Add existing to tracked

node_modules/@cesium/widgets/Source/Command.js

@@ -0,0 +1,42 @@
+import { DeveloperError } from "@cesium/engine";
+
+/**
+ * A Command is a function with an extra <code>canExecute</code> observable property to determine
+ * whether the command can be executed.  When executed, a Command function will check the
+ * value of <code>canExecute</code> and throw if false.
+ *
+ * This type describes an interface and is not intended to be instantiated directly.
+ * See {@link createCommand} to create a command from a function.
+ *
+ * @alias Command
+ * @constructor
+ */
+function Command() {
+  /**
+   * Gets whether this command can currently be executed.  This property is observable.
+   * @type {boolean}
+   * @default undefined
+   */
+  this.canExecute = undefined;
+
+  /**
+   * Gets an event which is raised before the command executes, the event
+   * is raised with an object containing two properties: a <code>cancel</code> property,
+   * which if set to false by the listener will prevent the command from being executed, and
+   * an <code>args</code> property, which is the array of arguments being passed to the command.
+   * @type {Event}
+   * @default undefined
+   */
+  this.beforeExecute = undefined;
+
+  /**
+   * Gets an event which is raised after the command executes, the event
+   * is raised with the return value of the command as its only parameter.
+   * @type {Event}
+   * @default undefined
+   */
+  this.afterExecute = undefined;
+
+  DeveloperError.throwInstantiationError();
+}
+export default Command;