Question

我制作了一个自定义视图，该视图来自xml布局。我添加了一个清除视图的按钮。现在我想在点击时清除画布区域。我在xml布局文件中添加了一个onClick事件。现在我如何在哪里添加清除整个视图/画布的代码？我刚刚添加了一小部分代码。（这不是清除任何东西）。我按顺序添加了我的活动，视图和布局文件。

public class CustomViewActivity extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

    }

    public void clearLine(View v) {

    new CustomView(CustomViewActivity.this, null).clearCanvas();        
  } 

}

public class CustomView extends View {

    private Paint paint = new Paint();
      private Path path = new Path();
      public Boolean clearCanvas = false;

      public CustomView(Context context, AttributeSet attrs, int defStyle) {
            super(context, attrs, defStyle);         
        }

    public CustomView(Context context,AttributeSet attrs ) {
        super(context,attrs);
        paint.setAntiAlias(true);
        paint.setColor(Color.BLUE);
        paint.setTextSize(20);
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeJoin(Paint.Join.ROUND);
        paint.setStrokeWidth(5f);
    }

    protected void onDraw(Canvas canvas) {
    if(clearCanvas)
        {  // Choose the colour you want to clear with.
            canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);
            //canvas.drawColor(0, Mode.CLEAR);
            clearCanvas = false;            
        }

        super.onDraw(canvas);
        canvas.drawText("Hello World", 5, 30, paint);
        canvas.drawPath(path, paint);

    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {

    //int action = event.getAction() & MotionEvent.ACTION_MASK;

       float eventX = event.getX();
        float eventY = event.getY();

        switch (event.getAction()) {
          case MotionEvent.ACTION_DOWN:
            path.moveTo(eventX, eventY);

            return true;
          case MotionEvent.ACTION_MOVE: 
            path.lineTo(eventX, eventY);
            break;
          case MotionEvent.ACTION_UP:
            // nothing to do 
           break;
          default:
            return false;
        }

        // Schedules a repaint.
        invalidate();
        return true;

    }
    public void clearCanvas(){

            clearCanvas = true;
            postInvalidate();
            //canvas.drawColor(0, Mode.CLEAR);

        }

}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >


           <com.example.CustomViewEvent.CustomView
               android:id="@+id/customView"
               android:layout_width="fill_parent"
               android:layout_height="wrap_content" />

           <Button
               android:id="@+id/button1"
               android:layout_width="wrap_content"
               android:layout_height="wrap_content"
               android:layout_alignParentBottom="true"
               android:layout_centerHorizontal="true"
               android:layout_marginBottom="28dp"
               android:onClick="clearLine"
               android:text="CLEAR" />

</RelativeLayout>

Answer 1

您需要做的是在onDraw方法中访问画布。

因此，如果您使用全局变量，请在按钮单击方法中将其设置为true。在OnDraw中，您可以检查其状态并在必要时清除画布。（然后将其设置为false，这样每次都不会这样做。）

请参阅以下代码了解用法。

public Boolean clearCanvas = false;

protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if(clearCanvas)
        {  // Choose the colour you want to clear with.
            canvas.drawColor(Color.TRANSPARENT, PorterDuff.Mode.CLEAR);
            clearCanvas = false;
        }
        canvas.drawPath(path, paint);       
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {

    float eventX = event.getX();
        float eventY = event.getY();

        switch (event.getAction()) {
          case MotionEvent.ACTION_DOWN:
            path.moveTo(eventX, eventY);

            return true;
          case MotionEvent.ACTION_MOVE: 
            path.lineTo(eventX, eventY);
            break;
          case MotionEvent.ACTION_UP:
            // nothing to do 
           break;
          default:
            return false;
        }

        // Schedules a repaint.
        invalidate();
        return true;

    }
// this is the method which will be invoked from main activity class for clearing whatever //is in the view/canvas 
         public void clearCanvas(){

            //canvas.drawColor(0, Mode.CLEAR);

            clearCanvas = true;
            invalidate();
        }

}

修改看看你的新代码我发现了一些问题。

我认为它不会清除你没有清除正确视图的事实。

首先，获取现有视图的实例。然后你可以清除它。而不是错误的非现有实例。

CustomView cv = (CustomView)findViewById(R.id.customView); cv.clearCanvas();

尝试invalidate();其他postInvalidate();一个人应该工作。

postInvalidate()用于在非UI线程上运行时。

清除画布区域

1 个答案: