自定义布局返回null

Question

我按照本指南了解如何在代码段Link

中设置按钮

问题是final MapWrapperLayout mapWrapperLayout = (MapWrapperLayout) findViewById(R.id.map_relative_layout); 在此行代码处返回null值和空指针异常：

        mapWrapperLayout.init(map, getPixelsFromDp(getApplicationContext(), 39 + 20)); 

这是我的主要活动的oncreate方法的代码，它扩展了Activity：

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

    final MapFragment mapFragment = (MapFragment)getFragmentManager().findFragmentById(R.id.map);
    final MapWrapperLayout mapWrapperLayout = (MapWrapperLayout) findViewById(R.id.map_relative_layout);
    final GoogleMap map = mapFragment.getMap();

    // MapWrapperLayout initialization
    // 39 - default marker height
    // 20 - offset between the default InfoWindow bottom edge and it's content bottom edge 
    mapWrapperLayout.init(map, getPixelsFromDp(getApplicationContext(), 39 + 20)); 

    // We want to reuse the info window for all the markers, 
    // so let's create only one class member instance
    this.infoWindow = (ViewGroup)getLayoutInflater().inflate(R.layout.infowindow, null);
    this.infoTitle = (TextView)infoWindow.findViewById(R.id.title);
    this.infoSnippet = (TextView)infoWindow.findViewById(R.id.snippet);
    this.infoButton = (Button)infoWindow.findViewById(R.id.button);

    // Setting custom OnTouchListener which deals with the pressed state
    // so it shows up 
    this.infoButtonListener = new OnInfoWindowElemTouchListener(infoButton,
            getResources().getDrawable(R.drawable.common_signin_btn_icon_dark),
            getResources().getDrawable(R.drawable.common_signin_btn_icon_disabled_dark)) 
    {
        @Override
        protected void onClickConfirmed(View v, Marker marker) {
            // Here we can perform some action triggered after clicking the button
            Toast.makeText(MainActivity.this, marker.getTitle() + "'s button clicked!", Toast.LENGTH_SHORT).show();
        }
    }; 
    this.infoButton.setOnTouchListener(infoButtonListener);


    map.setInfoWindowAdapter(new InfoWindowAdapter() {
        @Override
        public View getInfoWindow(Marker marker) {
            return null;
        }

        @Override
        public View getInfoContents(Marker marker) {
            // Setting up the infoWindow with current's marker info
            infoTitle.setText(marker.getTitle());
            infoSnippet.setText(marker.getSnippet());
            infoButtonListener.setMarker(marker);

            // We must call this to set the current marker and infoWindow references
            // to the MapWrapperLayout
            mapWrapperLayout.setMarkerWithInfoWindow(marker, infoWindow);
            return infoWindow;
        }
    });

    // Let's add a couple of markers
    map.addMarker(new MarkerOptions()
        .title("Prague")
        .snippet("Czech Republic")
        .position(new LatLng(50.08, 14.43)));

    map.addMarker(new MarkerOptions()
        .title("Paris")
        .snippet("France")
        .position(new LatLng(48.86,2.33)));

    map.addMarker(new MarkerOptions()
        .title("London")
        .snippet("United Kingdom")
        .position(new LatLng(51.51,-0.1)));
}

MapWrapperLayout与指南相同：

public class MapWrapperLayout extends RelativeLayout {
/**
 * Reference to a GoogleMap object 
 */
private GoogleMap map;

/**
 * Vertical offset in pixels between the bottom edge of our InfoWindow 
 * and the marker position (by default it's bottom edge too).
 * It's a good idea to use custom markers and also the InfoWindow frame, 
 * because we probably can't rely on the sizes of the default marker and frame. 
 */
private int bottomOffsetPixels;

/**
 * A currently selected marker 
 */
private Marker marker;

/**
 * Our custom view which is returned from either the InfoWindowAdapter.getInfoContents 
 * or InfoWindowAdapter.getInfoWindow
 */
private View infoWindow;    

public MapWrapperLayout(Context context) {
    super(context);
}

public MapWrapperLayout(Context context, AttributeSet attrs) {
    super(context, attrs);
}

public MapWrapperLayout(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
}

/**
 * Must be called before we can route the touch events
 */
public void init(GoogleMap map, int bottomOffsetPixels) {
    this.map = map;
    this.bottomOffsetPixels = bottomOffsetPixels;
}

