What is react-native component doing when the component is in background - react-native

Question

What is react-native component doing when the component is in background?
And What will react-native do when component re-active to foreground?
Like Page A navigate to Page B.
And Page B back to Page A.What happened to A and B?
Is there any ui update like onResume on Android or something?
Edit:
I have a TabNavagation with four tab.One of the tabs renders an Android UI Component which is a custom frameLayout(a listview is in the frameLayout).
When the frameLayout first rendered, the listview position is not correct(position is -4 item height).But when I navigate to a child page and goback, I found that the frameLayout is correct now and does't enter any android lifecycle.
So I'm wondering is it doing something background?

Answer 1

It is hard to tell without detail information on what type of navigation you are using but I can explain it considering you are using common navigation packages.
If your navigation is some sort of stack navigation, when you navigate/push a new screen to view the previous screen lays under the stack and doesn't un-mount. It is much like dealing a deck of card to a pile. It doesn't re-render unless the props or state of that specific component changes. When you navigate back the new screen un-mounts and the previous screen shows back.
Common packages lets you to pass props to the next screen and if you need to re-render a previous component a easy hack to achieve this by sending a function from the previous component and running it on componentWillUnmount life-cycle event of the current screen. I explained this sort of usage in this answer.

Answer 2

After debug many many times, I found that it is the react-navigation.
Four tabs in homepage with bottom tabbar.When I enter a child page with navigationOptions:{header:null}, the bottom bar is gone. And when I goback from the child page, the bottom bar has back meanwhile and the whole view has corrected.Seems the whole view is re-draw?So I called view.requestLayout() in createViewInstance().So the final result is the view need to requestLayout manual.
UPDATE:
answer top is not totally correct.The final problem is the custom view should measure self in its requestLayout.
I found the answer in https://github.com/facebook/react-native/issues/4990
#Override
public void requestLayout() {
super.requestLayout();
post(measureAndLayout);
}
private final Runnable measureAndLayout = new Runnable() {
#Override
public void run() {
measure(MeasureSpec.makeMeasureSpec(getWidth(), MeasureSpec.EXACTLY),
MeasureSpec.makeMeasureSpec(getHeight(), MeasureSpec.EXACTLY));
layout(getLeft(), getTop(), getRight(), getBottom());
}
};
And thanks for your answers.#bennygenel #Jouke