feat: allow apply condition to be a function (#4857)

docs/guide/api-plugin.md

@@ -414,6 +414,15 @@ function myPlugin() {
 }
 ```
 
+A function can also be used for more precise control:
+
+```js
+apply(config, { command }) {
+  // apply only on build but not for SSR
+  return command === 'build' && !config.build.ssr
+}
+```
+
 ## Rollup Plugin Compatibility
 
 A fair number of Rollup plugins will work directly as a Vite plugin (e.g. `@rollup/plugin-alias` or `@rollup/plugin-json`), but not all of them, since some plugin hooks do not make sense in an unbundled dev server context.

packages/vite/src/node/config.ts

@@ -285,7 +285,15 @@ export async function resolveConfig(
 
   // resolve plugins
   const rawUserPlugins = (config.plugins || []).flat().filter((p) => {
-    return p && (!p.apply || p.apply === command)
+    if (!p) {
+      return false
+    } else if (!p.apply) {
+      return true
+    } else if (typeof p.apply === 'function') {
+      return p.apply({ ...config, mode }, configEnv)
+    } else {
+      return p.apply === command
+    }
   }) as Plugin[]
   const [prePlugins, normalPlugins, postPlugins] =
     sortUserPlugins(rawUserPlugins)

packages/vite/src/node/plugin.ts

@@ -50,9 +50,9 @@ export interface Plugin extends RollupPlugin {
    */
   enforce?: 'pre' | 'post'
   /**
-   * Apply the plugin only for serve or for build.
+   * Apply the plugin only for serve or build, or on certain conditions.
    */
-  apply?: 'serve' | 'build'
+  apply?: 'serve' | 'build' | ((config: UserConfig, env: ConfigEnv) => boolean)
   /**
    * Modify vite config before it's resolved. The hook can either mutate the
    * passed-in config directly, or return a partial config object that will be