錯誤使用畫布繪製簡單的形狀和光標座標與OnTouchListener

Question

我正在測試程序，利用畫布onDraw方法繪製基本的線/形狀，並通過onTouch方法分配觸摸事件的座標。每當我運行該程序時，它都會顯示錯誤消息「應用程序意外停止，請重試。」

任何有關解決這個問題的見解非常感謝！

public class CustomDraw extends Activity implements OnTouchListener{ 
    StringBuilder builder = new StringBuilder(); 
    String text; 

    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     requestWindowFeature(Window.FEATURE_NO_TITLE); 
     getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, 
          WindowManager.LayoutParams.FLAG_FULLSCREEN); 
     RenderView rv = new RenderView(this); 
     rv.setOnTouchListener(this); 
     setContentView(rv); 

    } 
    class RenderView extends View {   
     Paint paint; 

    public RenderView(Context context) { 
     super(context); 
     paint = new Paint();    
    } 

    protected void onDraw(Canvas canvas) { 
     canvas.drawRGB(255, 255, 255);    
     paint.setColor(Color.RED); 
     canvas.drawLine(0, 0, canvas.getWidth()-1, canvas.getHeight()-1, paint); 

     paint.setStyle(Style.STROKE); 
     paint.setColor(0xff00ff00);    
     canvas.drawCircle(canvas.getWidth()/2, canvas.getHeight()/2, 40, paint); 

     paint.setStyle(Style.FILL); 
     paint.setColor(0x770000ff); 
     canvas.drawRect(100, 100, 200, 200, paint); 

     paint.setStyle(Style.FILL); 
     paint.setColor(Color.BLACK); 
     canvas.drawText(text, 10, 25, paint); 

     invalidate(); 
    } 
} 

    public boolean onTouch(View v, MotionEvent event) { 

     builder.setLength(0); 
     switch (event.getAction()) { 
     case MotionEvent.ACTION_DOWN: 
      builder.append("down, "); 
      break; 
     case MotionEvent.ACTION_MOVE: 
      builder.append("move, "); 
      break; 
     case MotionEvent.ACTION_CANCEL: 
      builder.append("cancle, "); 
      break; 
     case MotionEvent.ACTION_UP: 
      builder.append("up, "); 
      break; 
     } 

     builder.append(event.getX()); 
     builder.append(", "); 
     builder.append(event.getY()); 
     text = builder.toString(); 
     Log.d("TouchTest", text); 

     return true; 
    } 
} 

根據要求，這裏是日誌結果。

07-06 01:08:10.520: W/KeyCharacterMap(367): No keyboard for id 0 
07-06 01:08:10.520: W/KeyCharacterMap(367): Using default keymap: /system/usr/keychars/qwerty.kcm.bin 
07-06 01:08:12.391: D/AndroidRuntime(367): Shutting down VM 
07-06 01:08:12.391: W/dalvikvm(367): threadid=3: thread exiting with uncaught exception (group=0x4001b188) 
07-06 01:08:12.401: E/AndroidRuntime(367): Uncaught handler: thread main exiting due to uncaught exception 
07-06 01:08:12.411: E/AndroidRuntime(367): java.lang.NullPointerException 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.graphics.Canvas.drawText(Native Method) 
07-06 01:08:12.411: E/AndroidRuntime(367): at com.badlogic.androidgames.CustomDraw$RenderView.onDraw(CustomDraw.java:72) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.view.View.draw(View.java:6535) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.view.ViewGroup.drawChild(ViewGroup.java:1531) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1258) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.view.View.draw(View.java:6538) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.widget.FrameLayout.draw(FrameLayout.java:352) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.view.ViewGroup.drawChild(ViewGroup.java:1531) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.view.ViewGroup.dispatchDraw(ViewGroup.java:1258) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.view.View.draw(View.java:6538) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.widget.FrameLayout.draw(FrameLayout.java:352) 
07-06 01:08:12.411: E/AndroidRuntime(367): at com.android.internal.policy.impl.PhoneWindow$DecorView.draw(PhoneWindow.java:1830) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.view.ViewRoot.draw(ViewRoot.java:1349) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.view.ViewRoot.performTraversals(ViewRoot.java:1114) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.view.ViewRoot.handleMessage(ViewRoot.java:1633) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.os.Handler.dispatchMessage(Handler.java:99) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.os.Looper.loop(Looper.java:123) 
07-06 01:08:12.411: E/AndroidRuntime(367): at android.app.ActivityThread.main(ActivityThread.java:4363) 
07-06 01:08:12.411: E/AndroidRuntime(367): at java.lang.reflect.Method.invokeNative(Native Method) 
07-06 01:08:12.411: E/AndroidRuntime(367): at java.lang.reflect.Method.invoke(Method.java:521) 
07-06 01:08:12.411: E/AndroidRuntime(367): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860) 
07-06 01:08:12.411: E/AndroidRuntime(367): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618) 
07-06 01:08:12.411: E/AndroidRuntime(367): at dalvik.system.NativeStart.main(Native Method) 
07-06 01:08:12.421: I/dalvikvm(367): threadid=7: reacting to signal 3 
07-06 01:08:12.421: I/dalvikvm(367): Wrote stack trace to '/data/anr/traces.txt' 
07-06 01:08:17.540: I/Process(367): Sending signal. PID: 367 SIG: 9 

Answer 1

當應用程序啓動。

您的字符串文本尚未設定，並在您onDraw你有這樣的

canvas.drawText(text, 10, 25, paint); 

這將導致NullPointerException

從

http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/2.2_r1.1/android/graphics/Canvas.java#Canvas.0mNativeCanvas

1275 public void drawText(String text, int start, int end, float x, float y, 
1276       Paint paint) { 
1277  if ((start | end | (end - start) | (text.length() - end)) < 0) { 
1278   throw new IndexOutOfBoundsException(); 
1279  } 
1280  native_drawText(mNativeCanvas, text, start, end, x, y, 
1281      paint.mNativePaint); 
1282 } 

所以剛纔初始化文本在s撻

String text="initial"; 

Answer 2

當您第一次觸摸時，您需要將此語句設置爲小寫。目前它會繼續設置您的建設者在onTouch重置長度

builder.setLength(0); 

    switch (event.getAction()) { 
    case MotionEvent.ACTION_DOWN: 
     builder.setLength(0); 
     builder.append("down, "); 
     break;