All These Fragments

In the preceding sample, R. The container parameter passed to onCreateView is the parent ViewGroup from the activity's layout in which your fragment layout is inserted. The savedInstanceState parameter is a Bundle that provides data about the previous instance of the fragment, if the fragment is being resumed restoring state is discussed more in the section about Handling the Fragment Lifecycle.

The inflate method takes three arguments:. Now you've seen how to create a fragment that provides a layout. Next, you need to add the fragment to your activity. Usually, a fragment contributes a portion of UI to the host activity, which is embedded as a part of the activity's overall view hierarchy. There are two ways you can add a fragment to the activity layout:. In this case, you can specify layout properties for the fragment as if it were a view.

For example, here's the layout file for an activity with two fragments:. When the system creates this activity layout, it instantiates each fragment specified in the layout and calls the onCreateView method for each one, to retrieve each fragment's layout. Each fragment requires a unique identifier that the system can use to restore the fragment if the activity is restarted and which you can use to capture the fragment to perform transactions, such as remove it.

There are two ways to provide an ID for a fragment:. At any time while your activity is running, you can add fragments to your activity layout. You simply need to specify a ViewGroup in which to place the fragment. To make fragment transactions in your activity such as add, remove, or replace a fragment , you must use APIs from FragmentTransaction. You can get an instance of FragmentTransaction from your FragmentActivity like this:. You can then add a fragment using the add method, specifying the fragment to add and the view in which to insert it.

The first argument passed to add is the ViewGroup in which the fragment should be placed, specified by resource ID, and the second parameter is the fragment to add. Once you've made your changes with FragmentTransaction , you must call commit for the changes to take effect. To manage the fragments in your activity, you need to use FragmentManager. To get it, call getSupportFragmentManager from your activity.

Some things that you can do with FragmentManager include:. For more information about these methods and others, refer to the FragmentManager class documentation. As demonstrated in the previous section, you can also use FragmentManager to open a FragmentTransaction , which allows you to perform transactions, such as add and remove fragments. A great feature about using fragments in your activity is the ability to add, remove, replace, and perform other actions with them, in response to user interaction. Each set of changes that you commit to the activity is called a transaction and you can perform one using APIs in FragmentTransaction.

You can also save each transaction to a back stack managed by the activity, allowing the user to navigate backward through the fragment changes similar to navigating backward through activities. You can acquire an instance of FragmentTransaction from the FragmentManager like this:. Each transaction is a set of changes that you want to perform at the same time.

You can set up all the changes you want to perform for a given transaction using methods such as add , remove , and replace. Then, to apply the transaction to the activity, you must call commit. Before you call commit , however, you might want to call addToBackStack , in order to add the transaction to a back stack of fragment transactions.

This back stack is managed by the activity and allows the user to return to the previous fragment state, by pressing the Back button. For example, here's how you can replace one fragment with another, and preserve the previous state in the back stack:. In this example, newFragment replaces whatever fragment if any is currently in the layout container identified by the R.

By calling addToBackStack , the replace transaction is saved to the back stack so the user can reverse the transaction and bring back the previous fragment by pressing the Back button.

What do all the fragments do? : FFXVANE

FragmentActivity then automatically retrieve fragments from the back stack via onBackPressed. If you add multiple changes to the transaction—such as another add or remove —and call addToBackStack , then all changes applied before you call commit are added to the back stack as a single transaction and the Back button reverses them all together. The order in which you add changes to a FragmentTransaction doesn't matter, except:. If you don't call addToBackStack when you perform a transaction that removes a fragment, then that fragment is destroyed when the transaction is committed and the user cannot navigate back to it.

Whereas, if you do call addToBackStack when removing a fragment, then the fragment is stopped and is later resumed if the user navigates back. For each fragment transaction, you can apply a transition animation, by calling setTransition before you commit. Calling commit doesn't perform the transaction immediately. Rather, it schedules it to run on the activity's UI thread the "main" thread as soon as the thread is able to do so. If necessary, however, you may call executePendingTransactions from your UI thread to immediately execute transactions submitted by commit.

Doing so is usually not necessary unless the transaction is a dependency for jobs in other threads. You can commit a transaction using commit only prior to the activity saving its state when the user leaves the activity. If you attempt to commit after that point, an exception is thrown. This is because the state after the commit can be lost if the activity needs to be restored. For situations in which it's okay that you lose the commit, use commitAllowingStateLoss.

Although a Fragment is implemented as an object that's independent from a FragmentActivity and can be used inside multiple activities, a given instance of a fragment is directly tied to the activity that hosts it. Specifically, the fragment can access the FragmentActivity instance with getActivity and easily perform tasks such as find a view in the activity layout:. To share data, create a shared ViewModel, as outlined in the Share data between fragments section in the ViewModel guide.

If you need to propagate events that cannot be handled with a ViewModel, you can instead define a callback interface inside the fragment and require that the host activity implement it. When the activity receives a callback through the interface, it can share the information with other fragments in the layout as necessary.

For example, if a news application has two fragments in an activity—one to show a list of articles fragment A and another to display an article fragment B —then fragment A must tell the activity when a list item is selected so that it can tell fragment B to display the article. Then the activity that hosts the fragment implements the OnArticleSelectedListener interface and overrides onArticleSelected to notify fragment B of the event from fragment A. To ensure that the host activity implements this interface, fragment A's onAttach callback method which the system calls when adding the fragment to the activity instantiates an instance of OnArticleSelectedListener by casting the Activity that is passed into onAttach:.

