Little question about setParent method on transfromManager

[yicong123456]

I wanna combine some nodes who have already add into the scene to a root and apply the transform to the root directly so that I didnot need to apply the transform to each of them. And I just have a simple try to set the connection as below:

        val tmManager = engine.transformManager
        val rootEntity = EntityManager.get().create()
        scene.addEntity(rootEntity)

        val entity1 = EntityManager.get().create()
        scene.addEntity(entity1)

        val entity2 = EntityManager.get().create()
        scene.addEntity(entity2)

        val entity3 = EntityManager.get().create()
        scene.addEntity(entity3)

        val entity4 = EntityManager.get().create()
        scene.addEntity(entity4)

        tmManager.setParent(entity1, rootEntity)
        tmManager.setParent(entity2, rootEntity)
        tmManager.setParent(entity3, rootEntity)
        tmManager.setParent(entity4, rootEntity)

        val r1 = tmManager.getParent(entity1)
        val r2 = tmManager.getParent(entity2)
        val r3 = tmManager.getParent(entity3)
        val r4 = tmManager.getParent(entity4)

        Timber.i("delete after check: root entity: ${rootEntity}, e1: ${entity1}, e2: ${entity2}, e3:${entity3}, e4: ${entity4}, r1:${r1} , r2:${r2} ,r3:${r3} ,r4:${r4}")

and I'm confused that I did nothing except set rootEntity as the root to entity14, but the result is all of the r1r4 printed out is 0 after the code executed. This is the reuslt printed out: delete after check: root entity: 5359, e1: 5360, e2: 5361, e3:5362, e4: 5363, r1:0 , r2:0 ,r3:0 ,r4:0 .It shows that I failed to set entity1~4's parent to the rootEntity. May I know what's mistake I have made and thanks so much!