findViewById（）从片段内返回null-Android

Question

我正在尝试从MainActivity更新应用程序中的进度条，但是它不允许我引用片段中的任何内容。 如果我使用“片段”功能引用片段，但是片段将呈现在所有内容的下方（因为我的应用程序将片段切换到单个活动内）。 由于进度条，我尝试过的任何操作都会在运行时返回NullPointerException。

我的片段的.java文件：

 package com.example.adam.slypanel; import android.app.Fragment; import android.os.Bundle; import android.support.annotation.Nullable; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import android.widget.ProgressBar; import java.util.Random; /** * Created by root on 02/10/14. */ public class ServerStatusFragment extends Fragment{ public static ProgressBar cpuPercentage; public static ProgressBar ramPercentage; public static ProgressBar tempValue; public ServerStatusFragment() { } @Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { View ServerStatusView = inflater.inflate(R.layout.fragment_server_status, container, false); //Right here! D: cpuPercentage = (ProgressBar) ServerStatusView.findViewById(R.id.cpuUsageBar); ramPercentage = (ProgressBar) ServerStatusView.findViewById(R.id.ramUsageBar); tempValue = (ProgressBar) ServerStatusView.findViewById(R.id.tempBar); return (ServerStatusView); } //For testing -- Replace when needed public static int randInt(int min, int max) { // NOTE: Usually this should be a field rather than a method // variable so that it is not re-seeded every call. Random rand = new Random(); // nextInt is normally exclusive of the top value, // so add 1 to make it inclusive int randomNum = rand.nextInt((max - min) + 1) + min; return randomNum; } public void refresh() { cpuPercentage.setProgress(randInt(0, 100)); ramPercentage.setProgress(randInt(0, 100)); tempValue.setProgress(randInt(20, 100)); } } 

我尝试了各种不同的解决方案，到处环顾了一下，几乎到处都见到有人说要膨胀该片段，但是它膨胀了，我不知道为什么它不起作用...会有任何帮助非常感激！ 我仍然是Java的新手！ ：P我确定我一直是个白痴，只是看不到我做了什么...

附：我想从布局中获取的所有内容都存在于XML文件中，所以……不是那样！ ：P

这是我的fragment_server_status的XML

 <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="match_parent" android:layout_height="match_parent" android:id="@+id/Server_Status_Fragment" android:weightSum="1"> <RelativeLayout android:layout_width="match_parent" android:layout_height="match_parent"> <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:textAppearance="?android:attr/textAppearanceLarge" android:text="CPU Usage: " android:id="@+id/cpuUsageLabel" android:layout_alignParentTop="true" android:layout_alignParentLeft="true" android:layout_alignParentStart="true" android:layout_marginTop="30dp" android:layout_marginLeft="20dp" /> <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:textAppearance="?android:attr/textAppearanceLarge" android:text="RAM Usage: " android:id="@+id/ramUsageLabel" android:layout_toEndOf="@+id/tempLabel" android:layout_below="@+id/cpuUsageBar" android:layout_alignLeft="@+id/cpuUsageBar" android:layout_alignStart="@+id/cpuUsageBar" android:layout_marginTop="45dp" /> <TextView android:layout_width="wrap_content" android:layout_height="wrap_content" android:textAppearance="?android:attr/textAppearanceLarge" android:text="Temperature" android:id="@+id/tempLabel" android:layout_below="@+id/ramUsageBar" android:layout_alignRight="@+id/cpuUsageLabel" android:layout_alignEnd="@+id/cpuUsageLabel" android:layout_marginTop="52dp" /> <ProgressBar style="?android:attr/progressBarStyleHorizontal" android:layout_width="wrap_content" android:layout_height="25dp" android:id="@+id/cpuUsageBar" android:max="100" android:progress="25" android:indeterminate="false" android:layout_marginTop="40dp" android:layout_below="@+id/cpuUsageLabel" android:layout_alignLeft="@+id/cpuUsageLabel" android:layout_alignStart="@+id/cpuUsageLabel" /> <ProgressBar style="?android:attr/progressBarStyleHorizontal" android:layout_width="120dp" android:layout_height="25dp" android:id="@+id/ramUsageBar" android:max="100" android:progress="45" android:indeterminate="false" android:layout_toEndOf="@+id/tempBar" android:layout_centerVertical="true" android:layout_alignLeft="@+id/ramUsageLabel" android:layout_alignStart="@+id/ramUsageLabel" /> <ProgressBar style="?android:attr/progressBarStyleHorizontal" android:layout_width="120dp" android:layout_height="25dp" android:id="@+id/tempBar" android:max="100" android:progress="60" android:indeterminate="false" android:layout_marginTop="65dp" android:layout_below="@+id/tempLabel" android:layout_alignLeft="@+id/tempLabel" android:layout_alignStart="@+id/tempLabel" /> </RelativeLayout> </LinearLayout> 

我按照建议添加了OnViewCreated，然后运行它并... :(仍然是NullpointerException。

 01-09 16:26:39.571 7746-7746/com.example.adam.slypanel E/AndroidRuntime﹕ FATAL EXCEPTION: main Process: com.example.adam.slypanel, PID: 7746 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.adam.slypanel/com.example.adam.slypanel.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ProgressBar.setProgress(int)' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) at android.app.ActivityThread.access$800(ActivityThread.java:144) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5221) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:898) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ProgressBar.setProgress(int)' on a null object reference at com.example.adam.slypanel.ServerStatusFragment.refresh(ServerStatusFragment.java:55) at com.example.adam.slypanel.MainActivity$1.run(MainActivity.java:77) at com.example.adam.slypanel.MainActivity.onSectionAttached(MainActivity.java:107) at com.example.adam.slypanel.MainActivity$PlaceholderFragment.onAttach(MainActivity.java:234) at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:853) at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067) at android.app.BackStackRecord.run(BackStackRecord.java:833) at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1452) at android.app.Activity.performStart(Activity.java:5948) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2261)            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)            at android.app.ActivityThread.access$800(ActivityThread.java:144)            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)            at android.os.Handler.dispatchMessage(Handler.java:102)            at android.os.Looper.loop(Looper.java:135)            at android.app.ActivityThread.main(ActivityThread.java:5221)            at java.lang.reflect.Method.invoke(Native Method)            at java.lang.reflect.Method.invoke(Method.java:372)            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:898)            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693) 

Answer 1

似乎您需要在片段膨胀之前调用refresh 。 这意味着尚未调用onCreateView ，并且cpuPercentage仍为null。

如果确实需要从片段外部调用refresh ，请添加检查以查看是否设置了cpuPercentage ：

if (myFragment.cpuPercentage != null) {
    myFragment.refresh();
}

同样，将检查添加到每个其他字段，即ramPercentage和tempValue。

注意：在我看来，这些字段不应该是静态的。