android学习5——画图问题

重写View中的onDraw函数可以实现画图.代码如下:

 @Override
    public void onDraw(Canvas canvas) {
        Paint paint = new Paint();
        paint.setColor(Color.WHITE);
        canvas.drawText("Hello world", pX, pY, paint);
    }

要更新画的形状需要调用invalidate()函数.下面是点哪里在哪里画一行文字的代码.

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.view.MotionEvent;
import android.view.View;
public class MoveTextView extends View {
    private int pX;
    private int pY;
    public MoveTextView(Context context) {
        super(context);
    }
    @Override
    public void onDraw(Canvas canvas) {
        Paint paint = new Paint();
        paint.setColor(Color.WHITE);
        canvas.drawText("Hello world", pX, pY, paint);
    }
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        int x = (int) event.getX();
        int y = (int) event.getY();
        if (event.getAction() == MotionEvent.ACTION_DOWN) {
            pX = x;
            pY = y;
        }
        invalidate();
        return true;
    }
}