Add TestFlags provider and class (#132)

* Add TestFlags provider and class

* import order nit

* markets is an object and not array

* Use null coalescing op

src/hooks/useTestFlags.tsx

@@ -0,0 +1,46 @@
+import { createContext, useContext, useEffect, useState } from 'react';
+import { useLocation } from 'react-router-dom';
+import queryString from 'query-string';
+
+class TestFlags {
+  public queryParams: queryString.ParsedQuery<string>;
+
+  constructor() {
+    this.queryParams = {};
+  }
+
+  setQueryParams(flags: queryString.ParsedQuery<string>) {
+    this.queryParams = flags;
+  }
+
+  get displayInitializingMarkets() {
+    return !!this.queryParams.displayInitializingMarkets;
+  }
+}
+
+export const testFlags = new TestFlags();
+
+const useTestFlagsContext = () => {
+  const [queryParams, setQueryParams] = useState<queryString.ParsedQuery<string>>({});
+  const location = useLocation();
+
+  useEffect(() => {
+    const parsedQueryParams = queryString.parse(location.search.substring(-1));
+    testFlags.setQueryParams(parsedQueryParams);
+    setQueryParams(parsedQueryParams);
+  }, []);
+
+  return {
+    ...queryParams,
+  };
+};
+
+type TestFlagsContextType = ReturnType<typeof useTestFlagsContext>;
+const TestFlagsContext = createContext<TestFlagsContextType>({} as TestFlagsContextType);
+TestFlagsContext.displayName = 'TestFlags';
+
+export const TestFlagsProvider = ({ ...props }) => (
+  <TestFlagsContext.Provider value={useTestFlagsContext()} {...props} />
+);
+
+export const useTestFlags = () => useContext(TestFlagsContext);