WebView何时可以创建快照()?

loadContent load reload() 会补偿的。

show() 在加载内容之前。由于内容是异步加载的,因此完全有可能在调用之后的语句之前加载 负载 或 饰面。

然后,解决方法是将内容放在一个文件中,并调用WebEngines reload()

Path htmlFile = Files.createTempFile("snapshot-", ".html");
Files.writeString(htmlFile, html);

WebEngine engine = myWebView.getEngine();
engine.getLoadWorker().stateProperty().addListener(
    new ChangeListener<Worker.State>() {
        private boolean reloaded;

        @Override
        public void changed(ObservableValue<? extends Worker.State> obs,
                            Worker.State oldState,
                            Worker.State newState) {
            if (reloaded) {
                Image image = myWebView.snapshot(null, null);
                doStuffWithImage(image);

                try {
                    Files.delete(htmlFile);
                } catch (IOException e) {
                    log.log(Level.WARN, "Couldn't delete " + htmlFile, e);
                }
            } else {
                reloaded = true;
                engine.reload();
            }
        }
    });


engine.load(htmlFile.toUri().toString());

但因为你在使用 static 对于所有内容,都必须添加一些字段:

private static boolean reloaded;
private static volatile Path htmlFile;

/** Listens for a SUCCEEDED state to activate image capture **/
private static ChangeListener<Worker.State> stateListener = (ov, oldState, newState) -> {
    if (newState == Worker.State.SUCCEEDED) {
        if (reloaded) {
            WritableImage snapshot = webView.snapshot(new SnapshotParameters(), null);

            capture.set(SwingFXUtils.fromFXImage(snapshot, null));
            finished.set(true);
            stage.hide();

            try {
                Files.delete(htmlFile);
            } catch (IOException e) {
                log.log(Level.WARN, "Couldn't delete " + htmlFile, e);
            }
        } else {
            reloaded = true;
            webView.getEngine().reload();
        }
    }
};

然后每次加载内容时都必须重置它:

Path htmlFile = Files.createTempFile("snapshot-", ".html");
Files.writeString(htmlFile, html);

Platform.runLater(new Thread(() -> {
    try {
        reloaded = false;
        stage.show(); // JDK-8087569: will not capture without showing stage
        stage.toBack();
        webView.getEngine().load(htmlFile);
    }
    catch(Throwable t) {
        thrown.set(t);
    }
}));

注意,有更好的方法来执行多线程处理。您可以简单地使用 volatile 领域:

private static volatile boolean started;
private static volatile boolean finished = true;
private static volatile Throwable thrown;
private static volatile BufferedImage capture;

(布尔字段默认为false,对象字段默认为null。与C程序不同,这是Java的硬保证;没有未初始化的内存。)

与其在循环中轮询另一个线程中所做的更改,不如使用同步、锁或更高级别的类,如 CountDownLatch 它在内部使用这些东西:

private static final CountDownLatch initialized = new CountDownLatch(1);
private static volatile CountDownLatch finished;
private static volatile BufferedImage capture;
private static volatile Throwable thrown;
private static boolean reloaded;

private static volatile Path htmlFile;

// main javafx objects
private static WebView webView = null;
private static Stage stage = null;

private static ChangeListener<Worker.State> stateListener = (ov, oldState, newState) -> {
    if (newState == Worker.State.SUCCEEDED) {
        if (reloaded) {
            WritableImage snapshot = webView.snapshot(null, null);
            capture = SwingFXUtils.fromFXImage(snapshot, null);
            finished.countDown();
            stage.hide();

            try {
                Files.delete(htmlFile);
            } catch (IOException e) {
                log.log(Level.WARNING, "Could not delete " + htmlFile, e);
            }
        } else {
            reloaded = true;
            webView.getEngine().reload();
        }
    }
};

@Override
public void start(Stage primaryStage) {
    log.fine("Started JavaFX, creating WebView...");
    stage = primaryStage;
    primaryStage.setScene(new Scene(webView = new WebView()));

    Worker<Void> worker = webView.getEngine().getLoadWorker();
    worker.stateProperty().addListener(stateListener);

    webView.getEngine().setOnError(e -> {
        thrown = e.getException();
    });

    // Prevents JavaFX from shutting down when hiding window, useful for calling capture(...) in succession
    Platform.setImplicitExit(false);

    initialized.countDown();
}

public static BufferedImage capture(String html)
throws InterruptedException,
       IOException {

    htmlFile = Files.createTempFile("snapshot-", ".html");
    Files.writeString(htmlFile, html);

    if (initialized.getCount() > 0) {
        new Thread(() -> Application.launch(SnapshotRaceCondition2.class)).start();
        initialized.await();
    }

    finished = new CountDownLatch(1);
    thrown = null;

    Platform.runLater(() -> {
        reloaded = false;
        stage.show(); // JDK-8087569: will not capture without showing stage
        stage.toBack();
        webView.getEngine().load(htmlFile.toUri().toString());
    });

    finished.await();

    if (thrown != null) {
        throw new IOException(thrown);
    }

    return capture;
}

reloaded 未声明为volatile,因为它仅在JavaFX应用程序线程中访问。

为了适应调整大小以及底层快照行为,我(我们)提出了以下工作解决方案。注意,这些测试运行了2000x(Windows、macOS和Linux),提供了100%成功的随机WebView大小。

首先,我将引用一个JavaFX开发人员。这是引自一份私人(赞助的)错误报告:

“我假设您在FX AppThread上启动了调整大小操作,并且在达到SUCCEEDED状态后完成。在这种情况下,在我看来,在那一刻,等待2个脉冲(不阻塞FX AppThread)应该给webkit实现足够的时间进行更改,除非这导致JavaFX中的某些维度被更改,这可能会再次导致webkit中的维度被更改。

我正在考虑如何将这些信息输入JBS中的讨论,但我很肯定会有这样的答案:“只有当webcomponent稳定时,才应该拍摄快照”。所以为了预测这个答案,最好看看这个方法是否适合你。或者,如果它最终导致了其他问题,那么最好考虑一下这些问题,看看是否/如何在OpenJFX中修复它们。”

1. 默认情况下,javafx8使用默认值 600 如果高度正好 0 . WebView setMinHeight(1) , setPrefHeight(1) 为了避免这个问题。这不在下面的代码中,但值得一提的是,任何人都可以将其应用到他们的项目中。
3. 要防止快照空白错误,请利用快照回调,它还侦听脉冲。

// without this runlater, the first capture is missed and all following captures are offset
Platform.runLater(new Runnable() {
    public void run() {
        // start a new animation timer which waits for exactly two pulses
        new AnimationTimer() {
            int frames = 0;

            @Override
            public void handle(long l) {
                // capture at exactly two frames
                if (++frames == 2) {
                    System.out.println("Attempting image capture");
                    webView.snapshot(new Callback<SnapshotResult,Void>() {
                        @Override
                        public Void call(SnapshotResult snapshotResult) {
                            capture.set(SwingFXUtils.fromFXImage(snapshotResult.getImage(), null));
                            unlatch();
                            return null;
                        }
                    }, null, null);

                    //stop timer after snapshot
                    stop();
                }
            }
        }.start();
    }
});