Android Studio應用程序崩潰按鈕單擊

Question

最近我修復了一個錯誤，但現在我被困在清理畫布的繪製項目。我已經看過一些其他相同問題的主題，但它沒有解決我的問題。

package org.arkmap.klaas.testdrawing; 

import android.graphics.Color; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.widget.Button; 

public class WriteOnScreenActivity extends AppCompatActivity { 
    Button clear; 
    TouchEventView myView; 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_write_on_screen); 
     clear=(Button)findViewById(R.id.clearbutton); 
     myView = new TouchEventView(this, null); 
     clear.setOnClickListener(new View.OnClickListener() { 
      @Override 
      public void onClick(View vw) { 
       myView.clearCanvas(); 
      } 
     }); 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.menu_write_on_screen, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 

     //noinspection SimplifiableIfStatement 
     if (id == R.id.action_settings) { 
      return true; 
     } 

     return super.onOptionsItemSelected(item); 
    } 
} 

WriteOnScreenActivity.java

package org.arkmap.klaas.testdrawing; 

import android.content.Context; 
import android.graphics.Canvas; 
import android.graphics.Color; 
import android.graphics.Paint; 
import android.graphics.Path; 
import android.graphics.PorterDuff; 
import android.graphics.PorterDuffXfermode; 
import android.util.AttributeSet; 
import android.view.MotionEvent; 
import android.view.View; 
import java.util.Random; 
/** 
* Created by Startklaar on 19-1-2016. 
*/ 
public class TouchEventView extends View { 
    public Paint paint = new Paint(); 
    private Path path = new Path(); 
    public boolean cc = false; 

    Random rnd = new Random(); 

    public TouchEventView(Context ctx, AttributeSet attrs) { 
     super(ctx, attrs); 
     paint.setAntiAlias(true); 
     paint.setARGB(255, rnd.nextInt(256), rnd.nextInt(256), rnd.nextInt(256)); 
     //paint.setColor(Color.WHITE); 
     paint.setStrokeJoin(Paint.Join.ROUND); 
     paint.setStyle(Paint.Style.STROKE); 
     paint.setStrokeWidth(10f); 
    } 

    @Override 
    protected void onDraw(Canvas canvas) { 
     if (cc) { 
      path = new Path(); 
      Paint clearPaint = new Paint(); 
      clearPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.CLEAR)); 
      canvas.drawRect(0, 0, 0, 0, clearPaint); 
      cc = false; 
     } 
     canvas.drawPath(path, paint); 
    } 

    public void clearCanvas() 
    { 
     cc = true; 
     invalidate(); 
    } 

    @Override 
    public boolean onTouchEvent(MotionEvent event) { 
     float xPos = event.getX(); 
     float yPos = event.getY(); 

     switch(event.getAction()) { 
      case MotionEvent.ACTION_DOWN: 
       path.moveTo(xPos, yPos); 
       return true; 
      case MotionEvent.ACTION_MOVE: 
       path.lineTo(xPos,yPos); 
       break; 
      case MotionEvent.ACTION_UP: 
       break; 
      default: 
       return false; 
     } 

     invalidate(); 
     return true; 
    } 
} 

TouchEventView.java

希望在這裏得到一些幫助！

編輯：錯誤已修復，我唯一的問題是，該按鈕不起作用。

Answer 1

myView未被初始化。你應該設置它：

myView = (TouchEventView) findViewById(R.id.XXXX);