Fragment methods: attach(), detach(), remove(), re

问题:

I am very confused between these functions and their purposes. What I have observed that using replace() replaces the existing fragment with a new one. We can use addToBackStack(null) to put that fragment in back stack so we can go back to the previously shown fragment. Now when a fragment is added (or replaced) - onAttach() -> onCreate() etc.... methods of the fragment are called in order.

Now when we call remove() on the fragment from our activity, which functions of the fragment are called and in which order?

What does attach() and detach() do? Does detach() remove the fragment? And when these two attach() and detach() are used, which functions of the fragment are called and in which order??

Also, what happens on popBackStack()?? I mean which functions are called when we use popBackStack()on the fragment from our activity??

And when does onDestroy() called??

Thank you.

回答1:

Now when we call remove() on the fragment from our activity, which functions of the fragment are called and in which order?

Look at http://developer.android.com/reference/android/app/Fragment.html .

The order is: onPause(), onStop(), onDestroyView(), onDestroy(), onDetach()

What does attach() and detach() do? Does detach() remove the fragment? And when these two attach() and detach() are used, which functions of the fragment are called and in which order??

attach() and detach() is respectively associates or detaches the Fragment with/from the Activity. When attaching the Fragment, the onAttach() lifecycle method is called, when detaching, the onDetach() lifecycle method is called in the Fragment. For more information look at the link above.

Also, what happens on popBackStack()?? I mean which functions are called when we use popBackStack()on the fragment from our activity??

If the Fragment hasn't been destroyed, then on popBackStack() the onStart() and onResume() methods are called. If the Fragment has been destroyed previously, then the lifecycle methods will be called starting from onAttach(). It's the same as, when you press the back button on Activities.

回答2:

Just a note on popBackStack(). It doesn't pop a fragment, it pops a fragment transaction. So whatever the last fragment transaction was is reversed. If you were displaying a FragmentA and your transaction was:

fragmentTransaction.replace(R.id.your_layout, fragmentB);
fragmentTransaction.addToBackStack(null);

It would replace FragmentA with FragmentB, and add that transaction (not the fragment) to the back stack. If you then hit the back button, it pops the back stack and gets the transaction, which was "replace this FragmentA with a FragmentB". It then reverses that transaction. Backwards, the instruction is to replace whatever the current fragment is with FragmentA. If the original FragmentA still exists, it uses that one. If it's been destroyed, it makes a new one.

回答3:

Suppose fragment A and fragment B was added to a container with the following steps:

1. Added fragment A =>  .replace(R.id.container, fragmentA) => addToBackStack(null)
2. Added fragment B =>  .replace(R.id.container, fragmentB) => addToBackStack(null)
3. Removed fragment B => fragmentManager.popBackStack();

Callbacks when fm.popBackStack() is called:

FragmentB: onPause()
FragmentB: onStop()
FragmentB: onDestroy()
FragmentB: onDetach()
FragmentA: onCreateView()
FragmentA: onViewCreated()
FragmentA: onActivityCreated()
FragmentA: onStart()
FragmentA: onResume()

Explanation: Why while removing and destroying fragment B using popBackStack(), fragment A view was created again and resumed?

Ans: When you added fragment B, you used replace() and addToBackStack(), so all the fragments were removed from the container, and fragment B was added to the container. And, also this transaction was recorded in the Back stack. So, when fm.popBackStack() is called, first the transaction is popped out from the back stack and so the operations reverted itself, therefore fragment b is removed from the container and destroyed. And all other fragments get added, for our case fragment A's view is added to the container. Also noted that fragment A's onAttach & onCreate() is not called because it has already been created & attached to the MainActivity earlier.

Back press on fragment B does the same thing:

If you press the back button, it calls fm.popBackStack() and pops the transaction.