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+/* global __REACT_DEVTOOLS_GLOBAL_HOOK__ */
+
+import { options, Component } from 'preact';
+
+// Internal helpers from preact
+import { ATTR_KEY } from '../src/constants';
+import { isFunctionalComponent } from '../src/vdom/functional-component';
+
+/**
+ * Return a ReactElement-compatible object for the current state of a preact
+ * component.
+ */
+function createReactElement(component) {
+	return {
+		type: component.constructor,
+		key: component.key,
+		ref: null, // Unsupported
+		props: component.props
+	};
+}
+
+/**
+ * Create a ReactDOMComponent-compatible object for a given DOM node rendered
+ * by preact.
+ *
+ * This implements the subset of the ReactDOMComponent interface that
+ * React DevTools requires in order to display DOM nodes in the inspector with
+ * the correct type and properties.
+ *
+ * @param {Node} node
+ */
+function createReactDOMComponent(node) {
+	const childNodes = node.nodeType === Node.ELEMENT_NODE ?
+		Array.from(node.childNodes) : [];
+
+	const isText = node.nodeType === Node.TEXT_NODE;
+
+	return {
+		// --- ReactDOMComponent interface
+		_currentElement: isText ? node.textContent : {
+			type: node.nodeName.toLowerCase(),
+			props: node[ATTR_KEY]
+		},
+		_renderedChildren: childNodes.map(child => {
+			if (child._component) {
+				return updateReactComponent(child._component);
+			}
+			return updateReactComponent(child);
+		}),
+		_stringText: isText ? node.textContent : null,
+
+		// --- Additional properties used by preact devtools
+
+		// A flag indicating whether the devtools have been notified about the
+		// existence of this component instance yet.
+		// This is used to send the appropriate notifications when DOM components
+		// are added or updated between composite component updates.
+		_inDevTools: false,
+		node
+	};
+}
+
+/**
+ * Return the name of a component created by a `ReactElement`-like object.
+ *
+ * @param {ReactElement} element
+ */
+function typeName(element) {
+	if (typeof element.type === 'function') {
+		return element.type.displayName || element.type.name;
+	}
+	return element.type;
+}
+
+/**
+ * Return a ReactCompositeComponent-compatible object for a given preact
+ * component instance.
+ *
+ * This implements the subset of the ReactCompositeComponent interface that
+ * the DevTools requires in order to walk the component tree and inspect the
+ * component's properties.
+ *
+ * See https://github.com/facebook/react-devtools/blob/e31ec5825342eda570acfc9bcb43a44258fceb28/backend/getData.js
+ */
+function createReactCompositeComponent(component) {
+	const _currentElement = createReactElement(component);
+	const node = component.base;
+
+	let instance = {
+		// --- ReactDOMComponent properties
+		getName() {
+			return typeName(_currentElement);
+		},
+		_currentElement: createReactElement(component),
+		props: component.props,
+		state: component.state,
+		forceUpdate: component.forceUpdate.bind(component),
+		setState: component.setState.bind(component),
+
+		// --- Additional properties used by preact devtools
+		node
+	};
+
+	// React DevTools exposes the `_instance` field of the selected item in the
+	// component tree as `$r` in the console.  `_instance` must refer to a
+	// React Component (or compatible) class instance with `props` and `state`
+	// fields and `setState()`, `forceUpdate()` methods.
+	instance._instance = component;
+
+	// If the root node returned by this component instance's render function
+	// was itself a composite component, there will be a `_component` property
+	// containing the child component instance.
+	if (component._component) {
+		instance._renderedComponent = updateReactComponent(component._component);
+	} else {
+		// Otherwise, if the render() function returned an HTML/SVG element,
+		// create a ReactDOMComponent-like object for the DOM node itself.
+		instance._renderedComponent = updateReactComponent(node);
+	}
+
+	return instance;
+}
+
+/**
+ * Map of Component|Node to ReactDOMComponent|ReactCompositeComponent-like
+ * object.
+ *
+ * The same React*Component instance must be used when notifying devtools
+ * about the initial mount of a component and subsequent updates.
+ */
+let instanceMap = new Map();
+
+/**
+ * Update (and create if necessary) the ReactDOMComponent|ReactCompositeComponent-like
+ * instance for a given preact component instance or DOM Node.
+ *
+ * @param {Component|Node} componentOrNode
+ */
+function updateReactComponent(componentOrNode) {
+	const newInstance = componentOrNode instanceof Node ?
+		createReactDOMComponent(componentOrNode) :
+		createReactCompositeComponent(componentOrNode);
+	if (instanceMap.has(componentOrNode)) {
+		let inst = instanceMap.get(componentOrNode);
+		Object.assign(inst, newInstance);
+		return inst;
+	}
+	instanceMap.set(componentOrNode, newInstance);
+	return newInstance;
+}
+
+function nextRootKey(roots) {
+	return '.' + Object.keys(roots).length;
+}
+
+/**
+ * Find all root component instances rendered by preact in `node`'s children
+ * and add them to the `roots` map.