/**
 * Best to be called from either the InfoWindowAdapter.getInfoContents 
 * or InfoWindowAdapter.getInfoWindow. 
 */
public void setMarkerWithInfoWindow(Marker marker, View infoWindow) {
    this.marker = marker;
    this.infoWindow = infoWindow;
}

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
    boolean ret = false;
    // Make sure that the infoWindow is shown and we have all the needed references
    if (marker != null && marker.isInfoWindowShown() && map != null && infoWindow != null) {
        // Get a marker position on the screen
        Point point = map.getProjection().toScreenLocation(marker.getPosition());

        // Make a copy of the MotionEvent and adjust it's location
        // so it is relative to the infoWindow left top corner
        MotionEvent copyEv = MotionEvent.obtain(ev);
        copyEv.offsetLocation(
            -point.x + (infoWindow.getWidth() / 2), 
            -point.y + infoWindow.getHeight() + bottomOffsetPixels);

        // Dispatch the adjusted MotionEvent to the infoWindow
        ret = infoWindow.dispatchTouchEvent(copyEv);
    }
    // If the infoWindow consumed the touch event, then just return true.
    // Otherwise pass this event to the super class and return it's result
    return ret || super.dispatchTouchEvent(ev);
}

}

这是我的带有项目标签的xml文件

    <?xml version="1.0" encoding="utf-8"?>
<com.example.testmappa.MapWrapperLayout xmlns:android="http://schemas.android.com/apk/res/android"
          xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/map_relative_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity" >



<fragment
        android:id="@+id/map"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        class="com.google.android.gms.maps.MapFragment" />

</com.example.testmappa.MapWrapperLayout>

这是我的家庭活动布局：

<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"
tools:context=".MainActivity" >

<fragment
    android:id="@+id/map"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    class="com.google.android.gms.maps.MapFragment" />

最后我的logcat

    03-13 17:50:56.386: E/AndroidRuntime(21055): FATAL EXCEPTION: main
03-13 17:50:56.386: E/AndroidRuntime(21055): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.testmappa/com.example.testmappa.MainActivity}: java.lang.NullPointerException
03-13 17:50:56.386: E/AndroidRuntime(21055):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2249)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2299)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at android.app.ActivityThread.access$700(ActivityThread.java:154)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1284)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at android.os.Looper.loop(Looper.java:137)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at android.app.ActivityThread.main(ActivityThread.java:5306)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at java.lang.reflect.Method.invokeNative(Native Method)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at java.lang.reflect.Method.invoke(Method.java:511)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at dalvik.system.NativeStart.main(Native Method)
03-13 17:50:56.386: E/AndroidRuntime(21055): Caused by: java.lang.NullPointerException
03-13 17:50:56.386: E/AndroidRuntime(21055):    at com.example.testmappa.MainActivity.onCreate(MainActivity.java:44)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at android.app.Activity.performCreate(Activity.java:5255)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1097)
03-13 17:50:56.386: E/AndroidRuntime(21055):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2213)

任何人都可以解释我为什么会有这个错误？谢谢！

Answer 1

更改此

final MapWrapperLayout mapWrapperLayout = (MapWrapperLayout) findViewById(R.layout.map_relative_layout);

到

final MapWrapperLayout mapWrapperLayout = (MapWrapperLayout) findViewById(R.id.map_relative_layout);

findViewById查找当前虚增布局中包含id的视图。

编辑：

你有

setContentView(R.layout.activity_main);
final MapWrapperLayout mapWrapperLayout = (MapWrapperLayout) findViewById(R.id.map_relative_layout);

虽然activity_main.xml没有自定义布局。

Answer 2

检查它是否为＆＃39; null＆＃39;在你分配之后。如果是这种情况，那么您可能没有适当地设置当前内容视图。

setContentView(R.layout./*nameOfTheXmlFileContainingThisLayout.xml*/);

在致电

之前

final MapWrapperLayout mapWrapperLayout = (MapWrapperLayout) findViewById(R.id.map_relative_layout);

当您尝试访问MapWrapperLayout时，如果没有加载当前的xml内容视图，它将始终返回null，因为它查找的位置错误。