在使用android类的手写应用时，整体上都有这样一个印象：android的手写不流畅、不自然，和苹果应用比起来相差太远。本文结合作者亲身经历，介绍一下有效提高手写流畅度的几种方法：

1、未做任何处理的手写效果：

这是一个自定义的view，通过在onTouchEvent时间中捕获系统回调的触摸点信息，然后再onDraw方法里面刷新，可以明显地感觉到线条很生硬，并且在手写的过程中跟随感很差，反应迟钝，具体代码如下：

<喎"http://www.2cto.com/kf/ware/vc/" target="_blank" class="keylink">vcD4KPHByZSBjbGFzcz0="brush:java;">package com.mingy.paint.view;import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.graphics.Path;import android.util.AttributeSet;import android.view.MotionEvent;import android.view.View;public class PaintOrignalView extends View { public PaintOrignalView(Context context, AttributeSet attrs, int defStyle) { super(context, attrs, defStyle); initPaintView(); } public PaintOrignalView(Context context, AttributeSet attrs) { super(context, attrs); initPaintView(); } public PaintOrignalView(Context context) { super(context); initPaintView(); } public void clear( ){ if( null != mPath ){ mPath.reset( ); invalidate( ); } } private void initPaintView() { mPaint.setAntiAlias(true); mPaint.setColor(Color.BLACK); mPaint.setStyle(Paint.Style.STROKE); mPaint.setStrokeJoin(Paint.Join.ROUND); mPaint.setStrokeWidth(5f); } @Override protected void onDraw(Canvas canvas) { canvas.drawPath(mPath, mPaint); } @Override public boolean onTouchEvent(MotionEvent event) { float eventX = event.getX(); float eventY = event.getY(); switch (event.getAction()) { case MotionEvent.ACTION_DOWN: { mPath.moveTo(eventX, eventY); invalidate(); } return true; case MotionEvent.ACTION_MOVE: { mPath.lineTo(eventX, eventY); invalidate(); } break; case MotionEvent.ACTION_UP: { mPath.lineTo(eventX, eventY); invalidate(); } break; default: { } return false; } return true; } private Paint mPaint = new Paint(); private Path mPath = new Path();}
通过分析，发现效率低下的原因是：

（1）底层回调给onTouchEvent方法中的点太少（单位时间内点信息少导致跟随感差，快速手写时点之间距离过长）；

（2）捕获点信息后通知View刷新时，刷新不及时（刷新区域太大）；

结合查阅的MotionEvent和View的api文档，发现可以从如下两个方向着手来提高手写体验：

2、增加触摸点个数：

显然我们无法改善系统回调onTouchEvent的次数，所以只能通过插值的方式来增加触摸点个数，但遗憾的时通过插值计算出来的点是没有压力值的，不方便做笔锋效果，通过查阅MotionEvent的api文档发现，Android对触屏事件进行批量处理。传递给onTouchEvent()的每一个MotionEvent都包含上至前一个onTouchEvent()调用之间捕获的若干个坐标点。如果将这些点都加入到绘制中，可使手写效果更加平滑。Android Developers对MotionEvent的介绍如下：

将这些点取出来，跟随感有明显改善，并且随着单位时间内点数的增多，快速手写时点之间距离减小，看上去更为平滑：

修改后的代码如下：

package com.mingy.paint.view;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
public class PaintMorePointsView extends View {
public PaintMorePointsView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
initPaintView();
}
public PaintMorePointsView(Context context, AttributeSet attrs) {
super(context, attrs);
initPaintView();
}
public PaintMorePointsView(Context context) {
super(context);
initPaintView();
}
public void clear( ){
if( null != mPath ){
mPath.reset( );
invalidate( );
}
}
private void initPaintView() {
mPaint.setAntiAlias(true);
mPaint.setColor(Color.BLACK);
mPaint.setStyle(Paint.Style.STROKE);
mPaint.setStrokeJoin(Paint.Join.ROUND);
mPaint.setStrokeWidth(5f);
}
@Override protected void onDraw(Canvas canvas) {
canvas.drawPath(mPath, mPaint);
}
@Override public boolean onTouchEvent(MotionEvent event) {
float eventX = event.getX();
float eventY = event.getY();
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN: {
mPath.moveTo(eventX, eventY);
invalidate();
}
return true;
case MotionEvent.ACTION_MOVE: {
int historySize = event.getHistorySize();
for (int i = 0;
i < historySize;
i++) {
float historicalX = event.getHistoricalX(i);
float historicalY = event.getHistoricalY(i);
mPath.lineTo(historicalX, historicalY);
}
mPath.lineTo(eventX, eventY);
invalidate();
}
break;
case MotionEvent.ACTION_UP: {
int historySize = event.getHistorySize();
for (int i = 0;
i < historySize;
i++) {
float historicalX = event.getHistoricalX(i);
float historicalY = event.getHistoricalY(i);
mPath.lineTo(historicalX, historicalY);
}
mPath.lineTo(eventX, eventY);
invalidate();
}
break;
default: {
}
return false;
}
return true;
}
private Paint mPaint = new Paint();
private Path mPath = new Path();
}

