Avoid unnecessary re-render on CHECK_NETWORK_STATUS

ts/state/ducks/network.ts

@@ -3,6 +3,7 @@
 
 import { SocketStatus } from '../../types/SocketStatus';
 import { trigger } from '../../shims/events';
+import { assignWithNoUnnecessaryAllocation } from '../../util/assignWithNoUnnecessaryAllocation';
 
 // State
 
@@ -89,11 +90,12 @@ export function reducer(
   if (action.type === CHECK_NETWORK_STATUS) {
     const { isOnline, socketStatus } = action.payload;
 
-    return {
-      ...state,
+    // This action is dispatched frequently. We avoid allocating a new object if nothing
+    //   has changed to avoid an unnecessary re-render.
+    return assignWithNoUnnecessaryAllocation(state, {
       isOnline,
       socketStatus,
-    };
+    });
   }
 
   if (action.type === CLOSE_CONNECTING_GRACE_PERIOD) {

ts/test-both/util/assignWithNoUnnecessaryAllocation_test.ts

@@ -0,0 +1,101 @@
+// Copyright 2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { assert } from 'chai';
+
+import { assignWithNoUnnecessaryAllocation } from '../../util/assignWithNoUnnecessaryAllocation';
+
+describe('assignWithNoUnnecessaryAllocation', () => {
+  interface Person {
+    name?: string;
+    age?: number;
+  }
+
+  it('returns the same object if there are no modifications', () => {
+    const empty = {};
+    assert.strictEqual(assignWithNoUnnecessaryAllocation(empty, {}), empty);
+
+    const obj = {
+      foo: 'bar',
+      baz: 'qux',
+      und: undefined,
+    };
+    assert.strictEqual(assignWithNoUnnecessaryAllocation(obj, {}), obj);
+    assert.strictEqual(
+      assignWithNoUnnecessaryAllocation(obj, { foo: 'bar' }),
+      obj
+    );
+    assert.strictEqual(
+      assignWithNoUnnecessaryAllocation(obj, { baz: 'qux' }),
+      obj
+    );
+    assert.strictEqual(
+      assignWithNoUnnecessaryAllocation(obj, { und: undefined }),
+      obj
+    );
+  });
+
+  it('returns a new object if there are modifications', () => {
+    const empty: Person = {};
+    assert.deepEqual(
+      assignWithNoUnnecessaryAllocation(empty, { name: 'Bert' }),
+      { name: 'Bert' }
+    );
+    assert.deepEqual(assignWithNoUnnecessaryAllocation(empty, { age: 8 }), {
+      age: 8,
+    });
+    assert.deepEqual(
+      assignWithNoUnnecessaryAllocation(empty, { name: undefined }),
+      {
+        name: undefined,
+      }
+    );
+
+    const obj: Person = { name: 'Ernie' };
+    assert.deepEqual(
+      assignWithNoUnnecessaryAllocation(obj, { name: 'Big Bird' }),
+      {
+        name: 'Big Bird',
+      }
+    );
+    assert.deepEqual(assignWithNoUnnecessaryAllocation(obj, { age: 9 }), {
+      name: 'Ernie',
+      age: 9,
+    });
+    assert.deepEqual(
+      assignWithNoUnnecessaryAllocation(obj, { age: undefined }),
+      {
+        name: 'Ernie',
+        age: undefined,
+      }
+    );
+  });
+
+  it('only performs a shallow comparison', () => {
+    const obj = { foo: { bar: 'baz' } };
+    assert.notStrictEqual(
+      assignWithNoUnnecessaryAllocation(obj, { foo: { bar: 'baz' } }),
+      obj
+    );
+  });
+
+  it("doesn't modify the original object when there are no modifications", () => {
+    const empty = {};
+    assignWithNoUnnecessaryAllocation(empty, {});
+    assert.deepEqual(empty, {});
+
+    const obj = { foo: 'bar' };
+    assignWithNoUnnecessaryAllocation(obj, { foo: 'bar' });
+    assert.deepEqual(obj, { foo: 'bar' });
+  });
+
+  it("doesn't modify the original object when there are modifications", () => {
+    const empty: Person = {};
+    assignWithNoUnnecessaryAllocation(empty, { name: 'Bert' });
+    assert.deepEqual(empty, {});
+
+    const obj = { foo: 'bar' };
+    assignWithNoUnnecessaryAllocation(obj, { foo: 'baz' });
+    assert.deepEqual(obj, { foo: 'bar' });
+  });
+});

ts/util/assignWithNoUnnecessaryAllocation.ts

@@ -0,0 +1,32 @@
+// Copyright 2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { has } from 'lodash';
+
+/**
+ * This function is like `Object.assign` but won't create a new object if we don't need
+ * to. This is purely a performance optimization.
+ *
+ * This is useful in places where we don't want to create a new object unnecessarily,
+ * like in reducers where we might cause an unnecessary re-render.
+ *
+ * See the tests for the specifics of how this works.
+ */
+// We want this to work with any object, so we allow `object` here.
+// eslint-disable-next-line @typescript-eslint/ban-types
+export function assignWithNoUnnecessaryAllocation<T extends object>(
+  obj: Readonly<T>,
+  source: Readonly<Partial<T>>
+): T {
+  // We want to bail early so we use `for .. in` instead of `Object.keys` or similar.
+  // eslint-disable-next-line no-restricted-syntax
+  for (const key in source) {
+    if (!has(source, key)) {
+      continue;
+    }
+    if (!(key in obj) || obj[key] !== source[key]) {
+      return { ...obj, ...source };
+    }
+  }
+  return obj;
+}