Android:在 fragment 内绘制 Canvas

Question

我想在 Android 上的 Canvas 中绘制多个元素。但我用的是 fragment 。在阅读了类似的问题后(​​ Draw a line within a Fragment ， How to draw in a fragment ，...)，我在我的 fragment 中尝试了这段代码来简单地绘制一个矩形，但它不起作用:

public class ShowProfileFragment extends Fragment {


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {

    View rootView = inflater.inflate(R.layout.fragment_show_profils, container, false);

    RelativeLayout relativeLayout = (RelativeLayout) rootView.findViewById(R.id.rect);
    relativeLayout.addView(new Rectangle(getActivity()));

    return rootView;
}

private class Rectangle extends View {
    Paint paint = new Paint();

    public Rectangle(Context context) {
        super(context);
    }
    @Override
    public void onDraw(Canvas canvas) {
        paint.setColor(Color.GREEN);
        Rect rect = new Rect(20, 56, 200, 112);
        canvas.drawRect(rect, paint );
    }
}
}

由于这一行，我检索到 NullPointerException: relativeLayout.addView(new Rectangle(getActivity()));

getActivity() 的结果是我的 MainActivity。

完整的错误日志:

01-29 08:11:01.871    9966-9966/uie.app E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: uie.app, PID: 9966
java.lang.NullPointerException
        at info.uie.limux.ShowProfileFragment.onCreateView(ShowProfileFragment.java:25)
        at android.support.v4.app.Fragment.performCreateView(Fragment.java:1786)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:947)
        at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1126)
        at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:739)
        at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1489)
        at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:486)
        at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:1073)
        at android.support.v4.view.ViewPager.populate(ViewPager.java:919)
        at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1441)
        at android.view.View.measure(View.java:16628)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at android.view.View.measure(View.java:16628)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:327)
        at android.view.View.measure(View.java:16628)
        at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5125)
        at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
        at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2298)
        at android.view.View.measure(View.java:16628)
        at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1916)
        at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1113)
        at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1295)
        at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
        at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5622)
        at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
        at android.view.Choreographer.doCallbacks(Choreographer.java:574)
        at android.view.Choreographer.doFrame(Choreographer.java:544)
        at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
        at android.os.Handler.handleCallback(Handler.java:733)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:212)
        at android.app.ActivityThread.main(ActivityThread.java:5135)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:877)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:693)
        at dalvik.system.NativeStart.main(Native Method)

编辑:

这是我的fragment_show_rooms.xml的代码

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="#fa6a6a">


<RelativeLayout
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="51dp"
    android:background="#444"
    android:id="@+id/rect">

    </RelativeLayout>
</RelativeLayout>

Answer 1

(只是为了使其正确):

在fragment_show_profils.xml 文件中添加矩形RelativeLayout，而不是在fragment_show_rooms.xml 中。

:-)