show the Undo button only if there is something drawn

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

@@ -32,7 +32,7 @@ import butterknife.ButterKnife;
 import butterknife.OnClick;
 import yuku.ambilwarna.AmbilWarnaDialog;
 
-public class MainActivity extends AppCompatActivity {
+public class MainActivity extends AppCompatActivity implements MyCanvas.PathsChangedListener {
     private static final String TAG = MainActivity.class.getSimpleName();
     private static final String FOLDER_NAME = "images";
     private static final String FILE_NAME = "simple-draw.png";
@@ -40,6 +40,7 @@ public class MainActivity extends AppCompatActivity {
     private static final int STORAGE_PERMISSION = 1;
 
     @BindView(R.id.my_canvas) MyCanvas myCanvas;
+    @BindView(R.id.undo) View undoBtn;
     @BindView(R.id.color_picker) View colorPicker;
 
     private int color;
@@ -50,6 +51,7 @@ public class MainActivity extends AppCompatActivity {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
         ButterKnife.bind(this);
+        myCanvas.setListener(this);
 
         setColor(Color.BLACK);
     }
@@ -235,4 +237,9 @@ public class MainActivity extends AppCompatActivity {
         colorPicker.setBackgroundColor(color);
         myCanvas.setColor(color);
     }
+
+    @Override
+    public void pathsChanged(int cnt) {
+        undoBtn.setVisibility(cnt > 0 ? View.VISIBLE : View.GONE);
+    }
 }

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

@@ -22,6 +22,7 @@ public class MyCanvas extends View {
     private float curY;
     private float startX;
     private float startY;
+    private PathsChangedListener listener;
 
     public MyCanvas(Context context, AttributeSet attrs) {
         super(context, attrs);
@@ -37,6 +38,11 @@ public class MyCanvas extends View {
 
         paths = new LinkedHashMap<>();
         paths.put(path, paint.getColor());
+        pathsUpdated();
+    }
+
+    public void setListener(PathsChangedListener listener) {
+        this.listener = listener;
     }
 
     public void undo() {
@@ -49,6 +55,7 @@ public class MyCanvas extends View {
         }
 
         paths.remove(lastKey);
+        pathsUpdated();
         invalidate();
     }
 
@@ -93,7 +100,7 @@ public class MyCanvas extends View {
     private void actionUp() {
         path.lineTo(curX, curY);
 
-        // drawing dots on click
+        // draw a dot on click
         if (startX == curX && startY == curY) {
             path.lineTo(curX, curY + 2);
             path.lineTo(curX + 1, curY + 2);
@@ -101,9 +108,16 @@ public class MyCanvas extends View {
         }
 
         paths.put(path, paint.getColor());
+        pathsUpdated();
         path = new Path();
     }
 
+    private void pathsUpdated() {
+        if (listener != null && paths != null) {
+            listener.pathsChanged(paths.size());
+        }
+    }
+
     @Override
     public boolean onTouchEvent(MotionEvent event) {
         final float x = event.getX();
@@ -128,4 +142,8 @@ public class MyCanvas extends View {
         invalidate();
         return true;
     }
+
+    public interface PathsChangedListener {
+        void pathsChanged(int cnt);
+    }
 }

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

@@ -22,6 +22,7 @@
         android:layout_alignParentRight="true"
         android:layout_below="@id/color_picker"
         android:padding="@dimen/icon_padding"
-        android:src="@mipmap/undo"/>
+        android:src="@mipmap/undo"
+        android:visibility="gone"/>
 
 </RelativeLayout>