findFragmentById时发生NullpointerException

问题描述

我有一个活动，其中托管了一个片段.

I have an Activity which hosts a Fragment.

活动布局文件:

<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">

    <fragment class="com.my.ContentFragment"
        android:id="@+id/fragment_content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>
</FrameLayout>

活动的Java代码:

import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;

public class ContentActivity extends ActionBarActivity {

     @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);

            //data from  previous Activity
            Bundle data = getIntent().getExtras();

            Fragment contentFragment = getSupportFragmentManager()
                                                    .findFragmentById(R.id.fragment_content);

            //Pass data to fragment
            /*NullpointerException, contentFragment is null!*/
            contentFragment.setArguments(data);

            setContentView(R.layout.activity_main);
        }
...
}

我尝试在Activity的onCreate()中找到该片段，然后将一些数据传递给它.但是当我findFragmentById(...)时，出现了 NullpointerException ，为什么?

I try to find the fragment in onCreate() of Activity, and then pass some data to it. But when I findFragmentById(...), I got NullpointerException, Why?

推荐答案

在将片段添加到活动之前，您无法执行findFragmentById.制作setContentView时，Actvitiy会扩大布局并将其视图添加到当前窗口.在此布局中有Fragment的情况下，会将它与xml中提供的给定ID或标记添加到FragmentManager中.只有在这一刻，您才能在那找到它.因此，您要做的就是将findFragmentById移到setContentView下方.

You can't perform a findFragmentById before adding the fragment to your activity. When you make a setContentView the Actvitiy inflates the layout and adds its views to the current window. In the case of having a Fragment in this layout, it is added with the given Id or Tag provided in the xml to the FragmentManager. Only in this moment you can find it there. So what you have to do is move the findFragmentById below the setContentView.

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