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State diagram (Official documentation)

Mermaid state diagram official documentation as a prompt

System Message: # 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. ```mermaid-example --- title: Simple sample --- stateDiagram-v2 [*] --> Still Still --> [*] Still --> Moving Moving --> Still Moving --> Crash Crash --> [*] ``` Older renderer: ```mermaid-example stateDiagram [*] --> Still Still --> [*] Still --> Moving Moving --> Still Moving --> Crash Crash --> [*] ``` 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: ```mermaid-example stateDiagram-v2 stateId ``` Another way is by using the state keyword with a description as per below: ```mermaid-example stateDiagram-v2 state "This is a state description" as s2 ``` Another way to define a state with a description is to define the state id followed by a colon and the description: ```mermaid-example stateDiagram-v2 s2 : This is a state 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. ```mermaid-example stateDiagram-v2 s1 --> s2 ``` It is possible to add text to a transition to describe what it represents: ```mermaid-example stateDiagram-v2 s1 --> s2: A transition ``` ## 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. ```mermaid-example stateDiagram-v2 [*] --> s1 s1 --> [*] ``` ## 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: ```mermaid-example stateDiagram-v2 [*] --> First state First { [*] --> second second --> [*] } ``` You can do this in several layers: ```mermaid-example stateDiagram-v2 [*] --> First state First { [*] --> Second state Second { [*] --> second second --> Third state Third { [*] --> third third --> [*] } } } ``` You can also define transitions also between composite states: ```mermaid-example stateDiagram-v2 [*] --> First First --> Second First --> Third state First { [*] --> fir fir --> [*] } state Second { [*] --> sec sec --> [*] } state Third { [*] --> thi thi --> [*] } ``` _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 &lt;&lt;choice&gt;&gt;. ```mermaid-example stateDiagram-v2 state if_state <<choice>> [*] --> IsPositive IsPositive --> if_state if_state --> False: if n < 0 if_state --> True : if n >= 0 ``` ## Forks It is possible to specify a fork in the diagram using &lt;&lt;fork&gt;&gt; &lt;&lt;join&gt;&gt;. ```mermaid-example 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 --> [*] ``` ## 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. ```mermaid-example 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. ``` ## Concurrency As in plantUml you can specify concurrency using the -- symbol. ```mermaid-example 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 } ``` ## 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. ```mermaid-example stateDiagram direction LR [*] --> A A --> B B --> C state B { direction LR a --> b } B --> D ``` ## 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 ```mermaid stateDiagram-v2 [*] --> Still Still --> [*] %% this is a comment Still --> Moving Moving --> Still %% another comment Moving --> Crash Crash --> [*] ``` ## 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](https://www.w3.org/TR/CSS/#properties)_ 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 `yellow` ### 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: ```txt 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`: ```txt class Crash badBadEvent ``` Here is an example applying the `movement` style to the two states `Moving` and `Crash`: ```txt 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` ```mermaid-example 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 ``` #### 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 ```txt [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: ```mermaid-example 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 --> [*] ``` ## 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.) ```mermaid-example 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 --> [*] ```

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