<!-- 告訴系統：我要開懸浮窗啦！ -->
<uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW" />

// 檢查是不是Android 6.0以上的"高級游樂園"
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
    // 看看通行證有沒有過期
    if (!Settings.canDrawOverlays(this)) {
        // 申請權限
        Intent intent = new Intent(Settings.ACTION_MANAGE_OVERLAY_PERMISSION,
                Uri.parse("package:" + getPackageName()));
        startActivityForResult(intent, 1001);
    } else {
        showFloatingWindow(); // 亮出我們的懸浮窗！
    }
}

// 窗口管家（WindowManager）登場
windowManager = (WindowManager) getSystemService(WINDOW_SERVICE);

// 給懸浮窗穿件"衣服"
ImageView floatView = new ImageView(this);
floatView.setImageResource(R.drawable.ic_float); 

// 懸浮窗的"身份證信息"
WindowManager.LayoutParams params = new WindowManager.LayoutParams(
    ViewGroup.LayoutParams.WRAP_CONTENT,
    ViewGroup.LayoutParams.WRAP_CONTENT,
    Build.VERSION.SDK_INT >= Build.VERSION_CODES.O ? 
        WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY : // Android 8.0+用這個
        WindowManager.LayoutParams.TYPE_PHONE,                // 老版本用這個
    WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE, // 別搶焦點，做個安靜的美窗
    PixelFormat.TRANSLUCENT // 半透明效果更高級
);

// 初始位置：屏幕左上角+向下偏移100像素
params.gravity = Gravity.START | Gravity.TOP;
params.x = 0;
params.y = 100;

// 把懸浮窗"放"到屏幕上
windowManager.addView(floatView, params);

floatView.setOnTouchListener(new View.OnTouchListener() {
    private int startX, startY;
    private float touchX, touchY;

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN: // 手指按下
                startX = params.x;
                startY = params.y;
                touchX = event.getRawX(); // 記錄觸點X
                touchY = event.getRawY(); // 記錄觸點Y
                return true;

            case MotionEvent.ACTION_MOVE: // 手指滑動
                // 計算新位置
                params.x = startX + (int)(event.getRawX() - touchX);
                params.y = startY + (int)(event.getRawY() - touchY);
                windowManager.updateViewLayout(floatView, params);
                return true;

            case MotionEvent.ACTION_UP: // 手指松開
                autoAttachToEdge(); // 觸發吸邊效果
                return true;
        }
        return false;
    }
});

private void autoAttachToEdge() {
    int screenWidth = getResources().getDisplayMetrics().widthPixels;
    int viewCenterX = params.x + floatView.getWidth()/2;

    // 判斷離哪邊近
    if(viewCenterX < screenWidth/2) {
        params.x = 0; // 吸到左邊緣
    } else {
        params.x = screenWidth - floatView.getWidth(); // 吸到右邊緣
    }

    // 添加彈性動畫
    ValueAnimator animator = ValueAnimator.ofInt(params.x, targetX);
    animator.addUpdateListener(animation -> {
        params.x = (int) animation.getAnimatedValue();
        windowManager.updateViewLayout(floatView, params);
    });
    animator.setDuration(300).start();
}