Hierarchical state machines allow states and sub-states. Rules restrict which actions are allowed for each state.įinite state machines only allow one state at a time. User actions (also called triggers) cause the state machine to transition between states.
State machines come in different flavors, but they’re essentially a design pattern that represents a process that moves from one state to another. There are also some additional benefits such as preventing reentrancy of asynchronous methods and showing and hiding controls for specific states. It will also show you how to bind commands to the state machine.
This article will demonstrate how to design and implement a state machine by analyzing your application. Binding commands directly to the state machine provides a good solution to this tricky problem. Each new state requires additional conditions that need evaluation, so this strategy can quickly become overly complex.Ī state machine solves the “can execute” problem, as it restricts the actions allowed for a specific state. Typically, the “can execute” validation is implemented with a conditional expression using local variables such as IsLoading, CanDoXYZ, SomeObject != null and so on. The true challenge isn’t in executing the command, but ensuring it executes when the ViewModel is in a valid state for that action.
When a user interacts with that control, it executes the command.Īs always, the devil is in the details. Controls on the View bind to these properties. When you use the Model-View-ViewModel (MVVM) design pattern, a command is exposed on the ViewModel as a property that implements the ICommand interface. Windows Presentation Foundation (WPF) has a powerful commanding framework that lets you separate the UI and the command logic. Tarquin Vaughan-Scott | November 2014 | Get the Code Volume 29 Number 11 MVVM : WPF Commanding with the State Machine Pattern
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