Meta-modeling Concepts

The meta-meta-model in webgme describes the syntax for the meta-models. Below follows a short description and a video elaborating on each concept defined in the meta-meta-model. To follow this tutorial it is perfectly fine to skip Sets, Mixins, Aspects and Constraints.

This entire section can also be seen as a reference manual and it’s recommended to return to these when creating the meta-model.

Containment

  • Defines where in the containment hierarchy a node can be created

  • One-to-many relationship

    • The cardinality can be restricted (e.g 0..* means any number of children)

  • All nodes have a parent

    • Except the ROOT node which is the root of the Composition Tree

  • Containment is a strong relationship: if a parent node is deleted so are all its children

Click here to open the video in a browser..



Base Relationship/Inheritance

  • The base is a built-in pointer (one-to-one relationship)

  • All nodes have base pointer defined

  • All nodes (except the ROOT and FCO) have another node as their base

    • The FCO is the root of the Inheritance Tree (the ROOT lives outside!)

  • We can modify the value of base in the Meta Editor

  • The base/instance relationship is a strong relationship: if a base node is deleted so are all its instances

  • A node’s base is its prototype

    • Prototypal/Prototypical inheritance: asking for a property of a node traverses the base-chain till a value is reached

Click here to open the video in a browser..



Attributes

  • Textual or numerical information

  • Types: string, boolean, integer, float and asset

    • Assets can store a SHA-256 hash acting as an identifier for an artifact stored outside the model database

  • name is by default defined on the FCO

    • Is used to identify meta-nodes

  • Depending on the type, attributes can have different properties:

    • Default value - the value stored at the node defining the attribute
    • Read-only - the value can only be edited at meta-nodes
    • Enumeration, min, max, regex, multiline, etc.

Click here to open the video in a browser..



Pointers

  • Named one-to-one relation between two nodes, the owner and the target
  • Target of a pointer is either another node or NULL
  • base is a built-in pointer (inheritance)

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Connections

  • Not an actual concept of meta-meta-model
  • Using reserved named pointers, src and dst, we can create connections
  • Connections are a visualization technique on the canvas for nodes with pointers src and dst defined
  • We can assign attributes, children etc. to a connection and we can create any number of connections from/to a node
  • In the Property Editor we can see that it’s just two pointers
  • If any of the src or dst pointers are NULL, the connection appears as a regular box annotated with << Connection >>

Click here to open the video in a browser..



Sets

  • One-to-many relationship between a set owner and members

  • The cardinality can be restricted (e.g 0..* means any number of members)

  • Has special visualizer: Set membership

    • Similar to Composition - but members are visualized/edited

  • Meta-nodes are the members of the built-in set, MetaAspectSet, owned by the ROOT

Click here to open the video in a browser..



Mixins

  • Meta-nodes can be used by other meta-nodes as mixins

  • Meta-definitions are inherited from mixins

    • No actual data (e.g. attr values) of the mixin node is inherited

  • A meta-node can have more than one mixin (but only one base)

    • In case of colliding definitions, the base node has precedence
    • Collisions among mixins resolved based on GUID

Click here to open the video in a browser..



Aspects

  • Defines filtered views of your composition/containment (by selecting a set of valid children types)
  • Defined aspects show up as tabs in the Composition view

Click here to open the video in a browser..



Constraints

  • Functions defining constraints that cannot be captured by other meta-rules

  • Such custom constraints are evaluated at the server and by default turned off

    • To enable config.core.enableCustomConstraints = true;

  • Constraints will be evaluated for every node that is of the meta-type where

Click here to open the video in a browser..



Meta Properties

Meta properties are properties that are typically set on meta-nodes with main purpose to guide/constrain the end-user while modeling. In contrast to the other concepts they are not defined using the Meta Editor, instead they are set in META tab in the Property Editor.

../_images/meta_properties.png
isAbstract
An abstract node cannot be instantiated or copied
isPort
A port node will visually be elevated to the border of its parent.
validPlugins
A list of plugins that can accepts the node as active-node.
validVisualizers
Which visualizers should be listed for the node and which one should be opened when navigating to it.