Various TypeScript improvements (#2309)

- Use Mithril.Attributes as base for ComponentAttrs, remove =any from class signature for Component
- Convert Alert to TypeScript, introduce AlertAttrs interface
- Convert AlertManagerState to TypeScript, add overload signatures for `show`, introduce AlertState interface for stored Alerts.
- Set ComponentAttrs as default T for Component
- Make attrs in AlertAttrs optional
- Add AlertIdentifier interface, simplify show type signature
- Remove mithril patch shim, as all patches onto m are now deprecated
- Use Mithril.Static for shim

js/src/common/states/AlertManagerState.ts

@@ -0,0 +1,80 @@
+import Mithril from 'mithril';
+import Alert, { AlertAttrs } from '../components/Alert';
+
+/**
+ * Returned by `AlertManagerState.show`. Used to dismiss alerts.
+ */
+export type AlertIdentifier = number;
+
+export interface AlertState {
+  componentClass: typeof Alert;
+  attrs: AlertAttrs;
+  children: Mithril.Children;
+}
+
+export default class AlertManagerState {
+  protected activeAlerts: { [id: number]: AlertState } = {};
+  protected alertId = 0;
+
+  getActiveAlerts() {
+    return this.activeAlerts;
+  }
+
+  /**
+   * Show an Alert in the alerts area.
+   *
+   * @returns The alert's ID, which can be used to dismiss the alert.
+   */
+  show(children: Mithril.Children): AlertIdentifier;
+  show(attrs: AlertAttrs, children: Mithril.Children): AlertIdentifier;
+  show(componentClass: Alert, attrs: AlertAttrs, children: Mithril.Children): AlertIdentifier;
+
+  show(arg1: any, arg2?: any, arg3?: any) {
+    // Assigns variables as per the above signatures
+    let componentClass = Alert;
+    let attrs: AlertAttrs = {};
+    let children: Mithril.Children;
+
+    if (arguments.length == 1) {
+      children = arg1 as Mithril.Children;
+    } else if (arguments.length == 2) {
+      attrs = arg1 as AlertAttrs;
+      children = arg2 as Mithril.Children;
+    } else if (arguments.length == 3) {
+      componentClass = arg1 as typeof Alert;
+      attrs = arg2 as AlertAttrs;
+      children = arg3;
+    }
+
+    // Breaking Change Compliance Warning, Remove in Beta 15.
+    // This is applied to the first argument (attrs) because previously, the alert was passed as the first argument.
+    if (attrs === Alert || attrs instanceof Alert) {
+      // This is duplicated so that if the error is caught, an error message still shows up in the debug console.
+      console.error('The AlertManager can only show Alerts. Whichever extension triggered this alert should be updated to comply with beta 14.');
+      throw new Error('The AlertManager can only show Alerts. Whichever extension triggered this alert should be updated to comply with beta 14.');
+    }
+    // End Change Compliance Warning, Remove in Beta 15
+    this.activeAlerts[++this.alertId] = { children, attrs, componentClass };
+    m.redraw();
+
+    return this.alertId;
+  }
+
+  /**
+   * Dismiss an alert.
+   */
+  dismiss(key: AlertIdentifier): void {
+    if (!key || !(key in this.activeAlerts)) return;
+
+    delete this.activeAlerts[key];
+    m.redraw();
+  }
+
+  /**
+   * Clear all alerts.
+   */
+  clear(): void {
+    this.activeAlerts = {};
+    m.redraw();
+  }
+}