Android学习 开发文档（Training）04 - fragment

Building a Dynamic UI with Fragments

1. create a fragment

可以把fragment看作为activity的一个模块化的部分，它拥有自己的生命周期，接受自己的输入，并且在activity运行中可以添加或者删除。(sort of like a “sub activity” that you can reuse in different activities). This lesson shows how to extend the Fragment class using the Support Library so your app remains compatible with devices running system versions as low as Android 1.6.

1.1 create a fragment class

创建一个fragment，继承Fragment类，然后重写重要的生命周期来添加我们的代码逻辑（和activity类似）。不同的是Fragment必须使用oncreateView（）回调来定义布局。

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.ViewGroup;

public class ArticleFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.article_view, container, false);
    }
}

1.2 add a fragment to an activity using xml

each instance of a Fragment class must be associated with a parent FragmentActivity.
Note: FragmentActivity is a special activity provided in the Support Library to handle fragments on system versions older than API level 11. If the lowest system version you support is API level 11 or higher, then you can use a regular Activity.

fragment的xml布局文件

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

    <fragment android:name="com.example.android.fragments.HeadlinesFragment"
              android:id="@+id/headlines_fragment"
              android:layout_weight="1"
              android:layout_width="0dp"
              android:layout_height="match_parent" />

    <fragment android:name="com.example.android.fragments.ArticleFragment"
              android:id="@+id/article_fragment"
              android:layout_weight="2"
              android:layout_width="0dp"
              android:layout_height="match_parent" />

</LinearLayout>

2. building a flexible UI

Fragment类提供了方法允许我们在运行的时候动态的添加，移除和替代fragment。

2.1 add a fragment to an activity at runtime

为了执行添加和删除的事务，我们必须使用FragmentManager来创建一个FragmentTransaction。另外，我们需要添加初始的fragment到activity中。为了添加一个fragment我们的layout中必须包含一个view容器方便插入。在这里我们使用FrameLayout来表示fragment container。
res/layout/news_articles.xml:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/fragment_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

Inside your activity, call getSupportFragmentManager() to get a FragmentManager using the Support Library APIs. Then call beginTransaction() to create a FragmentTransaction and call add() to add a fragment.

You can perform multiple fragment transaction for the activity using the same FragmentTransaction. When you’re ready to make the changes, you must call commit().

import android.os.Bundle;
import android.support.v4.app.FragmentActivity;

public class MainActivity extends FragmentActivity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.news_articles);

        // Check that the activity is using the layout version with
        // the fragment_container FrameLayout
        if (findViewById(R.id.fragment_container) != null) {

            // However, if we're being restored from a previous state,
            // then we don't need to do anything and should return or else
            // we could end up with overlapping fragments.
            if (savedInstanceState != null) {
                return;
            }

            // Create a new Fragment to be placed in the activity layout
            HeadlinesFragment firstFragment = new HeadlinesFragment();

            // In case this activity was started with special instructions from an
            // Intent, pass the Intent's extras to the fragment as arguments
            firstFragment.setArguments(getIntent().getExtras());

            // Add the fragment to the 'fragment_container' FrameLayout
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.fragment_container, firstFragment).commit();
        }
    }
}

2.2 replace one fragment with another

如果需要撤销类的功能的话，就需要再must call addToBackStack() before you commit the FragmentTransaction。如果这么做了，那么移除的fragment就会再stopped状态，否则直接进行销毁。
Example of replacing one fragment with another:

// Create fragment and give it an argument specifying the article it should show
ArticleFragment newFragment = new ArticleFragment();
Bundle args = new Bundle();
args.putInt(ArticleFragment.ARG_POSITION, position);
newFragment.setArguments(args);

FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack so the user can navigate back
transaction.replace(R.id.fragment_container, newFragment);
transaction.addToBackStack(null);

// Commit the transaction
transaction.commit();

3. communicating with other fragment

All Fragment-to-Fragment communication is done through the associated Activity. Two Fragments should never communicate directly。
所有的frag和fragment之间的通信应该通过activity来实现的，两个fragment之间不应该直接通信。

3.1 define an interface

为了使fragment和它的activity进行通信，在Fragment类中定义一个接口，并且在activity中继承他。Fragment在它的onattach（）生命周期中获取这个接口并且调用接口中的方法。
Here is an example of Fragment to Activity communication:

public class HeadlinesFragment extends ListFragment {
    OnHeadlineSelectedListener mCallback;

    // Container Activity must implement this interface
    public interface OnHeadlineSelectedListener {
        public void onArticleSelected(int position);
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);

        // This makes sure that the container activity has implemented
        // the callback interface. If not, it throws an exception
        try {
            mCallback = (OnHeadlineSelectedListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString()
                    + " must implement OnHeadlineSelectedListener");
        }
    }

    ...
}

Now the fragment can deliver messages to the activity by calling the onArticleSelected() method (or other methods in the interface) using the mCallback instance of the OnHeadlineSelectedListener interface.

3.2 implement the interface

In order to receive event callbacks from the fragment, the activity that hosts it must implement the interface defined in the fragment class.

为了得到事件的回调，activity必须implement fragment中的接口

public static class MainActivity extends Activity
        implements HeadlinesFragment.OnHeadlineSelectedListener{
    ...

    public void onArticleSelected(int position) {
        // The user selected the headline of an article from the HeadlinesFragment
        // Do something here to display that article
    }
}

3.3 deliver a message to a fragment

activity可以直接通过findFragmentById()得到Fragment的实例，然后直接调用fragment的公共方法。

public static class MainActivity extends Activity
        implements HeadlinesFragment.OnHeadlineSelectedListener{
    ...

    public void onArticleSelected(int position) {
        // The user selected the headline of an article from the HeadlinesFragment
        // Do something here to display that article

        ArticleFragment articleFrag = (ArticleFragment)
                getSupportFragmentManager().findFragmentById(R.id.article_fragment);

        if (articleFrag != null) {
            // If article frag is available, we're in two-pane layout...

            // Call a method in the ArticleFragment to update its content
            articleFrag.updateArticleView(position);
        } else {
            // Otherwise, we're in the one-pane layout and must swap frags...

            // Create fragment and give it an argument for the selected article
            ArticleFragment newFragment = new ArticleFragment();
            Bundle args = new Bundle();
            args.putInt(ArticleFragment.ARG_POSITION, position);
            newFragment.setArguments(args);

            FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

            // Replace whatever is in the fragment_container view with this fragment,
            // and add the transaction to the back stack so the user can navigate back
            transaction.replace(R.id.fragment_container, newFragment);
            transaction.addToBackStack(null);

            // Commit the transaction
            transaction.commit();
        }
    }
}

至此，结束