Question

我使用ViewPager和自定义的PagerAdapter（StaticPagerAdapter扩展FragmentStatePagerAdapter）。问题是：当我用Fragment3更改Fragment1时，未调用onCreateView并且findViewById（R.id.placeholder）返回null

protected void onStart() {
    super.onStart();

    mAdapter.startUpdate(mPager);
    mAdapter.setFragment(0, new DummyFragment3());
    mAdapter.notifyDataSetChanged();
    mAdapter.finishUpdate(mPager);

    TextView tv = (TextView) this.findViewById(R.id.placeholder);
    tv.setText("PLACEHOLDER");
}

解决方案可以是移动

TextView tv = (TextView) this.findViewById(R.id.placeholder);
tv.setText("PLACEHOLDER");

在片段中的onCreateView上，但不幸的是我无法做到。

问题：如何确保在ViewPager中更改片段后立即调用onCreateView？

有什么建议吗？提前致谢！低于所有代码

MainActivity

public class MainActivity extends AppCompatActivity {

StaticPagerAdapter mAdapter;
 ViewPager mPager;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    mAdapter = new StaticPagerAdapter(getFragmentManager(),2);

    mAdapter.setTitle(0,"EXPENSES");
    mAdapter.setTitle(1,"CONTACTS");
    mAdapter.setFragment(0, new DummyFragment1());
    mAdapter.setFragment(1, new DummyFragment2());

    SlidingTabLayout mTabs = (SlidingTabLayout) findViewById(R.id.tabs);
    mTabs.setDistributeEvenly(true);

    mPager = (ViewPager) findViewById(R.id.pager);
    mPager.setAdapter(mAdapter);
    mTabs.setViewPager(mPager);
}

@Override
protected void onStart() {
    super.onStart();

    mAdapter.startUpdate(mPager);
    mAdapter.setFragment(0, new DummyFragment3());
    mAdapter.notifyDataSetChanged();
    mAdapter.finishUpdate(mPager);

    TextView tv = (TextView) this.findViewById(R.id.placeholder);
    tv.setText("PLACEHOLDER");
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
}

StaticPagerAdapter

import android.app.Fragment;
import android.app.FragmentManager;
import android.support.v13.app.FragmentStatePagerAdapter;


public class StaticPagerAdapter extends FragmentStatePagerAdapter {

    String[] titels;
    Fragment[] fragments;
    public StaticPagerAdapter(FragmentManager fm, int size) {
        super(fm);
        titels = new String[size];
        fragments = new Fragment[size];
    }

    @Override
    public Fragment getItem(int position) {
        if (fragments!=null & position< fragments.length)
            return fragments[position];
        return new Fragment();
    }

    @Override
    public CharSequence getPageTitle(int position) {
        if (titels!=null && position<titels.length)
            return titels[position];
        return "";
    }

    public void setTitle(int position, String title) {
        if (titels!=null && position<titels.length)
            titels[position]=title;
    }

    public void setFragment(int position, Fragment fragment) {
        if (fragments!=null && position<fragments.length)
            fragments[position]=fragment;
        this.notifyDataSetChanged();
    }

    @Override
    public int getCount() {
        return titels.length;
    }

    @Override
    public int getItemPosition(Object object) {
        return POSITION_NONE;
    }
}

DummyFragment1

public class DummyFragment1 extends Fragment{

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_dummy, container, false);
        //return super.onCreateView(inflater, container, savedInstanceState);

        ((TextView)view.findViewById(R.id.text)).setText("Fragment number 1");

        return view;


    }
}

LayoutFragment1

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

    <TextView
        android:id="@+id/text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="20dp"/>


</LinearLayout>

DummyFragment3

public class DummyFragment3 extends Fragment{

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_dummy, container, false);
        //return super.onCreateView(inflater, container, savedInstanceState);

        ((TextView)view.findViewById(R.id.text)).setText("Fragment number 3");

        return view;


    }

LayoutFragment3

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

    <TextView
        android:id="@+id/text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textSize="20dp"/>

    <TextView
        android:id="@+id/placeholder"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/text"
        android:textSize="20dp"
        />


</LinearLayout>

我还添加了一些Logs.d消息来检查订单

在活动中

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    mAdapter = new StaticPagerAdapter(getFragmentManager(),2);

    Log.d("msg","1");
    mAdapter.setTitle(0,"EXPENSES");
    mAdapter.setTitle(1,"CONTACTS");
    mAdapter.setFragment(0, new DummyFragment1());
    mAdapter.setFragment(1, new DummyFragment2());

    SlidingTabLayout mTabs = (SlidingTabLayout) findViewById(R.id.tabs);
    mTabs.setDistributeEvenly(true);

    mPager = (ViewPager) findViewById(R.id.pager);
    mPager.setAdapter(mAdapter);
    mTabs.setViewPager(mPager);
}

@Override
protected void onStart() {
    super.onStart();

    Log.d("msg", "2");
    mAdapter.startUpdate(mPager);
    mAdapter.setFragment(0, new DummyFragment3());
    mAdapter.notifyDataSetChanged();
    mAdapter.finishUpdate(mPager);

    Log.d("msg", "4");
    //TextView tv = (TextView) this.findViewById(R.id.placeholder);
    //tv.setText("PLACEHOLDER");
}

Fragment3

public class DummyFragment3 extends Fragment{

@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    Log.d("msg", "3");
    View view = inflater.inflate(R.layout.fragment_dummy, container, false);
    ((TextView)view.findViewById(R.id.text)).setText("Fragment number 3");

    return view;


}
}

但顺序是

D/msg: 1
D/msg: 2
D/msg: 4
D/msg: 3

我期待当我在PagerAdapter中更改Fragment时，会调用该Fragment的onCreateView，但这实际上会在以后发生。感谢所有输入

Answer 1

您可以通过在日志

中放置打印指令来验证进程的执行顺序

实施例

Log.d（＆＃34; msg＆＃34;，＆＃34; Throwable＆＃34;）;

输入不同的MSG并检查订单。

Answer 2

我刚刚找到了一个解决方案，我会在这里发布，以防有人遇到同样的问题。诀窍是在使用此序列更改Fragment时使用instantiateItem。

startUpdate将在FragmentManager中启动一个事务

instantiateItem将在FragmentManager的事务中附加或添加Fragment

finishUpdate将调用FragmentManager的executePendingTransactions，最后将最终调用片段的OnCreateView。

@Override
protected void onStart() {
    super.onStart();

    Log.d("msg", "2");

    Fragment tmp = new DummyFragment3();
    mAdapter.setFragment(0, tmp);
    mAdapter.startUpdate(mPager);
    mAdapter.instantiateItem(mPager, 0);
    mAdapter.finishUpdate(mPager);

    Log.d("msg", "4");
    TextView tv = (TextView) mPager.findViewById(R.id.placeholder);
    tv.setText("PLACEHOLDER");
}

最重要的是在FragmentPagerAdapter的实现中覆盖方法getItemId，并为每个要显示的片段返回一个唯一键。

@Override
public long getItemId(int position) {
    return getItem(position).hashCode();
}

原始的getItemId只返回位置，如果您不必更改ViewPager中的页面，这很好。

希望有所帮助

如何在ViewPager中更改Fragment时确保调用onCreateView？

2 个答案: