android - 使用 Canvas 通过 OnTouchListener 绘制简单形状和光标坐标时出错

Question

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。每当我运行该程序时，它都会显示“应用程序意外停止。请重试。”的错误消息。

非常感谢任何解决此问题的见解!

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

Answer 1

第一次touch需要把这条语句设置成小写。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;