Switching Fragments With Code

FragmentTransaction is a class that enables us to add and remove fragments programatically. It works with the backstack just like the one activities use. All we need is a ViewGroup to hold the fragments.

I'm going to list the steps required to work with FragmentTransaction instances. All the code snippets I'll write down will base on the assumption that in our layout xml file there is a FrameLayout with an id of fragmentcontainer that contains or will contain the fragments we will transact with and that the reference to the new fragment is named newFragment.
The steps required are:

1. Create a FragmentTransaction instance in your activity.
2. Specify the changes the transaction should include.
3. Add the transaction to the backstack.
4. Commit the transaction.

Let's look at each of these steps in detail.

Creating a fragment transaction

Fragment transactions are created inside activities. To create a FragmentTransaction, you call the method beginTransaction() on the result of calling getSupportFragmentManager().

You can create a FragmentTransaction with the code snippet below:

FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

Specifying Changes in a Fragment Transaction

There are different changes a FragmentTransaction can contain. Each change is specified by a method. Some of these methods are add(), replace(), and remove().

add() and replace()

The add() method adds a new fragment to the backstack. The code snippet below shows how to use the method:

transaction.add(R.id.fragmentcontainer, newFragment);

The replace() method replaces the existing fragment with a new one. The code snippet below shows how to use the method:

transaction.replace(R.id.fragmentcontainer, newFragment);

Code breakdown

The add() and replace() methods take two arguments. They are:

• The id of the ViewGroup that contains the fragment

• The new fragment that you want to add or replace the old fragment with.

In the code snippets above, transaction is a FragmentTransaction instance, fragmentcontainer is the id of the ViewGroup in our activity that contains the fragment, and newFragment is a reference to the new fragment that we want to add to the backstack or replace the current one with.

remove()

The remove() method removes a fragment from the fragment backstack.

transaction.remove(currentFragment);

Code breakdown

The remove method takes one argument - a reference to the fragment we want to remove. The code snippet above will remove the fragment referenced by currentFragment from the backstack.

Adding a FragmentTransaction to the Backstack

You add a FragmentTransaction to the backstack with the method addToBackStack().
This method takes one argument - a string that you use to label the transaction in case you need to retrieve the transaction programatically.

transaction.addToBackstack("transaction_id");

In the code above, the string "transaction_id" can be used later to retrieve the transaction. You can also pass null into the addToBackStack() method if you don't want to pass a string.

Committing a Transaction

You commit a FragmentTransaction by calling the method commit() on the transaction instance.

transaction.commit();

When you call commit(), the changes you saved in the FragmentTransaction are applied to your application.

Summary

To switch fragments programatically, take these steps:

1. Create a FragmentTransaction in your activity.

FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

1. Specify the changes the transaction should include.

transaction.add(R.id.fragmentcontainer, newFragment);

1. Add the transaction to the backstack.

transaction.addToBackstack("transaction_id");

1. Commit the transaction.

transaction.commit();

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Coming up next: Dynamic Fragments