implement Undo functionality

app/build.gradle

@@ -23,4 +23,5 @@ dependencies {
     compile fileTree(dir: 'libs', include: ['*.jar'])
     testCompile 'junit:junit:4.12'
     compile 'com.android.support:appcompat-v7:23.1.1'
+    compile 'com.jakewharton:butterknife:7.0.1'
 }

app/src/main/java/draw/simplemobiletools/com/MainActivity.java

@@ -1,13 +1,24 @@
 package draw.simplemobiletools.com;
 
-import android.support.v7.app.AppCompatActivity;
 import android.os.Bundle;
+import android.support.v7.app.AppCompatActivity;
+
+import butterknife.Bind;
+import butterknife.ButterKnife;
+import butterknife.OnClick;
 
 public class MainActivity extends AppCompatActivity {
+    @Bind(R.id.my_canvas) MyCanvas myCanvas;
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
+        ButterKnife.bind(this);
+    }
+
+    @OnClick(R.id.undo)
+    public void undo() {
+        myCanvas.undo();
     }
 }

app/src/main/java/draw/simplemobiletools/com/MyCanvas.java

@@ -36,12 +36,22 @@ public class MyCanvas extends View {
         paths.add(path);
     }
 
+    public void undo() {
+        if (paths.size() <= 0)
+            return;
+
+        paths.remove(paths.size() - 1);
+        invalidate();
+    }
+
     @Override
     protected void onDraw(Canvas canvas) {
         super.onDraw(canvas);
+
         for (Path p : paths) {
             canvas.drawPath(p, paint);
         }
+        canvas.drawPath(path, paint);
     }
 
     private void actionDown(float x, float y) {
@@ -60,15 +70,15 @@ public class MyCanvas extends View {
     private void actionUp() {
         path.lineTo(curX, curY);
 
-        // drawing dots
+        // drawing dots on click
         if (startX == curX && startY == curY) {
             path.lineTo(curX, curY + 2);
             path.lineTo(curX + 1, curY + 2);
             path.lineTo(curX + 1, curY);
         }
 
-        path = new Path();
         paths.add(path);
+        path = new Path();
     }
 
     @Override

app/src/main/res/layout/activity_main.xml

@@ -7,7 +7,16 @@
     tools:context="draw.simplemobiletools.com.MainActivity">
 
     <draw.simplemobiletools.com.MyCanvas
+        android:id="@+id/my_canvas"
         android:layout_width="match_parent"
         android:layout_height="match_parent"/>
 
+    <ImageView
+        android:id="@+id/undo"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_alignParentRight="true"
+        android:padding="@dimen/activity_margin"
+        android:src="@mipmap/undo"/>
+
 </RelativeLayout>