android - 从 fragment 访问 View

Question

如果我从 Java 代码中访问 textview，它就可以工作。所以我假设我在某个地方犯了一个错误。谁能看出问题所在？

package com.projectcaruso.naturalfamilyplaning;

import com.projectcaruso.naturalfamilyplanning.R;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;


public class HistoryFragment extends Fragment {

    @Override
    public void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        SQLHelper entry = new SQLHelper(getActivity());

        TextView tv = (TextView) getView().findViewById(R.id.tvSQLinfo);

        entry.open();
        String results = entry.getData();
        entry.close();

        tv.setText(results);

    }       

     @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            return inflater.inflate(R.layout.fragment_history, container, false);
        }

}

XML:

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

    <TableLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent">

        <TableRow>

            <TextView
                android:layout_width="fill_parent"
                android:layout_height="fill_parent"
                android:text="date" />

            <TextView
                android:layout_width="fill_parent"
                android:layout_height="fill_parent"
                android:text="temp" />

        </TableRow>
    </TableLayout>

    <TextView
       android:id="@+id/tvSQLinfo"
       android:layout_width="fill_parent"
       android:layout_height="fill_parent"
       android:text="Get info from db" />   
</LinearLayout>

日志:

05-12 22:55:01.799: E/AndroidRuntime(1983): FATAL EXCEPTION: main
05-12 22:55:01.799: E/AndroidRuntime(1983): java.lang.NullPointerException
05-12 22:55:01.799: E/AndroidRuntime(1983):     at com.projectcaruso.naturalfamilyplaning.HistoryFragment.onCreate(HistoryFragment.java:20)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at android.support.v4.app.Fragment.performCreate(Fragment.java:1437)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:877)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1088)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1444)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:429)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at android.os.Handler.handleCallback(Handler.java:725)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at android.os.Handler.dispatchMessage(Handler.java:92)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at android.os.Looper.loop(Looper.java:137)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at android.app.ActivityThread.main(ActivityThread.java:5041)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at java.lang.reflect.Method.invokeNative(Native Method)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at java.lang.reflect.Method.invoke(Method.java:511)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
05-12 22:55:01.799: E/AndroidRuntime(1983):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

问题是您对 getView() 的调用返回了 null。

这是为什么？
因为在onCreateView()之前调用了onCreate()。

查看 Fragment 生命周期以获取更多信息:http://developer.android.com/guide/components/fragments.html#Creating

解决方案:
将您的逻辑也移到 onCreateView() 方法中并保留对膨胀 View 的引用，而不是通过 getView() 调用它。它看起来像这样:

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_history, container, false);

    SQLHelper entry = new SQLHelper(getActivity());
    entry.open();
    String results = entry.getData();
    entry.close();

    TextView tv = (TextView) view.findViewById(R.id.tvSQLinfo);
    tv.setText(results);

    return view;
}