[build-tools] Keep default Android animation scales in device run sessions

.changeset/session-emulator-animations.md

@@ -0,0 +1,5 @@
+---
+'@expo/build-tools': patch
+---
+
+Keep the default Android animation scales on emulators that device run sessions start, so Reduce motion shows the real device default; builds and tests still disable animations.

packages/build-tools/src/steps/functions/__tests__/startAgentDeviceSession.test.ts

@@ -431,6 +431,7 @@ describe(createStartAgentDeviceSessionBuildFunction, () => {
         lcdWidth: 720,
         lcdHeight: 1600,
         lcdDensity: 300,
+        adjustAnimationScale: false,
       })
     );
     expect(downloadBuildAsync).toHaveBeenCalledWith(

packages/build-tools/src/steps/functions/__tests__/startAndroidEmulator.test.ts

@@ -377,6 +377,23 @@ describe(createStartAndroidEmulatorBuildFunction, () => {
     expect(mockedAndroidUtils.disableWindowAndTransitionAnimationsAsync).not.toHaveBeenCalled();
   });
 
+  it('disables animations outside a device run session', async () => {
+    await createStep().executeAsync();
+
+    expect(mockedAndroidUtils.disableWindowAndTransitionAnimationsAsync).toHaveBeenCalledWith(
+      expect.objectContaining({ serialId: 'emulator-default' })
+    );
+  });
+
+  it('keeps the default animation scales in a device run session', async () => {
+    const step = createStep({ count: 2 }, { DEVICE_RUN_SESSION_ID: 'session-id' });
+
+    await step.executeAsync();
+
+    expect(mockedAndroidUtils.waitForReadyAsync).toHaveBeenCalledTimes(3);
+    expect(mockedAndroidUtils.disableWindowAndTransitionAnimationsAsync).not.toHaveBeenCalled();
+  });
+
   it('fails early on Linux when emulator host support check fails', async () => {
     mockedSpawn.mockRejectedValueOnce(new Error('accel check failed') as any);
 

packages/build-tools/src/steps/functions/startAgentDeviceSession.ts

@@ -213,6 +213,8 @@ export function createStartAgentDeviceSessionBuildFunction(ctx: CustomBuildConte
                 lcdWidth: inputs.lcd_width.value as number | undefined,
                 lcdHeight: inputs.lcd_height.value as number | undefined,
                 lcdDensity: inputs.lcd_density.value as number | undefined,
+                // Keep Android's default animation scales, so the session shows the real device.
+                adjustAnimationScale: false,
                 logcatDirectory,
                 env,
                 logger: taskLogger,

packages/build-tools/src/steps/functions/startAndroidEmulator.ts

@@ -94,6 +94,9 @@ export function createStartAndroidEmulatorBuildFunction(): BuildFunction {
           lcdWidth: inputs.lcd_width.value as number | undefined,
           lcdHeight: inputs.lcd_height.value as number | undefined,
           lcdDensity: inputs.lcd_density.value as number | undefined,
+          // Device run sessions show the device to a person, so they keep Android's default
+          // animation scales. Only test and build jobs disable animations.
+          adjustAnimationScale: !env.DEVICE_RUN_SESSION_ID,
           logcatDirectory,
           env,
           logger,
@@ -201,6 +204,7 @@ export async function startAndroidEmulatorAsync({
   lcdWidth,
   lcdHeight,
   lcdDensity,
+  adjustAnimationScale = true,
   logcatDirectory,
   env,
   logger,
@@ -211,6 +215,11 @@ export async function startAndroidEmulatorAsync({
   lcdWidth?: number;
   lcdHeight?: number;
   lcdDensity?: number;
+  /**
+   * Set the window and transition animation scales to 0 after boot. Defaults to true.
+   * $ANDROID_EMULATOR_ADJUST_ANIMATION_SCALE can still turn the adjustment off.
+   */
+  adjustAnimationScale?: boolean;
   logcatDirectory: string;
   env: BuildStepEnv;
   logger: bunyan;
@@ -237,10 +246,13 @@ export async function startAndroidEmulatorAsync({
   }
 
   const shouldAdjustAnimationScale =
+    adjustAnimationScale &&
     env.ANDROID_EMULATOR_ADJUST_ANIMATION_SCALE !== 'false' &&
     env.ANDROID_EMULATOR_ADJUST_ANIMATION_SCALE !== '0';
 
-  if (!shouldAdjustAnimationScale) {
+  if (!adjustAnimationScale) {
+    logger.info('Keeping the default Android emulator animation scales.');
+  } else if (!shouldAdjustAnimationScale) {
     logger.info(
       'Skipping Android emulator animation scale adjustments because $ANDROID_EMULATOR_ADJUST_ANIMATION_SCALE is disabled.'
     );