碎片之间的通信
片段之间的所有通信都必须通过活动进行。没有活动，碎片不能相互通信。
其他资源
在这个例子中，我们有一个 MainActivity，它包含两个片段 SenderFragment 和 ReceiverFragment，分别用于发送和接收 message（在这种情况下是一个简单的字符串）。
SenderFragment 中的按钮启动发送消息的过程。ReceiverFragment 中的 TextView 在收到消息时会更新。
以下是 MainActivity 的片段，其中的注释解释了重要的代码行：
// Our MainActivity implements the interface defined by the SenderFragment. This enables
// communication from the fragment to the activity
public class MainActivity extends AppCompatActivity implements SenderFragment.SendMessageListener {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
/**
* This method is called when we click on the button in the SenderFragment
* @param message The message sent by the SenderFragment
*/
@Override
public void onSendMessage(String message) {
// Find our ReceiverFragment using the SupportFragmentManager and the fragment's id
ReceiverFragment receiverFragment = (ReceiverFragment)
getSupportFragmentManager().findFragmentById(R.id.fragment_receiver);
// Make sure that such a fragment exists
if (receiverFragment != null) {
// Send this message to the ReceiverFragment by calling its public method
receiverFragment.showMessage(message);
}
}
}
MainActivity 的布局文件在 LinearLayout 中托管两个片段：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.naru.fragmentcommunication.MainActivity">
<fragment
android:id="@+id/fragment_sender"
android:name="com.naru.fragmentcommunication.SenderFragment"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
tools:layout="@layout/fragment_sender" />
<fragment
android:id="@+id/fragment_receiver"
android:name="com.naru.fragmentcommunication.ReceiverFragment"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
tools:layout="@layout/fragment_receiver" />
</LinearLayout>
SenderFragment 暴露了一个界面 SendMessageListener，帮助 MainActivity 知道何时点击了 SenderFragment 中的 Button。
以下是 SenderFragment 的代码片段，解释了重要的代码行：
public class SenderFragment extends Fragment {
private SendMessageListener commander;
/**
* This interface is created to communicate between the activity and the fragment. Any activity
* which implements this interface will be able to receive the message that is sent by this
* fragment.
*/
public interface SendMessageListener {
void onSendMessage(String message);
}
/**
* API LEVEL >= 23
* <p>
* This method is called when the fragment is attached to the activity. This method here will
* help us to initialize our reference variable, 'commander' , for our interface
* 'SendMessageListener'
*
* @param context
*/
@Override
public void onAttach(Context context) {
super.onAttach(context);
// Try to cast the context to our interface SendMessageListener i.e. check whether the
// activity implements the SendMessageListener. If not a ClassCastException is thrown.
try {
commander = (SendMessageListener) context;
} catch (ClassCastException e) {
throw new ClassCastException(context.toString()
+ "must implement the SendMessageListener interface");
}
}
/**
* API LEVEL < 23
* <p>
* This method is called when the fragment is attached to the activity. This method here will
* help us to initialize our reference variable, 'commander' , for our interface
* 'SendMessageListener'
*
* @param activity
*/
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
// Try to cast the context to our interface SendMessageListener i.e. check whether the
// activity implements the SendMessageListener. If not a ClassCastException is thrown.
try {
commander = (SendMessageListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString()
+ "must implement the SendMessageListener interface");
}
}
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
// Inflate view for the sender fragment.
View view = inflater.inflate(R.layout.fragment_receiver, container, false);
// Initialize button and a click listener on it
Button send = (Button) view.findViewById(R.id.bSend);
send.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
// Sanity check whether we were able to properly initialize our interface reference
if (commander != null) {
// Call our interface method. This enables us to call the implemented method
// in the activity, from where we can send the message to the ReceiverFragment.
commander.onSendMessage("HELLO FROM SENDER FRAGMENT!");
}
}
});
return view;
}
}
SenderFragment 的布局文件：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:orientation="vertical">
<Button
android:id="@+id/bSend"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="SEND"
android:layout_gravity="center_horizontal" />
</LinearLayout>
ReceiverFragment 很简单，并公开了一个简单的公共方法来更新其 TextView。当 MainActivity 收到来自 SenderFragment 的消息时，它会调用 ReceiverFragment 的这种公共方法
以下是 ReceiverFragment 的代码片段，其中的注释解释了重要的代码行：
public class ReceiverFragment extends Fragment {
TextView tvMessage;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
// Inflate view for the sender fragment.
View view = inflater.inflate(R.layout.fragment_receiver, container, false);
// Initialize the TextView
tvMessage = (TextView) view.findViewById(R.id.tvReceivedMessage);
return view;
}
/**
* Method that is called by the MainActivity when it receives a message from the SenderFragment.
* This method helps update the text in the TextView to the message sent by the SenderFragment.
* @param message Message sent by the SenderFragment via the MainActivity.
*/
public void showMessage(String message) {
tvMessage.setText(message);
}
}
ReceiverFragment 的布局文件：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:orientation="vertical">
<TextView
android:id="@+id/tvReceivedMessage"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Waiting for message!" />
</LinearLayout>