//未定义
    public static final int WINDOWING_MODE_UNDEFINED = 0;
    //普通全屏窗口
    public static final int WINDOWING_MODE_FULLSCREEN = 1;
    //画中画
    public static final int WINDOWING_MODE_PINNED = 2;
    //分屏主窗口
    public static final int WINDOWING_MODE_SPLIT_SCREEN_PRIMARY = 3;
    //分屏副窗口
    public static final int WINDOWING_MODE_SPLIT_SCREEN_SECONDARY = 4;
    //自由窗口
    public static final int WINDOWING_MODE_FREEFORM = 5;
    
    //自由窗口模式里面，窗口支持放大缩小以及移动位置，原理是不断的更改Task的边界(用Rect表示)，
    //然后根据Task的边界来重新缩放Task，从而达到窗口缩放和拖动的作用。
    
    //如何以自由窗口模式启动应用?
    //在ActivityStarter拦截启动APP，以自由窗口模式启动
    frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java
    
    int startActivityInner(...)}{
    	...
    	computeLaunchParams(r, sourceRecord, targetTask);
    	...
    }
    
    private void computeLaunchParams(ActivityRecord r, ActivityRecord sourceRecord,
            Task targetTask) {
        final ActivityStack sourceStack = mSourceStack != null ? mSourceStack
                : mRootWindowContainer.getTopDisplayFocusedStack();
        if (sourceStack != null && sourceStack.inSplitScreenWindowingMode()
                && (mOptions == null
                        || mOptions.getLaunchWindowingMode() == WINDOWING_MODE_UNDEFINED)) {
            int windowingMode =
                    targetTask != null ? targetTask.getWindowingMode() : WINDOWING_MODE_UNDEFINED;
            if ((mLaunchFlags & FLAG_ACTIVITY_LAUNCH_ADJACENT) != 0) {
                if (sourceStack.inSplitScreenPrimaryWindowingMode()) {
                    windowingMode = WINDOWING_MODE_SPLIT_SCREEN_SECONDARY;
                } else if (sourceStack.inSplitScreenSecondaryWindowingMode()) {
                    windowingMode = WINDOWING_MODE_SPLIT_SCREEN_PRIMARY;
                }
            }

            if (mOptions == null) {
                mOptions = ActivityOptions.makeBasic();
            }
            mOptions.setLaunchWindowingMode(windowingMode);
        }

        mSupervisor.getLaunchParamsController().calculate(targetTask, r.info.windowLayout, r,
                sourceRecord, mOptions, PHASE_BOUNDS, mLaunchParams);
        mPreferredTaskDisplayArea = mLaunchParams.hasPreferredTaskDisplayArea()
                ? mLaunchParams.mPreferredTaskDisplayArea
                : mRootWindowContainer.getDefaultTaskDisplayArea();
        //add text
        mPreferredWindowingMode = 5;//mLaunchParams.mWindowingMode;
        mOptions.setLaunchBounds(new Rect(10,20,30,40));
        mOptions.setLaunchWindowingMode(5);
        //add text
    }

ActivityOptions副屏启动-CSDN博客

ActivityOptions - Android中文版 - API参考文档 (apiref.com)

支持多窗口模式 | Android Open Source Project (google.cn)

android freeform模式定制桌面系统_android freeform demo实例-CSDN博客

android framework实战开发之WINDOWING_MODE_FREEFORM自由窗口相关_android_learnframework-华为云开发者联盟 (csdn.net)