java - Android View 不响应绘图

Question

我目前正在 android 中试验我的第一个自定义 View ，并尝试通过 onTouchEvent 在 Canvas 上绘制点，但多次尝试后都失败了。 View 确实检测到我的触摸并在触摸时成功打印出 System.out.println 消息，但它仍然没有在 Canvas 上绘制。

经过多次尝试，我得出的结论是:

package com.techdigy.testapp;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

public class DrawingBoard extends View {
    Canvas canvas;
    Bitmap bmp;
    BitmapDrawable temp;

    public DrawingBoard(Context context, AttributeSet attributeSet) {
        super(context,attributeSet);
        // TODO Auto-generated constructor stub     
    }
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        bmp = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        canvas = new Canvas();
        }

    protected void onDraw(Canvas canvas) {
        //draw view
        }
    public boolean onTouchEvent(MotionEvent event) {
        //detect user touch
        float touchX = event.getX();
        float touchY = event.getY();
        Paint paint = new Paint();
        System.out.println("test");
        this.canvas.drawPoint(touchX, touchY, paint);
        temp = new BitmapDrawable(this.bmp);
        this.setBackground(this.temp);
        invalidate();
        return true;
        }
}

Answer 1

试试这个...

public class DrawingBoard extends View {
    private Bitmap bmp;
    private float touchX;
    private float touchY;
    private Paint paint;
    public DrawingBoard(Context context, AttributeSet attributeSet) {
        super(context, attributeSet);
        paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        paint.setColor(Color.GREEN);
        paint.setStyle(Paint.Style.STROKE);
    }

    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        bmp = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        setBackgroundDrawable(new BitmapDrawable(getResources(), bmp));
    }

    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawPoint(touchX, touchY, paint);
    }

    public boolean onTouchEvent(MotionEvent event) {
        // detect user touch
        touchX = event.getX();
        touchY = event.getY();
        System.out.println("test");
        invalidate();
        return true;
    }
}