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‎BT_basics/index.html

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<a href="#learn-by-example" title="Learn by example" class="md-nav__link">
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Learn by example
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<a href="#examples" title="Examples" class="md-nav__link">
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Examples
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</li>
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<a href="#learn-by-example" title="Learn by example" class="md-nav__link">
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Learn by example
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<a href="#examples" title="Examples" class="md-nav__link">
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Examples
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<p>For instance, in a service-oriented architecture, the leaves would contain
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the "client" code that communicate with the "server" that performs the
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operation.</p>
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<p>In the following example, we can see two Actions executed in a sequence,
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<code>DetectObject</code> and <code>GraspObject</code>.</p>
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<p><img alt="Leaf To Component Communication" src="../images/LeafToComponentCommunication.png" /></p>
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<p>The other nodes of the tree, those which are not leaves, control the
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<p>The other nodes of the tree, those which are <strong>not leaves</strong>, control the
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"flow of execution".</p>
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<p>To better understand how this flow takes place , imagine a signal, that we will further
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call "<strong>tick</strong>"; it is executed at the <strong>root</strong> of the tree and propagates through
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the branches until it reaches one or multiple leaves.</p>
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<p>The <code>tick()</code> callback returns a <code>NodeStatus</code> that will be either:</p>
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<p>To better understand how this control flow takes place , imagine a signal
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called "<strong>tick</strong>"; it is executed at the <strong>root</strong> of the tree and it propagates
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through the branches until it reaches one or multiple leaves.</p>
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<div class="admonition note">
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<p class="admonition-title">Note</p>
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<p>The word <strong>tick</strong> will be often used as a <em>verb</em> (to tick / to be ticked) and it means</p>
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<p>"To invoke the callback <code>tick()</code> called of a <code>TreeNode</code>".</p>
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</div>
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<p>Then a <code>TreeNode</code> is ticked, it returns a <code>NodeStatus</code> that can be either:</p>
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<ul>
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<li><strong>SUCCESS</strong></li>
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<li><strong>FAILURE</strong></li>
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and they needs more time to return a valid result.</p>
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<p>This C++ library provides also the status <strong>IDLE</strong>; it means that the node is ready to
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start.</p>
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<p>The result of a node is propagated back to the parent, that will decide
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<p>The result of a node is propagated back to its parent, that will decide
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which child should be ticked next or will return a result to its own parent.</p>
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<h2 id="types-of-nodes">Types of nodes</h2>
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<p><strong>ControlNodes</strong> are nodes which can have 1 to N children. Once a tick
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they are always atomic, i.e. they must not return RUNNING. They should not
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alter the state of the system.</p>
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<p><img alt="UML hierarchy" src="../images/TypeHierarchy.png" /></p>
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<h2 id="learn-by-example">Learn by example</h2>
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<h2 id="examples">Examples</h2>
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<p>To better understand how a BehaviorTrees work, let's focus on some practical
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examples. For the sake of simplicity we will not take into account what happens
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when an action returns RUNNING.</p>
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<ul>
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<li>If a child returns SUCCESS, tick the next one.</li>
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<li>If a child returns FAILURE, then no more children are ticked and the Sequence returns FAILURE.</li>
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<li>If all the children return SUCCESS, then the Sequence returns SUCCESS too.</li>
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<li>If <strong>all</strong> the children return SUCCESS, then the Sequence returns SUCCESS too.</li>
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</ul>
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<details class="warning"><summary>Exercise: find the bug! Expand to read the answer.</summary><p>If the action <strong>GrabBeer</strong> fails, the door of the
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fridge would remain open, since the last action <strong>CloseDoor</strong> is skipped.</p></details><h3 id="decorators">Decorators</h3>
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<div class="admonition warning">
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<p class="admonition-title">Have you spotted the bug?</p>
