在畫布上繪製文本並更新字符串值

Question

我是新來的android，並開始使用傳感器項目，在項目中，我無法在畫布上繪製文本當我使用 canvas.drawtext（「gary」， 200200，NULL）; 我的應用程序意外停止，實際上我想要在畫布中獲取加速度計的值。 請在我的代碼中提示我最新的錯誤，並指導我通過在畫布上獲取值。

package com.one2; 



import android.app.Activity; 

import android.content.Context; 

import android.graphics.Bitmap; 
import android.graphics.BitmapFactory; 

import android.graphics.Canvas; 
import android.hardware.Sensor; 
import android.hardware.SensorEvent; 
import android.hardware.SensorEventListener; 
import android.hardware.SensorManager; 
import android.os.Bundle; 
import android.view.SurfaceHolder; 
import android.view.SurfaceView; 

public class accelerometer extends Activity implements SensorEventListener { 
float x,y,z,sensorX ; 
float sensorY,sensorZ; 
Bitmap bmp; 
SensorManager sm ; 
MyBringBackSurface OurSurfaceView; 
static final float NS2S = 1.0f/1000000000.0f; 
float[] last_values = null; 
float[] velocity = null; 
float[] position = null; 
long last_timestamp = 0; 

public class MyBringBackSurface extends SurfaceView implements Runnable{ 

    SurfaceHolder ourHolder; 
    Thread ourThread = null; 
    boolean isRunning = false; 

    public MyBringBackSurface(Context context) { 
     super(context); 
     ourHolder = getHolder(); 

    } 
    public void pause(){ 
     isRunning = false; 
     while(true){ 
      try { 
       ourThread.join(); 
      } catch (InterruptedException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 
      break; 
     } 
    } 
    public void resume(){ 
     isRunning = true; 
     ourThread = new Thread(this); 
     ourThread.start(); 
    } 

    @Override 
    public void run() { 
     // TODO Auto-generated method stub 
     while(isRunning){ 
      if(!ourHolder.getSurface().isValid()) 
       continue;   
      Canvas canvas = ourHolder.lockCanvas(); 
      canvas.drawRGB(23, 33, 444); 
      canvas.drawText("gary",200 ,200 , null); 
      float cx = canvas.getWidth()/2; 
      float cy = canvas.getHeight()/2; 
      float fx = cy+sensorY; 
      float fy = cx+sensorX; 
      canvas.drawBitmap(bmp,-fx ,-fy,null); 

      ourHolder.unlockCanvasAndPost(canvas); 
     } 

    } 

} 



protected void onCreate(Bundle savedInstanceState) { 
    // TODO Auto-generated method stub 
    super.onCreate(savedInstanceState); 

    OurSurfaceView = new MyBringBackSurface(this); 
    OurSurfaceView.resume(); 
    setContentView(OurSurfaceView); 

    sm = (SensorManager)getSystemService(SENSOR_SERVICE); 
    if (sm.getSensorList(Sensor.TYPE_ACCELEROMETER).size()!= 0){ 
     Sensor s = sm.getSensorList(Sensor.TYPE_ACCELEROMETER).get(0); 
     sm.registerListener(this, s, SensorManager.SENSOR_DELAY_NORMAL); 
    } 

    bmp = BitmapFactory.decodeResource(getResources(), R.drawable.greenball); 

    x = y =z= sensorX = sensorY = sensorZ = 0; 
} 
@Override 
protected void onPause() { 
    // TODO Auto-generated method stub 
    sm.unregisterListener(this); 
    super.onPause(); 
} 
@Override 
public void onSensorChanged(SensorEvent e) { 
    // TODO Auto-generated method stub 
    try { 
     Thread.sleep(16); 
    } catch (InterruptedException e1) { 
     // TODO Auto-generated catch block 
     e1.printStackTrace(); 
    } 
    sensorX = e.values[0]; 
    sensorY = e.values[1]; 
    sensorZ= e.values[2]; 

    if(last_values != null){ 
      float dt = (e.timestamp - last_timestamp) * NS2S; 

      for(int index = 0; index < 3;++index){ 
       velocity[index] += (e.values[index] + last_values[index])/2 * dt; 
       position[index] += velocity[index] * dt; 
      } 
     } 
     else{ 
      last_values = new float[3]; 
      velocity = new float[3]; 
      position = new float[3]; 
      velocity[0] = velocity[1] = velocity[2] = 0f; 
      position[0] = position[1] = position[2] = 0f; 
     } 
     System.arraycopy(e.values, 0, last_values, 0, 3); 
     last_timestamp = e.timestamp; 
} 

@Override 
public void onAccuracyChanged(Sensor sensor, int accuracy) { 
    // TODO Auto-generated method stub 

} 

} 

09-05 09:21:11.241: E/AndroidRuntime(960): FATAL EXCEPTION: Thread-89 
09-05 09:21:11.241: E/AndroidRuntime(960): java.lang.NullPointerException 
09-05 09:21:11.241: E/AndroidRuntime(960): at android.graphics.Canvas.drawText(Canvas.java:1377) 
09-05 09:21:11.241: E/AndroidRuntime(960): at com.one2.accelerometer$MyBringBackSurface.run(accelerometer.java:65) 
09-05 09:21:11.241: E/AndroidRuntime(960): at java.lang.Thread.run(Thread.java:841) 

Answer 1

Canvas.drawText的第四參數必須是Paint對象。你不能通過null。所以，你應該創建一個新對象，並將它傳遞：

Paint paint = new Paint(); 
paint.setColor(Color.BLACK); 
canvas.drawText("gary",200,200,paint);