非静态内部类确实持有对其父类的引用。使 Fragment 内部类非静态的问题在于您始终持有对Activity的引用。GarbageCollector无法收集您的Activity。因此,如果例如方向发生变化,您可以“泄漏”活动。因为Fragment可能仍然存在并被插入到新的Activity中。
编辑:
由于有人问我一些例子,我开始写一个,在这样做时,我发现使用非静态片段时出现了更多问题:
- 它们不能在 xml 文件中使用,因为它们没有空的构造函数(它们可以有一个空的构造函数,但您通常通过这样做来实例化非静态嵌套类
myActivityInstance.new Fragment()
,这与仅调用空构造函数不同)
- 它们根本不能被重用——因为
FragmentManager
有时也会调用这个空的构造函数。如果您在某些事务中添加了片段。
因此,为了使我的示例正常工作,我必须添加
wrongFragment.setRetainInstance(true);
线以不使应用程序在方向更改时崩溃。
如果您执行此代码,您将有一个带有一些文本视图和 2 个按钮的活动 - 这些按钮会增加一些计数器。片段显示了他们认为他们的活动所具有的方向。一开始一切正常。但是在更改屏幕方向后,只有第一个 Fragment 可以正常工作 - 第二个仍然在其旧活动中调用东西。
我的活动课:
package com.example.fragmenttest;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentTransaction;
import android.content.res.Configuration;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.TextView;
public class WrongFragmentUsageActivity extends Activity
{
private String mActivityOrientation="";
private int mButtonClicks=0;
private TextView mClickTextView;
private static final String WRONG_FRAGMENT_TAG = "WrongFragment" ;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
int orientation = getResources().getConfiguration().orientation;
if (orientation == Configuration.ORIENTATION_LANDSCAPE)
{
mActivityOrientation = "Landscape";
}
else if (orientation == Configuration.ORIENTATION_PORTRAIT)
{
mActivityOrientation = "Portrait";
}
setContentView(R.layout.activity_wrong_fragement_usage);
mClickTextView = (TextView) findViewById(R.id.clicksText);
updateClickTextView();
TextView orientationtextView = (TextView) findViewById(R.id.orientationText);
orientationtextView.setText("Activity orientation is: " + mActivityOrientation);
Fragment wrongFragment = (WrongFragment) getFragmentManager().findFragmentByTag(WRONG_FRAGMENT_TAG);
if (wrongFragment == null)
{
wrongFragment = new WrongFragment();
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.add(R.id.mainView, wrongFragment, WRONG_FRAGMENT_TAG);
ft.commit();
wrongFragment.setRetainInstance(true); // <-- this is important - otherwise the fragment manager will crash when readding the fragment
}
}
private void updateClickTextView()
{
mClickTextView.setText("The buttons have been pressed " + mButtonClicks + " times");
}
private String getActivityOrientationString()
{
return mActivityOrientation;
}
@SuppressLint("ValidFragment")
public class WrongFragment extends Fragment
{
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
LinearLayout result = new LinearLayout(WrongFragmentUsageActivity.this);
result.setOrientation(LinearLayout.VERTICAL);
Button b = new Button(WrongFragmentUsageActivity.this);
b.setText("WrongFragmentButton");
result.addView(b);
b.setOnClickListener(new View.OnClickListener()
{
@Override
public void onClick(View v)
{
buttonPressed();
}
});
TextView orientationText = new TextView(WrongFragmentUsageActivity.this);
orientationText.setText("WrongFragment Activities Orientation: " + getActivityOrientationString());
result.addView(orientationText);
return result;
}
}
public static class CorrectFragment extends Fragment
{
private WrongFragmentUsageActivity mActivity;
@Override
public void onAttach(Activity activity)
{
if (activity instanceof WrongFragmentUsageActivity)
{
mActivity = (WrongFragmentUsageActivity) activity;
}
super.onAttach(activity);
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
LinearLayout result = new LinearLayout(mActivity);
result.setOrientation(LinearLayout.VERTICAL);
Button b = new Button(mActivity);
b.setText("CorrectFragmentButton");
result.addView(b);
b.setOnClickListener(new View.OnClickListener()
{
@Override
public void onClick(View v)
{
mActivity.buttonPressed();
}
});
TextView orientationText = new TextView(mActivity);
orientationText.setText("CorrectFragment Activities Orientation: " + mActivity.getActivityOrientationString());
result.addView(orientationText);
return result;
}
}
public void buttonPressed()
{
mButtonClicks++;
updateClickTextView();
}
}
请注意,onAttach
如果您想在不同的活动中使用您的片段,您可能不应该将活动投入其中- 但在这里它适用于示例。
activity_wrong_fragment_usage.xml:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".WrongFragmentUsageActivity"
android:id="@+id/mainView">
<TextView
android:id="@+id/orientationText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="" />
<TextView
android:id="@+id/clicksText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="" />
<fragment class="com.example.fragmenttest.WrongFragmentUsageActivity$CorrectFragment"
android:id="@+id/correctfragment"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</LinearLayout>