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<p>If the action <strong>GrabBeer</strong> fails, the door of the
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fridge would remain open, since the last action <strong>CloseFridge</strong> is skipped.</p>
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</div>
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<h3 id="decorators">Decorators</h3>
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<p>The goal of a <a href="../DecoratorNode/">DecoratorNode</a> is either to transform the result it received
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from the child, to terminate the child,
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or repeat ticking of the child, depending on the type of Decorator.</p>
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</pre></div>
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<p><strong>But</strong> there is an error. Can you find it?</p>
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<details class="warning"><summary>Exercise: find the bug! Expand to read the answer.</summary><p>If <strong>DoorOpen</strong> returns FAILURE, we have the desired behaviour.
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<p>But...</p>
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<div class="admonition warning">
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<p class="admonition-title">Have you spotted the bug?</p>
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<p>If <strong>DoorOpen</strong> returns FAILURE, we have the desired behaviour.
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But if it returns SUCCESS, the left branch fails and the entire Sequence
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is interrupted. </p></details><h3 id="second-controlnode-fallback">Second ControlNode: Fallback</h3>
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is interrupted.</p>
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<p>We will see later how we can improve this tree. </p>
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</div>
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<h3 id="second-controlnode-fallback">Second ControlNode: Fallback</h3>
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<p><a href="../FallbackNode/">FallbackNodes</a>, known also as <strong>"Selector"</strong>,
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are nodes that can express, as the name suggests, fallback strategies,
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ie. what to do next if a child returns FAILURE.</p>
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<p>In the next example, you can see how Sequence and Fallbacks can be combined:</p>
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<p><img alt="FallbackNodes" src="../images/FallbackBasic.png" /> </p>
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<blockquote>
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<p>In the door open?</p>
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<p>I not, try to open the door.</p>
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<p>Is the door open?</p>
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<p>If not, try to open the door.</p>
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<p>Otherwise, if you have a key, unlock and open the door.</p>
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<p>Otherwise, smash the door. </p>
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<p>If any of these actions succeeded, then enter the room.</p>
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<p>If <strong>any</strong> of these actions succeeded, then enter the room.</p>
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</blockquote>
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<h3 id="fetch-me-a-beer-revisited">"Fetch me a beer" revisited</h3>
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<p>We can now improve the "Fetch Me a Beer" example, which leaves the door open
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if the beer was not there.</p>
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<p>We use the color "green" to represent nodes which will return
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<p>We can now improve the "Fetch Me a Beer" example, which left the door open
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if the beer was not inside the fridge.</p>
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<p>We use the color "green" to represent nodes which return
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SUCCESS and "red" for those which return FAILURE. Black nodes are never executed. </p>
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<p><img alt="FetchBeer failure" src="../images/FetchBeerFails.png" /></p>
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<p>Let's create an alternative tree that closes the door even when <strong>GrabBeer</strong>
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returns FAILURE.</p>
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<p><img alt="FetchBeer failure" src="../images/FetchBeer.png" /></p>
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<p>Both the trees will close the door of the fridge, eventually, but:</p>
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<p>Both these trees will close the door of the fridge, eventually, but:</p>
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<ul>
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<li>the tree on the <strong>left</strong> side will always return SUCCESS if we managed to
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open and close the fridge.</li>
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<li>the tree on the <strong>right</strong> side will return SUCCESS if the beer was there,
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FAILURE otherwise.</li>
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<li>
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<p>the tree on the <strong>left</strong> side will always return SUCCESS if we managed to
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open and close the fridge.</p>
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</li>
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<li>
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<p>the tree on the <strong>right</strong> side will return SUCCESS if the beer was there,
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FAILURE otherwise.</p>
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</li>
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</ul>
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<p>Everything works as expected if <strong>GrabBeer</strong> returns SUCCESS.</p>
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<p><img alt="FetchBeer success" src="../images/FetchBeer2.png" /></p>

