

Overview

Annotations in cad-viewer (such as annotation circles, rectangles, cloud rectangles, sketches, etc.) are implemented as:

• Specialized commands derived from a common base class
• Entities created dynamically on a dedicated annotation layer
• Draw styles controlled by system variables instead of layer styles
• A temporary UI toolbar that modifies system variables during command execution

This approach ensures:

• Consistent behavior across all annotation commands
• Easy show/hide of all annotations
• AutoCAD-style workflow familiarity

Command Part

Base Command Class

All annotation-related commands inherit from the same base class:

• AcApBaseRevCmd

This base class centralizes common behaviors, such as:

• Annotation layer management
• System variable backup & restore
• Command lifecycle handling (start / end / cancel)

By inheriting from AcApBaseRevCmd, all annotation commands follow the same execution pattern and system-variable behavior.

Annotation Layer Management

All annotation entities are created on a dedicated annotation layer, which is:

• Created dynamically if it does not exist
• Used only during annotation command execution

Behind the scenes, this is achieved by manipulating the system variable:

• clayer (current layer)

Workflow

1. Before creating an annotation entity:

   • The original value of clayer is saved
   • clayer is switched to the annotation layer

2. Annotation entities are created

3. After command finishes (or is canceled):

   • clayer is restored to its original value

This guarantees:

• Annotations are always placed on the correct layer
• User layer state is not permanently modified

Show / Hide Annotations

Hiding or displaying annotations is implemented simply by:

• Hiding or displaying the annotation layer

No per-entity visibility logic is required.
This makes annotation visibility management efficient and consistent.

Entity Draw Style Resolution

Annotation entities do not use layer styles for their appearance.

Instead, their draw styles are resolved from system variables, including:

• cecolor — current entity color
• celweight — current entity line weight

This design allows:

• Each annotation to keep its visual style at creation time
• Layer style changes not affecting existing annotations

UI Part

Entity Draw Style Toolbar

When creating an annotation entity, the following UI component is shown:

• MlEntityDrawStyleToolbar

Behavior

• Appears at the top center of the viewer

• Allows users to modify:

  • Color
  • Line weight
  • Other entity draw styles

Because this toolbar overlaps with the file name display:

• The file name is temporarily hidden while the toolbar is visible

System Variable Binding

User interactions with MlEntityDrawStyleToolbar directly modify system variables, such as:

• cecolor
• celweight

These system variables are then used by annotation entities during creation, ensuring:

• What the user sees in the toolbar matches what is drawn
• Annotation behavior is consistent with AutoCAD-style workflows

System Variable Restore

After an annotation command finishes (successfully or not):

• Modified system variables (cecolor, celweight, etc.) are restored to their original values

This ensures:

• Annotation commands do not permanently affect global drawing state
• Users can safely switch between annotation and normal drawing commands

Summary

The annotation system in cad-viewer is built around three key ideas:

1. Command inheritance

   • All annotation commands share a common base implementation

2. Layer-based organization

   • All annotations live on a dynamically managed annotation layer

3. System-variable-driven styles

   • Draw styles are controlled via system variables, not layer styles

This architecture provides:

• Predictable command behavior
• Easy annotation management
• A familiar CAD-like user experience