如何缩放android xy绘图？

我是Android的初学者，我尝试在我的应用程序中缩放图表，但由于错误，我在下面编写的代码无法工作：

java.lang.NullPointerException: Attempt to invoke virtual method (android.view.View$OnTouchListener)' on a null object reference.

我不知道如何在代码中更正它

提前感谢你的帮助

    public class EcgChartActivity extends Activity implements OnTouchListener
    {


 private PointF minXY;
 private PointF maxXY;

//Class updating the data on the chart
private class EcgUpdater implements Observer {
    private final Plot plot;

    public EcgUpdater(Plot plot) {
        this.plot = plot;
    }

    @Override
    public void update(Observable arg0, Object arg1) {
        plot.redraw();
    }
}

private XYPlot ecgPlot;
private EcgUpdater ecgUpdater;
private Thread ecgPlotThread;
private LinkedHashMap<Integer, ChannelData> allChannelsData;
private EcgDataSource ecgDataSource;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_ecg_chart);


    this.ecgPlot = (XYPlot) findViewById(R.id.ecg_plot);

    this.ecgUpdater = new EcgUpdater(this.ecgPlot);

    this.ecgDataSource = BaseApp.getApp().getEcgDataSource();

    this.ecgPlotThread = new Thread(this.ecgDataSource);

    ecgPlot.setOnTouchListener(this);



     ecgPlot.getGraphWidget().setTicksPerRangeLabel(1);
     ecgPlot.getGraphWidget().setTicksPerDomainLabel(1);
     ecgPlot.getGraphWidget().setRangeValueFormat(
            new DecimalFormat("#######"));
    ecgPlot.getGraphWidget().setDomainValueFormat(
            new DecimalFormat("#######"));
    ecgPlot.getGraphWidget().setRangeLabelWidth(25);
    ecgPlot.setRangeLabel("");
    ecgPlot.setDomainLabel("");

    //Adding series to chart

    EcgPlotSeries channel1 = new EcgPlotSeries(this.ecgDataSource, 0, "Channel 1", Color.RED);
    this.ecgPlot.addSeries(channel1, channel1.getFormatter());

    EcgPlotSeries channel2 = new EcgPlotSeries(this.ecgDataSource, 1, "Channel 2", Color.BLUE);
    this.ecgPlot.addSeries(channel2, channel2.getFormatter());

    ecgPlot.redraw();
    ecgPlot.calculateMinMaxVals();
    minXY=new PointF(ecgPlot.getCalculatedMinX().floatValue(),ecgPlot.getCalculatedMinY().floatValue());
    maxXY=new PointF(ecgPlot.getCalculatedMaxX().floatValue(),ecgPlot.getCalculatedMaxY().floatValue());


    BaseApp.getApp().getEcgDataSource().addObserver(this.ecgUpdater);      

}


// Definition of the touch states
static final int NONE = 0;
static final int ONE_FINGER_DRAG = 1;
static final int TWO_FINGERS_DRAG = 2;
int mode = NONE;

PointF firstFinger;
float lastScrolling;
float distBetweenFingers;
float lastZooming;


@Override
public void onResume() {
    // Uruchomienie wątku rysującego wykres
    this.ecgPlotThread.start();
    super.onResume();
}


@Override
public void onPause() {
    BaseApp.getApp().getEcgDataSource().stopThread();
    super.onPause();
}



@Override
   public boolean onTouch(View v, MotionEvent event) {
        // TODO Auto-generated method stub

       switch (event.getAction() & MotionEvent.ACTION_MASK) {
       case MotionEvent.ACTION_DOWN:
           firstFinger = new PointF(event.getX(), event.getY());
           mode = ONE_FINGER_DRAG;
           break;
       case MotionEvent.ACTION_UP: 
           break;
       case MotionEvent.ACTION_POINTER_UP:
           Timer t = new Timer();
               t.schedule(new TimerTask() {

                @Override
                   public void run() {
                       while(Math.abs(lastScrolling)>1f || Math.abs(lastZooming-1)<1.01){ 
                       lastScrolling*=.8;
                       scroll(lastScrolling);
                       lastZooming+=(1-lastZooming)*.2;
                       zoom(lastZooming);
                       ecgPlot.setDomainBoundaries(minXY.x, maxXY.x, BoundaryMode.AUTO);
                       ecgPlot.redraw();
                   }
                   }


             }, 0);

             break;  

       case MotionEvent.ACTION_POINTER_DOWN: 
           distBetweenFingers = spacing(event);
           if (distBetweenFingers > 5f) {
               mode = TWO_FINGERS_DRAG;
           }
           break;
       case MotionEvent.ACTION_MOVE:
           if (mode == ONE_FINGER_DRAG) {
               PointF oldFirstFinger=firstFinger;
               firstFinger=new PointF(event.getX(), event.getY());
               lastScrolling=oldFirstFinger.x-firstFinger.x;
               scroll(lastScrolling);
               lastZooming=(firstFinger.y-oldFirstFinger.y)/ecgPlot.getHeight();
               if (lastZooming<0)
                   lastZooming=1/(1-lastZooming);
               else
                   lastZooming+=1;
               zoom(lastZooming);
               ecgPlot.setDomainBoundaries(minXY.x, maxXY.x, BoundaryMode.AUTO);
               ecgPlot.redraw();

           } else if (mode == TWO_FINGERS_DRAG) {
               float oldDist =distBetweenFingers; 
               distBetweenFingers=spacing(event);
               lastZooming=oldDist/distBetweenFingers;
               zoom(lastZooming);
               ecgPlot.setDomainBoundaries(minXY.x, maxXY.x, BoundaryMode.AUTO);
               ecgPlot.redraw();
           }
           break;
       }
       return true;

    }

    private void zoom(float scale) {
        float domainSpan = maxXY.x - minXY.x;
        float domainMidPoint = maxXY.x - domainSpan / 2.0f;
        float offset = domainSpan * scale / 2.0f;
        minXY.x=domainMidPoint- offset;
        maxXY.x=domainMidPoint+offset;
    }

    private void scroll(float pan) {
        float domainSpan = maxXY.x - minXY.x;
        float step = domainSpan / ecgPlot.getWidth();
        float offset = pan * step;
        minXY.x+= offset;
        maxXY.x+= offset;
    }

    @SuppressLint("FloatMath")
    private float spacing(MotionEvent event) {
        float x = event.getX(0) - event.getX(1);
        float y = event.getY(0) - event.getY(1);
        return FloatMath.sqrt(x * x + y * y);
    }

---

碰撞发生在这条线路上：

ecgPlot.setOnTouchListener(this);

这是因为“ecgPlot”变量为空

它被设置在线路上

this.ecgPlot = (XYPlot) findViewById(R.id.ecg_plot);

检查您的“R.layout.activity\u ecg\u图表”文件。

---

看起来它没有id为“R.id.ecg\u plot”的视图

谢谢您的回答。我检查了布局，实际上有一个你写的错误。我更正并重新开始，但出现了新的错误。我不知道，我怎样才能修复它们。感谢您的帮助。这是我的新错误：

java.lang.IllegalArgumentException:pointerIndex超出范围，位于android.view.MotionEvent.nativeGetAxisValue（本机方法）android.view.MotionEvent.getX（MotionEvent.java:1983）

您在处理多点触摸时遇到问题。我没有足够的经验来帮助你。另外，这个错误应该在另一个问题中提出，但我建议首先阅读更多关于如何使用多点触控的示例。

this.ecgPlot = (XYPlot) findViewById(R.id.ecg_plot);