feat: create <Resolve> component

docs/mock.md

@@ -11,6 +11,7 @@ Create a mock and implement it
 const Player = mock();
 
 // Now you can already use <Player>.
+<Player />
 
 // But implement it only later.
 Player.implement(RealPlayer);

src/Resolve/index.ts

@@ -0,0 +1,54 @@
+import {Component} from 'react';
+
+export interface IResolveProps {
+  promise: Promise<any>;
+  children?: (state: IResolveState) => React.ReactElement<any>;
+}
+
+export interface IResolveState {
+  pending: boolean;
+  value?: any;
+  error?: any;
+}
+
+export class Resolve extends Component<IResolveProps, any> {
+  mounted = false;
+
+  state: IResolveState = {
+    pending: true
+  };
+
+  constructor (props, context) {
+    super(props, context);
+
+    this.props.promise
+      .then((value) => {
+        if (this.mounted) {
+          this.setState({
+            pending: false,
+            value
+          });
+        }
+      })
+      .catch((error) => {
+        if (this.mounted) {
+          this.setState({
+            pending: false,
+            error
+          });
+        }
+      })
+  }
+
+  componentDidMount () {
+    this.mounted = true;
+  }
+
+  componentWillUnmount () {
+    this.mounted = false;
+  }
+
+  render () {
+    return this.props.children(this.state);
+  }
+}