Android实现一个简单的画板
思路：自定义CustomView，重写onTouch()，和onDraw()。在onTouch中识别触屏事件，并记录触屏发生的位置。为了在ACTION_DOWN发生后能够接收到后续的触屏事件，我们需要让onTouch()返回true。
在onDraw()中，将上一次触屏的Point和这一次触屏发生的Point，连成一条直线。因为invalidate()后，canvas被清空了，我们需要用mBitmap保存每一次绘画的结果。
http://dl.iteye.com/upload/attachment/0074/7089/c7cf6249-2a5c-3b0d-ae10-39519f212a6e.png
package com.ipjmc.hello;import android.content.Context;import android.graphics.Bitmap;import android.graphics.Canvas;import android.graphics.Paint;import android.util.AttributeSet;import android.view.MotionEvent;import android.view.View;public class CustomView extends View {private int mLastX, mLastY; //上次触屏的位置private int mCurrX, mCurrY; //当前触屏的位置private Bitmap mBitmap;//保存每次绘画的结果private Paint mPaint;public CustomView(Context context, AttributeSet attrs) {super(context, attrs);mPaint = new Paint();mPaint.setStrokeWidth(6);}@Overrideprotected void onDraw(Canvas canvas) {super.onDraw(canvas);int width = getWidth();int height = getHeight();if (mBitmap == null) {mBitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);}//先将结果画到Bitmap上Canvas tmpCanvas = new Canvas(mBitmap);tmpCanvas.drawLine(mLastX, mLastY, mCurrX, mCurrY, mPaint);//再把Bitmap画到canvas上canvas.drawBitmap(mBitmap, 0, 0, mPaint);}@Overridepublic boolean onTouchEvent(MotionEvent event) {mLastX = mCurrX;mLastY = mCurrY;mCurrX = (int) event.getX();mCurrY = (int) event.getY();switch (event.getAction()) {case MotionEvent.ACTION_DOWN:mLastX = mCurrX;mLastY = mCurrY;break;default:break;}invalidate();return true; //必须返回true}}
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