https://blog.csdn.net/hfy8971613/article/details/107201238

 private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
        ...
        boolean pausing = getDisplay().pauseBackStacks(userLeaving, next, false);
        if (mResumedActivity != null) {
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: Pausing " + mResumedActivity);
             // 暂停上一个Activity
            pausing |= startPausingLocked(userLeaving, false, next, false);
        }
        ...
        //这里next.attachedToProcess()，只有启动了的Activity才会返回true
        if (next.attachedToProcess()) {
            ...
            
            try {
                final ClientTransaction transaction =
                        ClientTransaction.obtain(next.app.getThread(), next.appToken);
                ...
                //启动了的Activity就发送ResumeActivityItem事务给客户端了，后面会讲到
                transaction.setLifecycleStateRequest(
                        ResumeActivityItem.obtain(next.app.getReportedProcState(),
                                getDisplay().mDisplayContent.isNextTransitionForward()));
                mService.getLifecycleManager().scheduleTransaction(transaction);
               ....
            } catch (Exception e) {
                ....
                mStackSupervisor.startSpecificActivityLocked(next, true, false);
                return true;
            }
            ....
        } else {
            ....
            if (SHOW_APP_STARTING_PREVIEW) {
                    //这里就是 冷启动时 出现白屏 的原因了：取根activity的主题背景 展示StartingWindow
                    next.showStartingWindow(null , false ,false);
                }
            // 继续当前Activity，普通activity的正常启动 关注这里即可
            mStackSupervisor.startSpecificActivityLocked(next, true, true);
        }
        return true;
    }