+ *
+ * @param {DOMElement} node
+ * @param {[key: string] => ReactDOMComponent|ReactCompositeComponent}
+ */
+function findRoots(node, roots) {
+	Array.from(node.childNodes).forEach(child => {
+		if (child._component) {
+			roots[nextRootKey(roots)] = updateReactComponent(child._component);
+		} else {
+			findRoots(child, roots);
+		}
+	});
+}
+
+/**
+ * Map of functional component name -> wrapper class.
+ */
+let functionalComponentWrappers = new Map();
+
+/**
+ * Wrap a functional component with a stateful component.
+ *
+ * preact does not record any information about the original hierarchy of
+ * functional components in the rendered DOM nodes. Wrapping functional components
+ * with a trivial wrapper allows us to recover information about the original
+ * component structure from the DOM.
+ *
+ * @param {VNode} vnode
+ */
+function wrapFunctionalComponent(vnode) {
+	const originalRender = vnode.nodeName;
+	const name = vnode.nodeName.name || '(Function.name missing)';
+	const wrappers = functionalComponentWrappers;
+	if (!wrappers.has(originalRender)) {
+		let wrapper = class extends Component {
+			render(props, state, context) {
+				return originalRender(props, context);
+			}
+		};
+
+		// Expose the original component name. React Dev Tools will use
+		// this property if it exists or fall back to Function.name
+		// otherwise.
+		wrapper.displayName = name;
+
+		wrappers.set(originalRender, wrapper);
+	}
+	vnode.nodeName = wrappers.get(originalRender);
+}
+
+/**
+ * Create a bridge for exposing preact's component tree to React DevTools.
+ *
+ * It creates implementations of the interfaces that ReactDOM passes to
+ * devtools to enable it to query the component tree and hook into component
+ * updates.
+ *
+ * See https://github.com/facebook/react/blob/59ff7749eda0cd858d5ee568315bcba1be75a1ca/src/renderers/dom/ReactDOM.js
+ * for how ReactDOM exports its internals for use by the devtools and
+ * the `attachRenderer()` function in
+ * https://github.com/facebook/react-devtools/blob/e31ec5825342eda570acfc9bcb43a44258fceb28/backend/attachRenderer.js
+ * for how the devtools consumes the resulting objects.
+ */
+function createDevToolsBridge() {
+	// The devtools has different paths for interacting with the renderers from
+	// React Native, legacy React DOM and current React DOM.
+	//
+	// Here we emulate the interface for the current React DOM (v15+) lib.
+
+	// ReactDOMComponentTree-like object
+	const ComponentTree = {
+		getNodeFromInstance(instance) {
+			return instance.node;
+		},
+		getClosestInstanceFromNode(node) {
+			while (node && !node._component) {
+				node = node.parentNode;
+			}
+			return node ? updateReactComponent(node._component) : null;
+		}
+	};
+
+	// Map of root ID (the ID is unimportant) to component instance.
+	let roots = {};
+	findRoots(document.body, roots);
+
+	// ReactMount-like object
+	//
+	// Used by devtools to discover the list of root component instances and get
+	// notified when new root components are rendered.