‎index.html

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</li>
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<a href="#the-problem" title="The problem" class="md-nav__link">
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The problem
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<a href="#what-is-a-behavior-tree" title="What is a Behavior Tree?" class="md-nav__link">
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What is a Behavior Tree?
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<a href="#what-is-a-behavior-tree" title="What is a Behavior Tree?" class="md-nav__link">
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What is a Behavior Tree?
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<a href="#ok-but-why-do-we-need-behaviortrees-or-fsm" title=""Ok, but WHY do we need BehaviorTrees (or FSM)?"" class="md-nav__link">
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"Ok, but WHY do we need BehaviorTrees (or FSM)?"
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<a href="#the-problem" title="The problem" class="md-nav__link">
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The problem
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What is a Behavior Tree?
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<a href="#what-is-a-behavior-tree" title="What is a Behavior Tree?" class="md-nav__link">
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What is a Behavior Tree?
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<a href="#ok-but-why-do-we-need-behaviortrees-or-fsm" title=""Ok, but WHY do we need BehaviorTrees (or FSM)?"" class="md-nav__link">
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"Ok, but WHY do we need BehaviorTrees (or FSM)?"
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to visualize, record, replay and analyze state transitions.</li>
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</ul>
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<p><img alt="ReadTheDocs" src="images/ReadTheDocs.png" /> </p>
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<h2 id="the-problem">The problem</h2>
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<p>Many software systems, being robotics a notable example, are inherently
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complex.</p>
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<p>The usual approach to manage complexity, heterogeneity and scalability is to
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use the concept of
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<a href="https://en.wikipedia.org/wiki/Component-based_software_engineering">Component Base Software Engineering</a>.</p>
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<p>Any existing middleware for robotics took this approach either informally or formally,
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being <a href="http://www.ros.org">ROS</a>, <a href="http://www.yarp.it">YARP</a> and
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<a href="http://www.servicerobotik-ulm.de">SmartSoft</a> some notable examples.</p>
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<p>A "good" software architecture should have the following characteristics:</p>
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<ul>
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<li>Modularity.</li>
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<li>Reusability of components.</li>
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<li>Composability.</li>
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<li>Good separation of concerns. </li>
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<p>If we don't keep these concepts in mind from the very beginning, we create
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software modules/components which are highly coupled to a particular application,
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instead of being reusable.</p>
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<p>Frequently, the concern of <strong>Coordination</strong> is mixed with <strong>Computation</strong>.
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In other words, people address the problems of coordinating actions and take decisions
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locally.</p>
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<p>The business logic becomes "spread" in many locations and it is <strong>hard for the developer
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to reason about it and to debug errors</strong> in the control flow.</p>
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<p>To achieve strong separation of concerns it is better to centralize
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the business logic in a single location. </p>
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<p><strong>Finite State Machines</strong> were created specifically with this goal in mind, but in
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the recent years <strong>Behavior Trees</strong> gained popularity, especially in the game industry.</p>
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<h2 id="what-is-a-behavior-tree">What is a Behavior Tree?</h2>
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<p>A Behavior Tree (<strong>BT</strong>) is a way to structure the switching between different
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tasks in an autonomous agent, such as a robot or a virtual entity in a computer game.</p>
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<p>BTs are a very efficient way of creating complex systems that are both modular and reactive.
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These properties are crucial in many applications, which has led to the spread
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of BT from computer game programming to many branches of AI and Robotics. </p>
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<p>If you are already familiar with Finite State Machines (<strong>FSM</strong>), you will
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easily grasp most of the concepts but, hopefully, you will find that BTs
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easily grasp most of the concepts but, hopefully, you will find that BTs
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are more expressive and easier to reason about.</p>
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<p>The main advantages of Behavior Trees, when compared to FSMs are:</p>
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<ul>
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"vocabulary" with his/her own custom nodes.</p>
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</li>
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</ul>
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<h2 id="ok-but-why-do-we-need-behaviortrees-or-fsm">"Ok, but WHY do we need BehaviorTrees (or FSM)?"</h2>
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<p>Many software systems, being robotics a notable example, are inherently
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complex.</p>
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<p>The usual approach to manage complexity, heterogeneity and scalability is to
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use the concept of
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<a href="https://en.wikipedia.org/wiki/Component-based_software_engineering">Component Base Software Engineering</a>.</p>
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<p>Any existing middleware for robotics took this approach either informally or formally,
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being <a href="http://www.ros.org">ROS</a>, <a href="http://www.yarp.it">YARP</a> and
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<a href="http://www.servicerobotik-ulm.de">SmartSoft</a> some notable examples.</p>
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<p>A "good" software architecture should have the following characteristics:</p>
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<ul>
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<li>Modularity.</li>
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<li>Reusability of components.</li>
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<li>Composability.</li>
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<li>Good separation of concerns. </li>
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</ul>
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<p>If we don't keep these concepts in mind from the very beginning, we create
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software modules/components which are highly coupled to a particular application,
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instead of being reusable.</p>
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<p>Frequently, the concern of <strong>Coordination</strong> is mixed with <strong>Computation</strong>.
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In other words, people address the problems of coordinating actions and take decisions
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locally.</p>
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<p>The business logic becomes "spread" in many locations and it is <strong>hard for the developer
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to reason about it and to debug errors</strong> in the control flow.</p>
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<p>To achieve strong separation of concerns it is better to centralize
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the business logic in a single location. </p>
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<p><strong>Finite State Machines</strong> were created specifically with this goal in mind, but in
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the recent years <strong>Behavior Trees</strong> gained popularity, especially in the game industry.</p>
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‎search/search_index.json

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‎sitemap.xml

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‎tutorial_A_create_trees/index.html

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It must return a NodeStatus, i.e. RUNNING, SUCCESS or FAILURE. </p>
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</li>
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<li>
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<p>The method <strong>halt()</strong> is used to stop an asynchronous Action. ApproachObject
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<p>The method <strong>halt()</strong> is used to stop an <strong>asynchronous Action</strong>. ApproachObject
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and then load the XML from file or text.</p>
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<p>The identifier used in the XML must coincide with those used to register
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the TreeNodes.</p>
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<p>The attribute "name" represent the name of the instance and it is optional.</p>
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<p>The attribute "name" represents the name of the instance and it is optional.</p>
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<div class="codehilite"><pre><span></span><span class="cp">#include</span> <span class="cpf">&quot;behavior_tree_core/xml_parsing.h&quot;</span><span class="cp"></span>
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<span class="cp">#include</span> <span class="cpf">&quot;Blackboard/blackboard_local.h&quot;</span><span class="cp"></span>
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<span class="cp">#include</span> <span class="cpf">&quot;dummy_nodes.h&quot;</span><span class="cp"></span>

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