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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.

Code:
mermaid
---
title: Simple sample
---
stateDiagram-v2
    [*] --> Still
    Still --> [*]

    Still --> Moving
    Moving --> Still
    Moving --> Crash
    Crash --> [*]
null

Older renderer:

Code:
mermaid
stateDiagram
    [*] --> Still
    Still --> [*]

    Still --> Moving
    Moving --> Still
    Moving --> Crash
    Crash --> [*]
null

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:

Code:
mermaid
stateDiagram-v2
    stateId
null

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

Code:
mermaid
stateDiagram-v2
    state "This is a state description" as s2
null

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

Code:
mermaid
stateDiagram-v2
    s2 : This is a state description
null

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.

Code:
mermaid
stateDiagram-v2
    s1 --> s2
null

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

Code:
mermaid
stateDiagram-v2
    s1 --> s2: A transition
null

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.

Code:
mermaid
stateDiagram-v2
    [*] --> s1
    s1 --> [*]
null

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:

Code:
mermaid
stateDiagram-v2
    [*] --> First
    state First {
        [*] --> second
        second --> [*]
    }
null

You can do this in several layers:

Code:
mermaid
stateDiagram-v2
    [*] --> First

    state First {
        [*] --> Second

        state Second {
            [*] --> second
            second --> Third

            state Third {
                [*] --> third
                third --> [*]
            }
        }
    }
null

You can also define transitions also between composite states:

Code:
mermaid
stateDiagram-v2
    [*] --> First
    First --> Second
    First --> Third

    state First {
        [*] --> fir
        fir --> [*]
    }
    state Second {
        [*] --> sec
        sec --> [*]
    }
    state Third {
        [*] --> thi
        thi --> [*]
    }
null

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>>.

Code:
mermaid
stateDiagram-v2
    state if_state <<choice>>
    [*] --> IsPositive
    IsPositive --> if_state
    if_state --> False: if n < 0
    if_state --> True : if n >= 0
null

Forks

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

Code:
mermaid
   stateDiagram-v2
    state fork_state <<fork>>
      [*] --> fork_state
      fork_state --> State2
      fork_state --> State3

      state join_state <<join>>
      State2 --> join_state
      State3 --> join_state
      join_state --> State4
      State4 --> [*]
null

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.

Code:
mermaid
    stateDiagram-v2
        State1: The state with a note
        note right of State1
            Important information! You can write
            notes.
        end note
        State1 --> State2
        note left of State2 : This is the note to the left.
null

Concurrency

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

Code:
mermaid
stateDiagram-v2
    [*] --> Active

    state Active {
        [*] --> NumLockOff
        NumLockOff --> NumLockOn : EvNumLockPressed
        NumLockOn --> NumLockOff : EvNumLockPressed
        --
        [*] --> CapsLockOff
        CapsLockOff --> CapsLockOn : EvCapsLockPressed
        CapsLockOn --> CapsLockOff : EvCapsLockPressed
        --
        [*] --> ScrollLockOff
        ScrollLockOff --> ScrollLockOn : EvScrollLockPressed
        ScrollLockOn --> ScrollLockOff : EvScrollLockPressed
    }
null

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.

Code:
mermaid
stateDiagram
    direction LR
    [*] --> A
    A --> B
    B --> C
    state B {
      direction LR
      a --> b
    }
    B --> D
null

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

Code:
mermaid
stateDiagram-v2
    [*] --> Still
    Still --> [*]
%% this is a comment
    Still --> Moving
    Moving --> Still %% another comment
    Moving --> Crash
    Crash --> [*]
null

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:

  1. Cannot be applied to start or end states
  2. 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:

  1. use the class keyword to apply a classDef style to one or more states in a single statement, or
  2. 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:

text
    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:

text
class Crash badBadEvent

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

text
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

Code:
mermaid
   stateDiagram
   direction TB

   accTitle: This is the accessible title
   accDescr: This is an accessible description

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

   [*]--> Still
   Still --> [*]
   Still --> Moving
   Moving --> Still
   Moving --> Crash
   Crash --> [*]

   class Still notMoving
   class Moving, Crash movement
   class Crash badBadEvent
   class end badBadEvent
null

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

text
[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:

Code:
mermaid
stateDiagram
   direction TB

   accTitle: This is the accessible title
   accDescr: This is an accessible description

   classDef notMoving fill:white
   classDef movement font-style:italic;
   classDef badBadEvent fill:#f00,color:white,font-weight:bold,stroke-width:2px,stroke:yellow

   [*] --> Still:::notMoving
   Still --> [*]
   Still --> Moving:::movement
   Moving --> Still
   Moving --> Crash:::movement
   Crash:::badBadEvent --> [*]
null

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.)

Code:
mermaid
stateDiagram
    classDef yourState font-style:italic,font-weight:bold,fill:white

    yswsii: Your state with spaces in it
    [*] --> yswsii:::yourState
    [*] --> SomeOtherState
    SomeOtherState --> YetAnotherState
    yswsii --> YetAnotherState
    YetAnotherState --> [*]
null