单击按钮时使画布无效？

Question

当我单击一个按钮时，我试图使我的画布无效。我有一个带有按钮的布局，下面我有一个画布视图。当我点击一个按钮时，圆圈应该隐藏或显示。现在在代码中，我只能使我的画布失效一次。当我第一次按下按钮时，它起作用了。但是当我按第二次时它不起作用。示例：如果我按隐藏，则显示它有效。但是当我再次按下隐藏按钮时它不起作用。当我单击一个按钮时，我希望 CanvasView 每次都无效。不仅是第一次。

public class CanvasWithButtonsActivity extends Activity {
    boolean showCircle = true;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(layout());

        findViewById(R.id.buttonHide).setOnClickListener(
                new OnClickListener() {      
                public void onClick(View v) {
                    //HIDE
                    showCircle = false;
                    //How do i invalidate my canvas from here?
                    CanvasView cv = new CanvasView(getApplicationContext());
                        cv.invalidate();
                }  
        });

        findViewById(R.id.buttonShow).setOnClickListener(
                new OnClickListener() {      
                public void onClick(View v) {
                    //SHOW
                    showCircle = true;
                    //How do i invalidate my canvas from here?
                    CanvasView cv = new CanvasView(getApplicationContext());
                        cv.invalidate();
                }  
        });
    }

    public RelativeLayout layout(){
        RelativeLayout mainLayout = new RelativeLayout(this);
        mainLayout.setLayoutParams(new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.FILL_PARENT,
                LinearLayout.LayoutParams.FILL_PARENT, 1));
        mainLayout.setBackgroundColor(Color.WHITE);

        View buttonLayout = LayoutInflater.from(getBaseContext()).inflate(R.layout.main, null);
        buttonLayout.setLayoutParams(new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.FILL_PARENT,
                LinearLayout.LayoutParams.FILL_PARENT, 1));
        CanvasView cv = new CanvasView(getApplicationContext());
        mainLayout.addView(cv);
        mainLayout.addView(buttonLayout);

        return mainLayout;
    }

    private class CanvasView extends View{

        public CanvasView(Context context) {
            super(context);
            // TODO Auto-generated constructor stub
        }

        protected void onDraw(Canvas myCanvas){
            Paint myPaint = new Paint();
            myPaint.setColor(Color.BLUE);
            if(showCircle == true)
                myCanvas.drawCircle(myCanvas.getWidth()/2, 100, 20, myPaint);
        }
    }
}

如果有人能解决这个问题，它会解决我的许多问题。

Answer 1

您似乎正在按钮处理程序中创建一个新的画布视图实例

CanvasView cv = new CanvasView(getApplicationContext());
cv.invalidate();

您可能想保留对它的引用，不是吗？

编辑： 在您的 layout() 方法中，您创建一个视图实例。这里的 cv 是对它的引用。它对您的方法来说是本地的：

CanvasView cv = new CanvasView(getApplicationContext());
mainLayout.addView(cv);
mainLayout.addView(buttonLayout);

您想让它成为全球性的（即类字段）。基本上是一个 Java 实例变量（比如你的布尔“showCircle”标志）。您可以在侦听器代码中使用它，就像更改布尔“showCircle”标志一样。

我还建议阅读更多关于使用 XML 创建布局和使用 findViewById 方法定位视图的信息。

EDIT2：如果您使用实例变量，您的代码将如下所示。 （我在记事本中输入的可能错误）：

public class CanvasWithButtonsActivity extends Activity {
    boolean showCircle = true;
    private CanvasView mCanvasView;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(layout());

        findViewById(R.id.buttonHide).setOnClickListener(
                new OnClickListener() {      
                public void onClick(View v) {
                    //HIDE
                    showCircle = false;
                    //How do i invalidate my canvas from here?
                    if (mCanvasView != null) {
                        mCanvasView.invalidate();
                    }
                }  
        });

        findViewById(R.id.buttonShow).setOnClickListener(
                new OnClickListener() {      
                public void onClick(View v) {
                    //SHOW
                    showCircle = true;
                    //How do i invalidate my canvas from here?
                    if (mCanvasView != null) {
                        mCanvasView.invalidate();
                    }
                }  
        });
    }

    public RelativeLayout layout(){
        RelativeLayout mainLayout = new RelativeLayout(this);
        mainLayout.setLayoutParams(new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.FILL_PARENT,
                LinearLayout.LayoutParams.FILL_PARENT, 1));
        mainLayout.setBackgroundColor(Color.WHITE);

        View buttonLayout = LayoutInflater.from(getBaseContext()).inflate(R.layout.main, null);
        buttonLayout.setLayoutParams(new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.FILL_PARENT,
                LinearLayout.LayoutParams.FILL_PARENT, 1));
        mCanvasView = new CanvasView(getApplicationContext());
        mainLayout.addView(mCanvasView);
        mainLayout.addView(buttonLayout);

        return mainLayout;
    }

    private class CanvasView extends View{

        public CanvasView(Context context) {
            super(context);
            // TODO Auto-generated constructor stub
        }

        protected void onDraw(Canvas myCanvas){
            Paint myPaint = new Paint();
            myPaint.setColor(Color.BLUE);
            if(showCircle == true)
                myCanvas.drawCircle(myCanvas.getWidth()/2, 100, 20, myPaint);
        }
    }
}