Find element by AndroidUIAutomator

Hi, I am using AndroidDriver findElementByAndoridUIAutomator to find elements. The situation is

A-> parent
         |_ a with text I could get => child of A
B-> parent with Id I could get.
         |_ b => child of B

A and B are on the same level. I am trying to find element b by using the following search string:
new UiSelector().text("text of a").instance(0).fromParent(new UiSelector().instance(0).fromParent(new UiSelector(),resourceId("id of B").instance(0))); But the result is “No element found”…

Any helps about the search string? Thank you so much.