feat: value change listener

fixes #308

src/main/java/de/f0rce/signaturepad/SignaturePad.java

@@ -46,6 +46,12 @@ private void updateImage(ImageEncode event) {
     this.imageUri = event.getImage();
     this.type = event.getType();
     this.isEmpty = event.isEmpty();
+
+    if (!this.imageUri.equals(event.getImage())) {
+      this.fireEvent(
+          new ValueChangedEvent(
+              event.getSource(), true, event.getImage(), event.getType(), event.isEmpty()));
+    }
   }
 
   /** Clears the widget. */
@@ -363,7 +369,11 @@ public boolean isReadOnly() {
    */
   public void setImage(String uri) {
     this.getElement().setProperty("img", uri);
+    if (!this.imageUri.equals(uri)) {
+      this.fireEvent(new ValueChangedEvent(this, false, uri, this.type, false));
+    }
     this.imageUri = uri;
+    this.isEmpty = false;
   }
 
   /**
@@ -426,4 +436,16 @@ public void setClearButtonVisible(boolean clearButtonVisible) {
   public boolean isClearButtonVisible() {
     return this.clearButtonVisible;
   }
+
+  /**
+   * Add a listener to the editor, which listens to when the value changed.
+   *
+   * <p>Check {@link ValueChangedEvent} for all available returned values.
+   *
+   * @param listener {@link ComponentEventListener}
+   * @return {@link Registration}
+   */
+  public Registration addValueChangeListener(ComponentEventListener<ValueChangedEvent> listener) {
+    return this.addListener(ValueChangedEvent.class, listener);
+  }
 }

src/main/java/de/f0rce/signaturepad/ValueChangedEvent.java

@@ -0,0 +1,64 @@
+package de.f0rce.signaturepad;
+
+import java.nio.charset.StandardCharsets;
+import java.util.Base64;
+import com.vaadin.flow.component.ComponentEvent;
+
+/** @author David "F0rce" Dodlek */
+public class ValueChangedEvent extends ComponentEvent<SignaturePad> {
+
+  /** */
+  private static final long serialVersionUID = 1280906391516162475L;
+
+  private String image;
+  private String type;
+  private boolean isEmpty;
+
+  public ValueChangedEvent(
+      SignaturePad source, boolean fromClient, String image, String type, boolean isEmpty) {
+    super(source, fromClient);
+    this.image = image;
+    this.type = type;
+    this.isEmpty = isEmpty;
+  }
+
+  /**
+   * Returns the dataUrl of the encoded image.
+   *
+   * @return {@link String}
+   */
+  public String getImage() {
+    return this.image;
+  }
+
+  /**
+   * Returns the dataurl of the encoded image as Base64 decoded byte array.
+   *
+   * @return byte[]
+   */
+  public byte[] getImageBase64() {
+    if (this.image.equals("") || this.isEmpty) {
+      return null;
+    }
+    String split = this.image.split(",")[1];
+    return Base64.getDecoder().decode(split.getBytes(StandardCharsets.UTF_8));
+  }
+
+  /**
+   * Returns the type, the image has been encoded with.
+   *
+   * @return {@link String}
+   */
+  public String getType() {
+    return this.type;
+  }
+
+  /**
+   * Returns if the signature is empty.
+   *
+   * @return boolean
+   */
+  public boolean isEmpty() {
+    return this.isEmpty;
+  }
+}