+	const Mount = {
+		_instancesByReactRootID: roots,
+
+		// Stub - React DevTools expects to find this method and replace it
+		// with a wrapper in order to observe new root components being added
+		_renderNewRootComponent(/* instance, ... */) { }
+	};
+
+	// ReactReconciler-like object
+	const Reconciler = {
+		// Stubs - React DevTools expects to find these methods and replace them
+		// with wrappers in order to observe components being mounted, updated and
+		// unmounted
+		mountComponent(/* instance, ... */) { },
+		performUpdateIfNecessary(/* instance, ... */) { },
+		receiveComponent(/* instance, ... */) { },
+		unmountComponent(/* instance, ... */) { }
+	};
+
+	/** Notify devtools that a new component instance has been mounted into the DOM. */
+	const componentAdded = component => {
+		const instance = updateReactComponent(component);
+		if (isRootComponent(component)) {
+			instance._rootID = nextRootKey(roots);
+			roots[instance._rootID] = instance;
+			Mount._renderNewRootComponent(instance);
+		}
+		visitNonCompositeChildren(instance, childInst => {
+			childInst._inDevTools = true;
+			Reconciler.mountComponent(childInst);
+		});
+		Reconciler.mountComponent(instance);
+	};
+
+	/** Notify devtools that a component has been updated with new props/state. */
+	const componentUpdated = component => {
+		const prevRenderedChildren = [];
+		visitNonCompositeChildren(instanceMap.get(component), childInst => {
+			prevRenderedChildren.push(childInst);
+		});
+
+		// Notify devtools about updates to this component and any non-composite
+		// children
+		const instance = updateReactComponent(component);
+		Reconciler.receiveComponent(instance);
+		visitNonCompositeChildren(instance, childInst => {
+			if (!childInst._inDevTools) {
+				// New DOM child component
+				childInst._inDevTools = true;
+				Reconciler.mountComponent(childInst);
+			} else {
+				// Updated DOM child component
+				Reconciler.receiveComponent(childInst);
+			}
+		});
+
+		// For any non-composite children that were removed by the latest render,
+		// remove the corresponding ReactDOMComponent-like instances and notify
+		// the devtools
+		prevRenderedChildren.forEach(childInst => {
+			if (!document.body.contains(childInst.node)) {
+				instanceMap.delete(childInst.node);
+				Reconciler.unmountComponent(childInst);
+			}
+		});
+	};
+
+	/** Notify devtools that a component has been unmounted from the DOM. */
+	const componentRemoved = component => {
+		const instance = updateReactComponent(component);
+		visitNonCompositeChildren(childInst => {
+			instanceMap.delete(childInst.node);
+			Reconciler.unmountComponent(childInst);
+		});
+		Reconciler.unmountComponent(instance);
+		instanceMap.delete(component);
+		if (instance._rootID) {
+			delete roots[instance._rootID];
+		}
+	};
+
+	return {
+		componentAdded,
+		componentUpdated,
+		componentRemoved,
+
+		// Interfaces passed to devtools via __REACT_DEVTOOLS_GLOBAL_HOOK__.inject()
+		ComponentTree,
+		Mount,
+		Reconciler
+	};
+}
+
+/**
+ * Return `true` if a preact component is a top level component rendered by
+ * `render()` into a container Element.
+ */
+function isRootComponent(component) {
+	return !component.base.parentElement || !component.base.parentElement[ATTR_KEY];
+}
+
+/**
+ * Visit all child instances of a ReactCompositeComponent-like object that are
+ * not composite components (ie. they represent DOM elements or text)
+ *
+ * @param {Component} component
+ * @param {(Component) => void} visitor
+ */
+function visitNonCompositeChildren(component, visitor) {
+	if (component._renderedComponent) {
+		if (!component._renderedComponent._component) {
+			visitor(component._renderedComponent);
+			visitNonCompositeChildren(component._renderedComponent, visitor);
+		}
+	} else if (component._renderedChildren) {
+		component._renderedChildren.forEach(child => {
+			visitor(child);
+			if (!child._component) visitNonCompositeChildren(child, visitor);
+		});
+	}
+}
+
+/**
+ * Create a bridge between the preact component tree and React's dev tools
+ * and register it.
+ *
+ * After this function is called, the React Dev Tools should be able to detect
+ * "React" on the page and show the component tree.
+ *
+ * This function hooks into preact VNode creation in order to expose functional
+ * components correctly, so it should be called before the root component(s)
+ * are rendered.
+ *
+ * Returns a cleanup function which unregisters the hooks.
+ */
+export function initDevTools() {
+	if (typeof __REACT_DEVTOOLS_GLOBAL_HOOK__ === 'undefined') {
+		// React DevTools are not installed
+		return;
+	}
+
+	// Hook into preact element creation in order to wrap functional components
+	// with stateful ones in order to make them visible in the devtools
+	const nextVNode = options.vnode;
+	options.vnode = (vnode) => {
+		if (isFunctionalComponent(vnode)) wrapFunctionalComponent(vnode);
+		if (nextVNode) return nextVNode(vnode);
+	};
+
+	// Notify devtools when preact components are mounted, updated or unmounted
+	const bridge = createDevToolsBridge();
+
+	const nextAfterMount = options.afterMount;
+	options.afterMount = component => {
+		bridge.componentAdded(component);
+		if (nextAfterMount) nextAfterMount(component);
+	};
+
+	const nextAfterUpdate = options.afterUpdate;
+	options.afterUpdate = component => {
+		bridge.componentUpdated(component);
+		if (nextAfterUpdate) nextAfterUpdate(component);
+	};
+
+	const nextBeforeUnmount = options.beforeUnmount;
+	options.beforeUnmount = component => {
+		bridge.componentRemoved(component);
+		if (nextBeforeUnmount) nextBeforeUnmount(component);
+	};
+
+	// Notify devtools about this instance of "React"
+	__REACT_DEVTOOLS_GLOBAL_HOOK__.inject(bridge);
+
+	return () => {
+		options.afterMount = nextAfterMount;
+		options.afterUpdate = nextAfterUpdate;
+		options.beforeUnmount = nextBeforeUnmount;
+	};
+}