State diagrams

“A state diagram is a type of diagram used in computer science and related fields to describe the behavior of systems. State diagrams require that the system described is composed of a finite number of states; sometimes, this is indeed the case, while at other times this is a reasonable abstraction.” Wikipedia

Mermaid can render state diagrams. The syntax tries to be compliant with the syntax used in plantUml as this will make it easier for users to share diagrams between mermaid and plantUml.

Older renderer:

In state diagrams systems are described in terms of states and how one state can change to another state via a transition. The example diagram above shows three states: Still, Moving and Crash. You start in the Still state. From Still you can change to the Moving state. From Moving you can change either back to the Still state or to the Crash state. There is no transition from Still to Crash. (You can’t crash if you’re still.)

States

A state can be declared in multiple ways. The simplest way is to define a state with just an id:

Another way is by using the state keyword with a description as per below:

Another way to define a state with a description is to define the state id followed by a colon and the description:

Transitions

Transitions are path/edges when one state passes into another. This is represented using text arrow, “-->”.

When you define a transition between two states and the states are not already defined, the undefined states are defined with the id from the transition. You can later add descriptions to states defined this way.

It is possible to add text to a transition to describe what it represents:

Start and End

There are two special states indicating the start and stop of the diagram. These are written with the [*] syntax and the direction of the transition to it defines it either as a start or a stop state.

Composite states

In a real world use of state diagrams you often end up with diagrams that are multidimensional as one state can have several internal states. These are called composite states in this terminology.

In order to define a composite state you need to use the state keyword followed by an id and the body of the composite state between {}. See the example below:

You can do this in several layers:

You can also define transitions also between composite states:

You can not define transitions between internal states belonging to different composite states

Choice

Sometimes you need to model a choice between two or more paths, you can do so using <<choice>>.

Forks

It is possible to specify a fork in the diagram using <<fork>> <<join>>.

Notes

Sometimes nothing says it better than a Post-it note. That is also the case in state diagrams.

Here you can choose to put the note to the right of or to the left of a node.

Concurrency

As in plantUml you can specify concurrency using the – symbol.

Setting the direction of the diagram

With state diagrams you can use the direction statement to set the direction which the diagram will render like in this example.

Comments

Comments can be entered within a state diagram chart, which will be ignored by the parser. Comments need to be on their own line, and must be prefaced with %% (double percent signs). Any text after the start of the comment to the next newline will be treated as a comment, including any diagram syntax

Styling with classDefs

As with other diagrams (like flowcharts), you can define a style in the diagram itself and apply that named style to a state or states in the diagram.

These are the current limitations with state diagram classDefs:

Cannot be applied to start or end states
Cannot be applied to or within composite states

These are in development and will be available in a future version.

You define a style using the classDef keyword, which is short for “class definition” (where “class” means something like a CSS class) followed by a name for the style, and then one or more property-value pairs. Each property-value pair is a valid CSS property name followed by a colon (:) and then a value.

Here is an example of a classDef with just one property-value pair:

    classDef movement font-style:italic;

where

the name of the style is movement
the only property is font-style and its value is italic

If you want to have more than one property-value pair then you put a comma (,) between each property-value pair.

Here is an example with three property-value pairs:

    classDef badBadEvent fill:#f00,color:white,font-weight:bold,stroke-width:2px,stroke:yellow

where

the name of the style is badBadEvent
the first property is fill and its value is #f00
the second property is color and its value is white
the third property is font-weight and its value is bold
the fourth property is stroke-width and its value is 2px
the fifth property is stroke and its value is yello

Apply classDef styles to states

There are two ways to apply a classDef style to a state:

use the class keyword to apply a classDef style to one or more states in a single statement, or
use the ::: operator to apply a classDef style to a state as it is being used in a transition statement (e.g. with an arrow to/from another state)

1. `class` statement

A class statement tells Mermaid to apply the named classDef to one or more classes. The form is:

    class [one or more state names, separated by commas] [name of a style defined with classDef]

Here is an example applying the badBadEvent style to a state named Crash:

class Crash badBadEvent

Here is an example applying the movement style to the two states Moving and Crash:

class Moving, Crash movement

Here is a diagram that shows the examples in use. Note that the Crash state has two classDef styles applied: movement and badBadEvent

2. `:::` operator to apply a style to a state

You can apply a classDef style to a state using the ::: (three colons) operator. The syntax is

[state]:::[style name]

You can use this in a diagram within a statement using a class. This includes the start and end states. For example:

Spaces in state names

Spaces can be added to a state by first defining the state with an id and then referencing the id later.

In the following example there is a state with the id yswsii and description Your state with spaces in it. After it has been defined, yswsii is used in the diagram in the first transition ([*] --> yswsii) and also in the transition to YetAnotherState (yswsii --> YetAnotherState).
(yswsii has been styled so that it is different from the other states.)