Avoid unnecessary re-renders (#8673)

* Only set the height on the message component if it ends up being greater than the maxHeight to avoid re-renders

* Cache the teams theme to avoid unnecessary computation and re-renders

* useDidUpdate instead of useEffect to avoid setting the theme preference without need

* add comment on clear cache

app/components/post_list/post/body/message/message.tsx

@@ -67,7 +67,12 @@ const Message = ({currentUser, isHighlightWithoutNotificationLicensed, highlight
     const blockStyles = getMarkdownBlockStyles(theme);
     const textStyles = getMarkdownTextStyles(theme);
 
-    const onLayout = useCallback((event: LayoutChangeEvent) => setHeight(event.nativeEvent.layout.height), []);
+    const onLayout = useCallback((event: LayoutChangeEvent) => {
+        const h = event.nativeEvent.layout.height;
+        if (h > maxHeight) {
+            setHeight(event.nativeEvent.layout.height);
+        }
+    }, [maxHeight]);
     const onPress = () => setOpen(!open);
 
     const channelMentions = useMemo(() => {

app/context/theme/index.tsx

@@ -6,6 +6,7 @@ import React, {type ComponentType, createContext, useEffect, useState} from 'rea
 import {Appearance} from 'react-native';
 
 import {Preferences} from '@constants';
+import useDidUpdate from '@hooks/did_update';
 import {queryThemePreferences} from '@queries/servers/preference';
 import {observeCurrentTeamId} from '@queries/servers/system';
 import {setThemeDefaults, updateThemeIfNeeded} from '@utils/theme';
@@ -33,22 +34,43 @@ export function getDefaultThemeByAppearance(): Theme {
 export const ThemeContext = createContext(getDefaultThemeByAppearance());
 const {Consumer, Provider} = ThemeContext;
 
+const themeCache = new Map<string, Theme>();
+let clearingThemeCache = false;
+const clearThemeCache = () => {
+    if (clearingThemeCache) {
+        return;
+    }
+    clearingThemeCache = true;
+    themeCache.clear();
+    setTimeout(() => {
+        // We set this timeout to avoid clearing the cache multiple times
+        // this would happen as we have a Provider for each screen in the stack
+        // and when the themes changes we only want to invalidate the cache once
+        clearingThemeCache = false;
+    }, 300);
+};
+
 const getTheme = (teamId: string | undefined, themes: PreferenceModel[]): Theme => {
-    if (teamId) {
-        const teamTheme = themes.find((t) => t.name === teamId) || themes[0];
-        if (teamTheme?.value) {
-            try {
-                const theme = setThemeDefaults(JSON.parse(teamTheme.value));
-                return theme;
-            } catch {
-                // no theme change
-            }
-        }
+    if (teamId && themeCache.has(teamId)) {
+        return themeCache.get(teamId)!;
     }
 
-    const defaultTheme = getDefaultThemeByAppearance();
+    const newTheme = (() => {
+        if (teamId) {
+            const teamTheme = themes.find((t) => t.name === teamId) || themes[0];
+            if (teamTheme?.value) {
+                try {
+                    return setThemeDefaults(JSON.parse(teamTheme.value));
+                } catch {
+                    // no theme change
+                }
+            }
+        }
+        return getDefaultThemeByAppearance();
+    })();
 
-    return defaultTheme;
+    themeCache.set(teamId || 'default', newTheme);
+    return newTheme;
 };
 
 const ThemeProvider = ({currentTeamId, children, themes}: Props) => {
@@ -69,7 +91,11 @@ const ThemeProvider = ({currentTeamId, children, themes}: Props) => {
         updateThemeIfNeeded(theme);
     }, [theme]);
 
-    useEffect(() => {
+    useDidUpdate(() => {
+        clearThemeCache();
+    }, [themes]);
+
+    useDidUpdate(() => {
         setTheme(getTheme(currentTeamId, themes));
     }, [currentTeamId, themes]);
 

app/screens/settings/display_theme/display_theme.tsx

@@ -1,7 +1,7 @@
 // Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
 // See LICENSE.txt for license information.
 
-import React, {useCallback, useMemo, useState, useEffect} from 'react';
+import React, {useCallback, useMemo, useState} from 'react';
 
 import {savePreference} from '@actions/remote/preference';
 import SettingContainer from '@components/settings/container';
@@ -9,6 +9,7 @@ import {Preferences} from '@constants';
 import {useServerUrl} from '@context/server';
 import {useTheme} from '@context/theme';
 import useAndroidHardwareBackHandler from '@hooks/android_back_handler';
+import useDidUpdate from '@hooks/did_update';
 import {popTopScreen} from '@screens/navigation';
 
 import CustomTheme from './custom_theme';
@@ -30,16 +31,6 @@ const DisplayTheme = ({allowedThemeKeys, componentId, currentTeamId, currentUser
 
     const close = () => popTopScreen(componentId);
 
-    useEffect(() => {
-        const differentTheme = theme.type?.toLowerCase() !== newTheme?.toLowerCase();
-
-        if (!differentTheme) {
-            close();
-            return;
-        }
-        setThemePreference();
-    }, [newTheme]);
-
     const setThemePreference = useCallback(() => {
         const allowedTheme = allowedThemeKeys.find((tk) => tk === newTheme);
         const themeJson = Preferences.THEMES[allowedTheme as ThemeKey] || initialTheme;
@@ -51,7 +42,17 @@ const DisplayTheme = ({allowedThemeKeys, componentId, currentTeamId, currentUser
             value: JSON.stringify(themeJson),
         };
         savePreference(serverUrl, [pref]);
-    }, [allowedThemeKeys, currentTeamId, theme.type, serverUrl, newTheme]);
+    }, [allowedThemeKeys, initialTheme, currentTeamId, currentUserId, serverUrl, newTheme]);
+
+    useDidUpdate(() => {
+        const differentTheme = theme.type?.toLowerCase() !== newTheme?.toLowerCase();
+
+        if (!differentTheme) {
+            close();
+            return;
+        }
+        setThemePreference();
+    }, [close, newTheme, setThemePreference, theme.type]);
 
     const onAndroidBack = () => {
         setThemePreference();