Android屏幕手写签名的原理就是把手机屏幕当作画板，把用户手指当作画笔，手指在屏幕上在屏幕上划来划去，屏幕就会显示手指的移动轨迹，就像画笔在画板上写字一样。实现手写签名需要结合绘图的路径工具Path，在有按下动作时调用Path对象的moveTo方法，将路径起始点移动到触摸点；在有移动操作时调用Path对象的quadTo方法，将记录本次触摸点与上次触摸点之间的路径；在有移动操作与提起动作时调用Canvas对象的drawPath方法，将本次触摸绘制在画布上。

layout/activity_signature.xml界面布局代码如下：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 xmlns:app="http://schemas.android.com/apk/res-auto"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:orientation="vertical"
 android:padding="5dp">

 <ScrollView
 android:layout_width="match_parent"
 android:layout_height="wrap_content">

 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:orientation="vertical">

 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:orientation="horizontal">

 <Button
  android:id="@+id/btn_add_signature"
  android:layout_width="0dp"
  android:layout_height="wrap_content"
  android:layout_weight="1"
  android:text="开始签名"
  android:textColor="@color/black"
  android:textSize="17sp" />

 <Button
  android:id="@+id/btn_reset_signature"
  android:layout_width="0dp"
  android:layout_height="match_parent"
  android:layout_weight="1"
  android:text="重置"
  android:textColor="@color/black"
  android:textSize="17sp" />

 <Button
  android:id="@+id/btn_revoke_signature"
  android:layout_width="0dp"
  android:layout_height="match_parent"
  android:layout_weight="1"
  android:text="回退"
  android:textColor="@color/black"
  android:textSize="17sp" />

 <Button
  android:id="@+id/btn_end_signature"
  android:layout_width="0dp"
  android:layout_height="wrap_content"
  android:layout_weight="1"
  android:text="结束签名"
  android:textColor="@color/black"
  android:textSize="17sp" />
 </LinearLayout>

 <com.fukaimei.touchevent.widget.SignatureView
 android:id="@+id/view_signature"
 android:layout_width="match_parent"
 android:layout_height="200dp"
 android:background="@color/white"
 app:paint_color="#0000aa"
 app:stroke_width="3" />

 <LinearLayout
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:orientation="horizontal">

 <Button
  android:id="@+id/btn_save_signature"
  android:layout_width="0dp"
  android:layout_height="wrap_content"
  android:layout_weight="1"
  android:text="保存图片文件"
  android:textColor="@color/black"
  android:textSize="17sp" />
 </LinearLayout>

 <ImageView
 android:id="@+id/iv_signature_new"
 android:layout_width="match_parent"
 android:layout_height="200dp"
 android:background="@color/white"
 android:scaleType="fitCenter" />
 </LinearLayout>
 </ScrollView>

</LinearLayout>

SignatureActivity.java逻辑代码如下：

package com.fukaimei.touchevent;

import android.graphics.Bitmap;
import android.os.Bundle;
import android.os.Handler;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.ImageView;
import android.widget.Toast;

import com.fukaimei.touchevent.filedialog.dialog.FileSaveFragment;
import com.fukaimei.touchevent.util.BitmapUtil;
import com.fukaimei.touchevent.widget.SignatureView;

public class SignatureActivity extends AppCompatActivity implements
 OnClickListener, FileSaveFragment.FileSaveCallbacks {
 private SignatureView view_signature;
 private ImageView iv_signature_new;
 private Bitmap mBitmap;

 @Override
 protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.activity_signature);
 view_signature = (SignatureView) findViewById(R.id.view_signature);
 iv_signature_new = (ImageView) findViewById(R.id.iv_signature_new);
 findViewById(R.id.btn_add_signature).setOnClickListener(this);
 findViewById(R.id.btn_end_signature).setOnClickListener(this);
 findViewById(R.id.btn_reset_signature).setOnClickListener(this);
 findViewById(R.id.btn_revoke_signature).setOnClickListener(this);
 findViewById(R.id.btn_save_signature).setOnClickListener(this);
 }

 @Override
 public void onClick(View v) {
 if (v.getId() == R.id.btn_save_signature) {
 if (mBitmap == null) {
 Toast.makeText(this, "请先开始然后结束签名", Toast.LENGTH_LONG).show();
 return;
 }
 FileSaveFragment.show(this, "jpg");
 } else if (v.getId() == R.id.btn_add_signature) {
 view_signature.setDrawingCacheEnabled(true);
 } else if (v.getId() == R.id.btn_reset_signature) {
 view_signature.clear();
 } else if (v.getId() == R.id.btn_revoke_signature) {
 view_signature.revoke();
 } else if (v.getId() == R.id.btn_end_signature) {
 if (view_signature.isDrawingCacheEnabled() != true) {
 Toast.makeText(this, "请先开始签名", Toast.LENGTH_LONG).show();
 } else {
 mBitmap = view_signature.getDrawingCache();
 iv_signature_new.setImageBitmap(mBitmap);
 mHandler.postDelayed(mResetCache, 100);
 }
 }
 }

 private Handler mHandler = new Handler();
 private Runnable mResetCache = new Runnable() {
 @Override
 public void run() {
 view_signature.setDrawingCacheEnabled(false);
 view_signature.setDrawingCacheEnabled(true);
 }
 };

 @Override
 public boolean onCanSave(String absolutePath, String fileName) {
 return true;
 }

 @Override
 public void onConfirmSave(String absolutePath, String fileName) {
 String path = String.format("%s/%s", absolutePath, fileName);
 BitmapUtil.saveBitmap(path, mBitmap, "jpg", 80);
 Toast.makeText(this, "成功保存图片文件：" + path, Toast.LENGTH_LONG).show();
 }

}

Demo程序运行效果界面截图如下：

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