3、减少每次刷新的区域：

通过2改善了手写流畅度和平滑度，但是还可以做进一步改善，通过减小每次刷新的区域（使用invalidate(Rect rect)方法），可以提高刷新的效率，上面的代码都是对整个view进行刷新，当view过大（比如填充整个屏幕）时，手写过程中还是能够感觉到迟钝的现象，改善后的效果如下：

代码如下：

package com.mingy.paint.view;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.RectF;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;
public class PaintInvalidateRectView extends View {
public PaintInvalidateRectView(Context context, AttributeSet attrs, int defStyle) {
super(context, attrs, defStyle);
initPaintView();
}
public PaintInvalidateRectView(Context context, AttributeSet attrs) {
super(context, attrs);
initPaintView();
}
public PaintInvalidateRectView(Context context) {
super(context);
initPaintView();
}
public void clear() {
if (null != mPath) {
mPath.reset();
invalidate();
}
}
private void initPaintView() {
mPaint.setAntiAlias(true);
mPaint.setColor(Color.BLACK);
mPaint.setStyle(Paint.Style.STROKE);
mPaint.setStrokeJoin(Paint.Join.ROUND);
mPaint.setStrokeWidth(5f);
}
@Override protected void onDraw(Canvas canvas) {
canvas.drawPath(mPath, mPaint);
}
@Override public boolean onTouchEvent(MotionEvent event) {
float eventX = event.getX();
float eventY = event.getY();
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN: {
mPath.moveTo(eventX, eventY);
mLastTouchX = eventX;
mLastTouchY = eventY;
}
return true;
case MotionEvent.ACTION_MOVE: case MotionEvent.ACTION_UP: {
resetDirtyRect(eventX, eventY);
int historySize = event.getHistorySize();
for (int i = 0;
i < historySize;
i++) {
float historicalX = event.getHistoricalX(i);
float historicalY = event.getHistoricalY(i);
getDirtyRect(historicalX, historicalY);
mPath.lineTo(historicalX, historicalY);
}
mPath.lineTo(eventX, eventY);
invalidate((int) (mDirtyRect.left - HALF_STROKE_WIDTH), (int) (mDirtyRect.top - HALF_STROKE_WIDTH), (int) (mDirtyRect.right + HALF_STROKE_WIDTH), (int) (mDirtyRect.bottom + HALF_STROKE_WIDTH));
mLastTouchX = eventX;
mLastTouchY = eventY;
}
break;
default: return false;
}
return true;
}
private void getDirtyRect(float historicalX, float historicalY) {
if (historicalX < mDirtyRect.left) {
mDirtyRect.left = historicalX;
}
else if (historicalX > mDirtyRect.right) {
mDirtyRect.right = historicalX;
}
if (historicalY < mDirtyRect.top) {
mDirtyRect.top = historicalY;
}
else if (historicalY > mDirtyRect.bottom) {
mDirtyRect.bottom = historicalY;
}
}
private void resetDirtyRect(float eventX, float eventY) {
mDirtyRect.left = Math.min(mLastTouchX, eventX);
mDirtyRect.right = Math.max(mLastTouchX, eventX);
mDirtyRect.top = Math.min(mLastTouchY, eventY);
mDirtyRect.bottom = Math.max(mLastTouchY, eventY);
}
private static final float STROKE_WIDTH = 5f;
private static final float HALF_STROKE_WIDTH = STROKE_WIDTH / 2;
private float mLastTouchX = 0;
private float mLastTouchY = 0;
private final RectF mDirtyRect = new RectF();
private Paint mPaint = new Paint();
private Path mPath = new Path();
}

后记：
由于Android的消息传递机制问题，驱动层传递给上层的点由于延时会丢失一部分，导致上层应用获取的点相对于系统给的点大大减少，虽然人眼在1秒钟内只要看到超过24帧就不能看出卡顿的现象，但由于刷新机制和由于其它原因（比如：UI线程阻塞）导致看上去不连续。本文通过增加触摸点、减少刷新区域后，手写效率和效果得到明显改善，当然通过软件的进一步处理，手写效果还能得到进一步改善，这需要通过软件做插值处理（压力值也可以考虑通过插值算法算出来），具体在后面介绍。