显示/隐藏 ImageView 的可见性时，Android 应用程序崩溃 [英] Android app crashes when showing/hiding visiblity of ImageView

问题描述

我一直在尝试创建我的第一个 android 应用，是的...遇到了应用崩溃...

I've been trying to create my first android app, and yes... got stuck up with an app crash...

我的片段(主)中有两个 ImageView，在相对布局中将 imageview 1 与 imageview2 重叠.我想要做的是，如果有 Internet 连接，imageview1(不是 imageview 2)应该会显示出来.如果没有互联网连接，反之亦然. 根据教程，我创建了一个单独的类来检测连接:

I had two ImageViews in my fragment (main), overlapping imageview 1 with imageview2 in a relative layout. What I wanted to do was, imageview1 (not imageview 2) should be showing up if there's an Internet connection. and vice versa if no internet connection. As per a tutorial, i created a separate class for detecting connection:

   package com.mypackage.myapp;

import android.content.Context;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;

public class ConnectionDetector {

    private Context _context;

    public ConnectionDetector(Context context){
        this._context = context;
    }

    public boolean isConnectingToInternet(){
        ConnectivityManager connectivity = (ConnectivityManager) _context.getSystemService(Context.CONNECTIVITY_SERVICE);
          if (connectivity != null)
          {
              NetworkInfo[] info = connectivity.getAllNetworkInfo();
              if (info != null)
                  for (int i = 0; i < info.length; i++)
                      if (info[i].getState() == NetworkInfo.State.CONNECTED)
                      {
                          return true;
                      }

          }
          return false;
    }
}

并在我的 mainactivity.class 中调用了该类，使用布尔变量为两个连接实例(连接/断开连接)创建了一个 if 语句...最后，我在 if 语句中的每个 ImageView 中使用了 setVisibility()创建().这是代码:

And called up the class in my mainactivity.class, made an if statement for two instances of connection (connected/disconnected) with a boolean variable... finally, i used setVisibility() for each ImageViews in the if statement in onCreate(). here's the code:

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    } 
    Intent cd = getIntent();
    ConnectionDetector cdr = new ConnectionDetector(getApplicationContext());
    boolean isInternetPresent = cdr.isConnectingToInternet();
    ImageView glss2 = (ImageView)findViewById(R.id.glass2);
    ImageView glss1 = (ImageView) findViewById(R.id.glass1);
    if (isInternetPresent) {
        glss1.setVisibility(View.VISIBLE);
        glss2.setVisibility(View.GONE);
    } else {
       glss1.setVisibility(View.GONE);
       glss2.setVisibility(View.VISIBLE);
    }
}

另外，如果你想要我的布局文件:

Additionally, if you want my layout file:

<RelativeLayout 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:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:background="@drawable/backrepeatlinen"
tools:context="com.mypackage.myapp.MainActivity$PlaceholderFragment" >

<ImageView
    android:id="@+id/glass1"
    android:layout_width="160dp"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:src="@drawable/glass" android:contentDescription="@string/hello_world"/>

<ImageView
    android:id="@+id/glass2"
    android:layout_width="150dp"
    android:layout_height="wrap_content"
    android:layout_alignBottom="@+id/glass1"
    android:layout_centerHorizontal="true"
    android:contentDescription="@string/hello_world"
    android:src="@drawable/broken" />

每次，当我在平板电脑上运行程序时，它都会在活动中崩溃.我尝试删除连接检测器和 if 语句，活动运行良好.

Everytime, when I run the program in my tablet, it crashes at the activity. i tried removing the connection detector and if statement thing, the activity moves well.

唯一的问题在于 if 语句中的 setVisiblity()，因为当删除它时，其他一切都正常工作.有没有可能解决?或者我们可以用不同的方法控制两个图像视图的可见性吗?

the only problem is on the setVisiblity() in the if statement, because when removing it, everything else works properly. Is it possible to solve? or can we controll the visiblity of two imageviews with a different method?.

日志猫:

05-21 11:05:38.432: E/filePathInTheme(23435): fallback to res
05-21 11:05:38.462: E/filePathInTheme(23435): fallback to res
05-21 11:05:38.482: E/filePathInTheme(23435): fallback to res
05-21 11:05:38.652: E/filePathInTheme(23435): fallback to res
05-21 11:05:38.802: I/Adreno200-EGLSUB(23435): <ConfigWindowMatch:2081>: Format RGBA_8888.
05-21 11:05:38.812: D/memalloc(23435): /dev/pmem: Mapped buffer base:0x50c32000 size:11960320 offset:10485760 fd:54
05-21 11:05:39.122: D/memalloc(23435): /dev/pmem: Mapped buffer base:0x51a3b000 size:13434880 offset:11960320 fd:57
05-21 11:05:41.542: E/filePathInTheme(23435): fallback to res
05-21 11:05:41.572: W/dalvikvm(23435): threadid=1: thread exiting with uncaught exception (group=0x40a659f0)
05-21 11:05:41.582: E/AndroidRuntime(23435): FATAL EXCEPTION: main
05-21 11:05:41.582: E/AndroidRuntime(23435): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.lemonaade.watchr/com.lemonaade.watchr.MainActivity}: java.lang.NullPointerException
05-21 11:05:41.582: E/AndroidRuntime(23435):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1961)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1986)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at android.app.ActivityThread.access$600(ActivityThread.java:123)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1152)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at android.os.Handler.dispatchMessage(Handler.java:99)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at android.os.Looper.loop(Looper.java:137)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at android.app.ActivityThread.main(ActivityThread.java:4450)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at java.lang.reflect.Method.invokeNative(Native Method)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at java.lang.reflect.Method.invoke(Method.java:511)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:787)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:554)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at dalvik.system.NativeStart.main(Native Method)
05-21 11:05:41.582: E/AndroidRuntime(23435): Caused by: java.lang.NullPointerException
05-21 11:05:41.582: E/AndroidRuntime(23435):    at com.lemonaade.watchr.MainActivity.onCreate(MainActivity.java:35)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at android.app.Activity.performCreate(Activity.java:4465)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
05-21 11:05:41.582: E/AndroidRuntime(23435):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1925)
05-21 11:05:41.582: E/AndroidRuntime(23435):    ... 11 more

谢谢.

推荐答案

请将此代码移到setContentView()下方.

ImageView glss1 = (ImageView) findViewById(R.id.glass1);
ImageView glss2 = (ImageView)findViewById(R.id.glass2);

在设置视图的内容之前，您正在查找视图.这就是它不起作用的原因.

You are finding view before you set the content of the view. thats why it is not working.

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