reduce java-based cpu usage when scanning

src/main/java/com/sparrowwallet/sparrow/control/QRScanDialog.java

@@ -3,6 +3,7 @@ package com.sparrowwallet.sparrow.control;
 import com.github.sarxos.webcam.WebcamEvent;
 import com.github.sarxos.webcam.WebcamListener;
 import com.github.sarxos.webcam.WebcamResolution;
+import com.github.sarxos.webcam.WebcamUpdater;
 import com.sparrowwallet.drongo.ExtendedKey;
 import com.sparrowwallet.drongo.KeyDerivation;
 import com.sparrowwallet.drongo.OutputDescriptor;
@@ -39,6 +40,7 @@ import javafx.geometry.Insets;
 import javafx.scene.Node;
 import javafx.scene.control.*;
 import javafx.scene.layout.*;
+import javafx.util.Duration;
 import org.controlsfx.glyphfont.Glyph;
 import org.controlsfx.tools.Borders;
 import org.slf4j.Logger;
@@ -67,6 +69,7 @@ public class QRScanDialog extends Dialog<QRScanDialog.Result> {
 
     private static final Pattern PART_PATTERN = Pattern.compile("p(\\d+)of(\\d+) (.+)");
 
+    private static final int SCAN_PERIOD_MILLIS = 100;
     private final ObjectProperty<WebcamResolution> webcamResolutionProperty = new SimpleObjectProperty<>(WebcamResolution.VGA);
 
     private final DoubleProperty percentComplete = new SimpleDoubleProperty(0.0);
@@ -79,7 +82,9 @@ public class QRScanDialog extends Dialog<QRScanDialog.Result> {
             webcamResolutionProperty.set(WebcamResolution.HD);
         }
 
-        this.webcamService = new WebcamService(webcamResolutionProperty.get(), new QRScanListener());
+        this.webcamService = new WebcamService(webcamResolutionProperty.get(), new QRScanListener(), new ScanDelayCalculator());
+        webcamService.setPeriod(Duration.millis(SCAN_PERIOD_MILLIS));
+        webcamService.setRestartOnFailure(false);
         WebcamView webcamView = new WebcamView(webcamService);
 
         final DialogPane dialogPane = new QRScanDialogPane();
@@ -703,4 +708,10 @@ public class QRScanDialog extends Dialog<QRScanDialog.Result> {
             super(message, cause);
         }
     }
+
+    public static class ScanDelayCalculator implements WebcamUpdater.DelayCalculator {
+        public long calculateDelay(long snapshotDuration, double deviceFps) {
+            return Math.max(SCAN_PERIOD_MILLIS - snapshotDuration, 0L);
+        }
+    }
 }

src/main/java/com/sparrowwallet/sparrow/control/WebcamService.java

@@ -3,6 +3,7 @@ package com.sparrowwallet.sparrow.control;
 import com.github.sarxos.webcam.Webcam;
 import com.github.sarxos.webcam.WebcamListener;
 import com.github.sarxos.webcam.WebcamResolution;
+import com.github.sarxos.webcam.WebcamUpdater;
 import com.google.zxing.*;
 import com.google.zxing.client.j2se.BufferedImageLuminanceSource;
 import com.google.zxing.common.HybridBinarizer;
@@ -10,7 +11,7 @@ import javafx.beans.property.BooleanProperty;
 import javafx.beans.property.ObjectProperty;
 import javafx.beans.property.SimpleBooleanProperty;
 import javafx.beans.property.SimpleObjectProperty;
-import javafx.concurrent.Service;
+import javafx.concurrent.ScheduledService;
 import javafx.concurrent.Task;
 import javafx.embed.swing.SwingFXUtils;
 import javafx.scene.image.Image;
@@ -19,16 +20,24 @@ import java.awt.image.BufferedImage;
 import java.util.Arrays;
 import java.util.concurrent.TimeUnit;
 
-public class WebcamService extends Service<Image> {
+public class WebcamService extends ScheduledService<Image> {
     private WebcamResolution resolution;
     private final WebcamListener listener;
-    private BooleanProperty opening = new SimpleBooleanProperty(false);
+    private final WebcamUpdater.DelayCalculator delayCalculator;
+    private final BooleanProperty opening = new SimpleBooleanProperty(false);
 
     private final ObjectProperty<Result> resultProperty = new SimpleObjectProperty<>(null);
 
