Subgraphs are MetaEdit+’s way to enable arbitrary depth of models, by linking a model element with a subgraph describing it in more detail. There are two kinds of semantically different link types: decompositions and explosions. Both link types are language-dependent and are available only if the language definition allows them.

Decomposition provides a semantically stricter link type between an object and a subgraph and can be used to build model hierarchies based on arbitrary leveling (as in Figure 3–46). Only zero or one decomposition is allowed for an object, and applies in all graphs containing that object. A graph can however be the target of a decomposition of multiple objects. Decomposition links are normally between graphs of the same type.

Each object, relationship or role in a graph can also be linked to other graphs via explosion. The explosion link allows selecting one element from the graph and explodes that to a new graph. An object (or a relationship or a role) can have a different set of explosions in each graph where it is used. Similarly, the same graph can be used as the target of multiple elements in multiple graphs. Explosion is often used between different graph types.

You can find out which object types can be decomposed or exploded into which graph types by selecting Help | Graph Type... in the Diagram, Matrix or Table Editor, and then the ‘Subgraphs’ help item.

The set of all existing subgraph links from this graph can be found from the Info Tool for the graph. The subgraphs of the current selected object can be seen from the status bar at the bottom of the Diagram Editor window.

To open or create a subgraph link, either hold down the Ctrl-key and double-click the element you want to create the subgraph link for, or select Open Subgraph from its popup menu. If there is only one subgraph attached to the element, that subgraph will be opened. If there are several possible subgraphs to choose from, a simplified version of the Subgraph management dialog will open, prompting to select one (as in Figure 3–47).

If there are currently no existing subgraphs, the Subgraphs dialog will open and allow you to create a new subgraph link (as shown in Figure 3–48).

The Subgraphs dialog for opening and creating subgraphs provides a list of possible candidate subgraphs and subgraph creation operations, sorted by the graph type and graph name. If both decomposition and explosion links are available for the current selection both of these choices will be presented separately as well. For example, the list in Figure 3–48 reads as follows: a user can either create a subgraph link to a new Communication, Activity, Sequence or State Machine Diagram, or link to an existing diagram, either the “Ball Game Behaviour - Design” Sequence Diagram or the “States of the Ball” State Machine Diagram. For Communication, Activity and Sequence diagrams only one type of subgraph link is possible (explosion in this case) whereas for State Machine both decomposition and explosion links are available. Select an existing subgraph or a creation operation from the list either by double-clicking it or by selecting and accepting with OK.

By default a new subgraph will be opened as Diagram – if you need some other representation like Matrix or Table or want to leave it unopened, check the appropriate radio button in the Create as box.

The simplified versions of the Subgraphs dialog also have a Manage button at the top of the dialog. Pressing the button will expand the dialog to the complete subgraphs management dialog as described next.

The Subgraphs dialog (as shown in Figure 3–49) lets you manage the subgraph links attached to an element. To open the Subgraphs dialog, select an element and choose Manage Subgraphs... from its popup menu.

The subgraphs currently attached to the element are listed on the right side of the dialog, whereas the list on the left shows possible subgraph candidates and the operations for creating new ones. To attach a subgraph candidate, select it from the Candidate subgraphs list and click the > button between the lists. Similarly, to remove an existing link to a subgraph, select it from the list of Current subgraphs and press the < button.

To create a new subgraph and link to it, select either ‘Create new explosion...’ or ‘Create new decomposition...’ for the required graph type from the Candidate subgraphs list (please note that the available link types depend on whether or not the language definition allows them). When creating a new subgraph, you also need to define which kind of representation will be created and opened for it. The default is Diagram but you can also choose Matrix or Table – or to not create or open a representation yet – by selecting the appropriate radio button at the bottom of the dialog.

Double-clicking an item on either list is an indication that you want to open the item. Double-clicking on the Current subgraphs list simply opens the selected item. Double-clicking on the Candidate subgraphs list attaches the selection – either an existing subgraph or a newly created one – as a subgraph before opening it.

To close the Subgraphs dialog, press the Close button in the lower right corner.