java - 单击按钮会使 Canvas 无效吗？

我试图使我的画布无效，当我单击一个按钮时。我有一个带有按钮的布局，下面有一个画布视图。当我单击按钮时，圆圈应该隐藏或显示。现在，在代码中，我只能使画布无效一次。当我第一次按下按钮时，它起作用。但是当我第二次按下时它不起作用。示例：如果我按隐藏，则表明它有效。但是，当我再次按下“隐藏”按钮时，它不起作用。当我单击按钮时，我希望CanvasView每次都失效。不只是第一次。

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);
        }
    }
}

如果有人对这个问题有解决方案，那将解决我的许多问题。

最佳答案

您似乎在按钮处理程序中创建了一个新的画布视图实例

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

您可能想参考一下，不是吗？

编辑：
在您的layout（）方法中，您将创建一个视图实例。简历这里是对它的引用。它是您的方法的本地方法：

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

您想使其成为全局（即“类”字段）。基本上是Java实例变量（例如您的布尔值“ showCircle”标志）。您可以从侦听器代码中使用它，就像更改布尔值“ showCircle”标志一样。

另外，我建议阅读更多有关使用XML创建布局和使用findViewById方法查找视图的信息。

EDIT2：如果您实例变量您的代码将看起来像这样。 （我在记事本中键入的可能错误）：

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);
        }
    }
}