我正在开发一个测试程序，该程序利用Canvas onDraw方法绘制基本线条/形状，并使用onTouch方法分配触摸事件的坐标。每当我运行程序时，它都会显示错误消息“应用程序已意外停止。请再试一次。”
非常感谢任何解决这个问题的见解！
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;
}
}
根据要求，这是日志结果。
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
答案 0 :(得分：1)
第一次触摸时，您需要将此语句设置为小写。目前，它将继续在onTouch中为您的构建器设置重置长度
builder.setLength(0);
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
builder.setLength(0);
builder.append("down, ");
break;
答案 1 :(得分：0)
申请开始时。
您的字符串文字尚未设置，并且在您onDraw中有此
canvas.drawText(text, 10, 25, paint);
这会导致NullPointerException
来自
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 }
所以只需在开始时初始化您的文字
String text="initial";