-    public WebcamService(WebcamResolution resolution, WebcamListener listener) {
+    private static final int QR_SAMPLE_PERIOD_MILLIS = 400;
+
+    private Webcam cam;
+    private long lastQrSampleTime;
+
+    public WebcamService(WebcamResolution resolution, WebcamListener listener, WebcamUpdater.DelayCalculator delayCalculator) {
         this.resolution = resolution;
         this.listener = listener;
+        this.delayCalculator = delayCalculator;
+        this.lastQrSampleTime = System.currentTimeMillis();
     }
 
     @Override
@@ -36,8 +45,9 @@ public class WebcamService extends Service<Image> {
         return new Task<Image>() {
             @Override
             protected Image call() throws Exception {
-                Webcam cam = Webcam.getWebcams(1, TimeUnit.MINUTES).get(0);
                 try {
+                    if(cam == null) {
+                        cam = Webcam.getWebcams(1, TimeUnit.MINUTES).get(0);
                         cam.setCustomViewSizes(resolution.getSize());
                         cam.setViewSize(resolution.getSize());
                         if(!Arrays.asList(cam.getWebcamListeners()).contains(listener)) {
@@ -45,26 +55,42 @@ public class WebcamService extends Service<Image> {
                         }
 
                         opening.set(true);
-                    cam.open();
+                        cam.open(true, delayCalculator);
                         opening.set(false);
-                    while(!isCancelled()) {
+                    }
-                        if(cam.isImageNew()) {
+
                     BufferedImage bimg = cam.getImage();
-                            updateValue(SwingFXUtils.toFXImage(bimg, null));
+                    Image image = SwingFXUtils.toFXImage(bimg, null);
+                    updateValue(image);
+
+                    if(System.currentTimeMillis() > (lastQrSampleTime + QR_SAMPLE_PERIOD_MILLIS)) {
                         readQR(bimg);
+                        lastQrSampleTime = System.currentTimeMillis();
                     }
-                    }
+
-                    return getValue();
+                    return image;
                 } finally {
                     opening.set(false);
-                    if(!cam.close()) {
-                        cam.close();
-                    }
                 }
             }
         };
     }
 
+    @Override
+    public void reset() {
+        cam = null;
+        super.reset();
+    }
+
+    @Override
+    public boolean cancel() {
+        if(cam != null && !cam.close()) {
+            cam.close();
+        }
+
+        return super.cancel();
+    }
+
     private void readQR(BufferedImage bufferedImage) {
         LuminanceSource source = new BufferedImageLuminanceSource(bufferedImage);
         BinaryBitmap bitmap = new BinaryBitmap(new HybridBinarizer(source));

src/main/java/com/sparrowwallet/sparrow/control/WebcamView.java

@@ -1,7 +1,10 @@
 package com.sparrowwallet.sparrow.control;
 
+import javafx.beans.property.ObjectProperty;
+import javafx.beans.property.SimpleObjectProperty;
 import javafx.scene.Node;
 import javafx.scene.control.Label;
+import javafx.scene.image.Image;
 import javafx.scene.image.ImageView;
 import javafx.scene.layout.Region;
 import org.slf4j.Logger;
@@ -16,6 +19,8 @@ public class WebcamView {
 
     private final Label statusPlaceholder;
 
+    private final ObjectProperty<Image> imageProperty = new SimpleObjectProperty<>(null);
+
     public WebcamView(WebcamService service) {
         this.service = service ;
         this.imageView = new ImageView();
@@ -23,22 +28,32 @@ public class WebcamView {
         // make the cam behave like a mirror:
         imageView.setScaleX(-1);
 
+        service.valueProperty().addListener((observable, oldValue, newValue) -> {
+            if(newValue != null) {
+                imageProperty.set(newValue);
+            }
+        });
+
         this.statusPlaceholder = new Label();
         this.view = new Region() {
             {
                 service.stateProperty().addListener((obs, oldState, newState) -> {
                     switch (newState) {
                         case READY:
+                            if(imageProperty.get() == null) {
                                 statusPlaceholder.setText("Initializing");
                                 getChildren().setAll(statusPlaceholder);
+                            }
                             break ;
                         case SCHEDULED:
+                            if(imageProperty.get() == null) {
                                 statusPlaceholder.setText("Waiting");
                                 getChildren().setAll(statusPlaceholder);
+                            }
                             break ;
                         case RUNNING:
                             imageView.imageProperty().unbind();
-                            imageView.imageProperty().bind(service.valueProperty());
+                            imageView.imageProperty().bind(imageProperty);
                             getChildren().setAll(imageView);
                             break ;
                         case CANCELLED: