lucasmrdt · October 29, 2018 16:34 · Oct 29, 2018
diff --git a/ReactContext.js b/ReactContext.js
@@ -0,0 +1,118 @@
+// @flow
+
+import React from "react";
+
+type UpdaterType = () => void;
+type GlobalListenersType = { [key: string]: UpdaterType };
+type ScopedListenersType = { [props: string]: Set<UpdaterType> };
+
+/**
+ * eg.
+ *
+ * context = new Context({ foo: 'bar' });
+ *
+ * const Child = context.withContext(props => <p>props.foo</p>);
+ *
+ * class Parent extends Context.Provider {
+ *  componentDidMount() {
+ *    this.setState({ foo: 'bar2 });
+ *  }
+ *
+ *  render() {
+ *    <Child /> // Will display bar2.
+ *  }
+ * }
+ */
+class Context {
+  globalListeners: GlobalListenersType = {};
+  scopedListeners: ScopedListenersType = {};
+
+  Provider: React.ComponentType<any> = React.PureComponent;
+
+  constructor(state: Object) {
+    this.state = state;
+
+    this.Provider.prototype.setState = this.setState;
+    this.Provider.prototype.state = this.state;
+  }
+
+  removeListenerForKey = (updater: UpdaterType, key: string) => {
+    if (typeof this.scopedListeners[key] === "object") {
+      this.scopedListeners[key].delete(updater);
+    }
+  };
+
+  removeListener = (updater: UpdaterType) => {
+    if (this.globalListeners[updater]) {
+      delete this.globalListeners[updater];
+    }
+    Object.keys(this.scopedListeners).forEach(
+      this.removeListenerForKey.bind(null, updater)
+    );
+  };
+
+  addListenerForKey = (updater: UpdaterType, key: string) => {
+    const listenersForKey = this.scopedListeners[key];
+
+    if (typeof listenersForKey === "object") {
+      listenersForKey.add(updater);
+    } else {
+      this.scopedListeners[key] = new Set([updater]);
+    }
+  };
+
+  addListener = (updater: UpdaterType, keys: Array<string>) => {
+    if (!keys || keys.length === 0) {
+      this.globalListeners[updater] = { updater };
+    } else {
+      keys.forEach(this.addListenerForKey.bind(null, updater));
+    }
+  };
+
+  setState = (state: Object | Function) => {
+    const updatedComponent = new Set();
+
+    if (typeof state === 'function') {
+      state = state(this.state);
+    }
+
+    Object.assign(this.state, state);
+    Object.keys(state).forEach((key: string) => {
+      const listenersForKey = this.scopedListeners[key];
+      if (!listenersForKey) return;
+
+      listenersForKey.forEach((updater: UpdaterType) => {
+        if (!updatedComponent.has(updater)) {
+          updatedComponent.add(updater);
+          updater();
+        }
+      });
+    });
+  };
+
+  withContext = (...keys: Array<string>) => {
+    return component => {
+      const addListener = this.addListener;
+      const removeListener = this.removeListener;
+      const state = this.state;
+      const factoriedComponent = React.createFactory(component);
+
+      class Consumer extends React.PureComponent {
+        componentDidMount() {
+          addListener(this.forceUpdate.bind(this), keys);
+        }
+
+        componentWillUnmount() {
+          removeListener(this.forceUpdate.bind(this));
+        }
+
+        render() {
+          return factoriedComponent({ ...this.props, ...state });
+        }
+      }
+      return Consumer;
+    };
+  };
+}
+
+export default Context;