If the activity hasn't implemented the interface, then the fragment throws a ClassCastException. On success, the mListener member holds a reference to activity's implementation of OnArticleSelectedListener , so that fragment A can share events with the activity by calling methods defined by the OnArticleSelectedListener interface. For example, if fragment A is an extension of ListFragment , each time the user clicks a list item, the system calls onListItemClick in the fragment, which then calls onArticleSelected to share the event with the activity:.

The id parameter passed to onListItemClick is the row ID of the clicked item, which the activity or other fragment uses to fetch the article from the application's ContentProvider.

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More information about using a content provider is available in the Content Providers document. Your fragments can contribute menu items to the activity's Options Menu and, consequently, the app bar by implementing onCreateOptionsMenu. In order for this method to receive calls, however, you must call setHasOptionsMenu during onCreate , to indicate that the fragment would like to add items to the Options Menu. Otherwise, the fragment doesn't receive a call to onCreateOptionsMenu. Any items that you then add to the Options Menu from the fragment are appended to the existing menu items.

The fragment also receives callbacks to onOptionsItemSelected when a menu item is selected. You can also register a view in your fragment layout to provide a context menu by calling registerForContextMenu. When the user opens the context menu, the fragment receives a call to onCreateContextMenu. When the user selects an item, the fragment receives a call to onContextItemSelected.

Although your fragment receives an on-item-selected callback for each menu item it adds, the activity is first to receive the respective callback when the user selects a menu item. If the activity's implementation of the on-item-selected callback does not handle the selected item, then the event is passed to the fragment's callback.

This is true for the Options Menu and context menus. For more information about menus, see the Menus developer guide and the App Bar training class. The effect of the activity lifecycle on the fragment lifecycle. Managing the lifecycle of a fragment is a lot like managing the lifecycle of an activity.

Like an activity, a fragment can exist in three states:. Also like an activity, you can preserve the UI state of a fragment across configuration changes and process death using a combination of onSaveInstanceState Bundle , ViewModel , and persistent local storage. The most significant difference in lifecycle between an activity and a fragment is how one is stored in its respective back stack. An activity is placed into a back stack of activities that's managed by the system when it's stopped, by default so that the user can navigate back to it with the Back button, as discussed in Tasks and Back Stack.

However, a fragment is placed into a back stack managed by the host activity only when you explicitly request that the instance be saved by calling addToBackStack during a transaction that removes the fragment. Otherwise, managing the fragment lifecycle is very similar to managing the activity lifecycle; the same practices apply.

Adam argues that we can overcome most of these challenges if we think about Fragments in a couple of different ways:. First, Fragments, like all the other android components , are simply composable entry points into an application. In fact, they are the same as Activities in terms of abstraction.

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The smaller and smarter we write Fragments, the less they are impacted by their surroundings. Second, Fragments are not fancy views. In practice, Fragments should depend on views and respond to events that happen within views. Likewise, views should have no knowledge of Fragments. Fragments should coordinate between other Android components and be the enforcer of application business logic. But after years of trying to incorporate Fragments, developers started to grow restless and longed for an alternative. In , roughly three years after the addition of Fragments to the Android Framework, developers from the company Square wrote a rather scathing blog post advocating against using Fragments.

In addition to some of the problems with Fragments we have already discussed in this post, Square cited problems debugging the Fragment Manager and exceptions that can occur when fragments are recreated. But instead of just pointing out the flaws with Fragments, Square decided to do something about it and built tools to make it easier to create applications without Fragments - Flow and Mortar.

A screen needs only to know with which View it will be associated with and any methods needed to update the view. Flow also gives you a basic navigation stack, independent from the Activity and FragmentManager backstacks and much simpler. This allows you to navigate and persist state when moving back and forth between screens. If Flow encapsulates the UI and provides a navigation stack, where do we put our business logic? What if we need to hook into the lifecycle? Mortar is a library that uses the MVP pattern to wrap up business logic into presenters and interact with views.

Each presenter is scoped, thanks to Dagger , so that resources are cleaned up if a presenter gets destroyed. Mortar, also, gives you a very scaled back lifecycle to manage the life of a presenter. Lots of boilerplate code. It has been a year since the last minor update to Mortar. Flow finally hit 1.

Fragments of Heraclitus

In the wake of the post, it seemed as though two communities instantly formed: If your app is structured using this pattern, then you might be able to switch back to using multiple activities. If you are not using this pattern and are just trying to move away from Fragments, you could consider a custom view instead. There are challenges, however, with both of these approaches.

In the case of switching to multiple activities, one area of interest we would need to pay extra attention to is how we optimize interfaces to support different form factors such as tablets. Android gives us the ability to qualify resources for the particular screen sizes, which would work great for simple layouts. But what if you wanted to implement a master-detail interface where the experience is different between devices? You may find yourself coding special logic to handle each form factor or even creating entirely different activities to solve the problem.

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But this approach can lead to interfaces that are tedious and difficult to maintain. One of the nice things about Fragments is they allow us to isolate our UI components so activities could focus on everything else. Could custom views give us the same benefits? The short answer is yes, unless we have to worry about the state of our UI. Remember, one of the goals of Activities and Fragments as an extension is that they always stay in the state where you left them. There are several items to consider if you decide to implement custom views:. View State In the case of orientation changes, the view will be destroyed and recreated.

The goal of both of these libraries is to eliminate the boilerplate needed to create a Parcelable. Managing the backstack How do you handle the back button? Depending on the use case, you may want to emulate the behavior you get with Fragments. For example, say you wanted to navigate between two views within a single activity and return to the same state using the android back button or toolbar. The FragmentManager allowed us to push Fragments onto a historical stack and manually call popbackstack , returning to the previous state, whenever the user pushes the back button.