PlaceHyperlink.java

package com.headexplodes.gwt.link;

import com.google.common.annotations.VisibleForTesting;
import com.google.gwt.dom.client.NativeEvent;
import com.google.gwt.event.dom.client.ClickEvent;
import com.google.gwt.event.dom.client.ClickHandler;
import com.google.gwt.inject.client.binder.GinBinder;
import com.google.gwt.place.shared.Place;
import com.google.gwt.place.shared.PlaceChangeRequestEvent;
import com.google.gwt.place.shared.PlaceController;
import com.google.gwt.place.shared.PlaceHistoryMapper;
import com.google.gwt.user.client.ui.Anchor;
import com.google.gwt.user.client.ui.Hyperlink;
import com.google.inject.Provider;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import javax.inject.Inject;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

/**
 * This widget lets us link to another Place within our application. Using an {@link Anchor} or an {@link Hyperlink}
 * would cause problems with GWT's history management and more specifically with aborted {@link PlaceChangeRequestEvent}s.
 * <p/>
 * See Thomas Broyer's explanations in
 * <a href="https://groups.google.com/forum/#!msg/google-web-toolkit/IpQQyeI1-Zc">this thread</a>
 * and <a href="http://stackoverflow.com/a/7835854/142983">this StackOverflow answer</a> for more details.
 * <p/>
 * We set the {@code href} of the anchor to make sure browsers style it correctly (mouse pointer), but also to support
 * opening the link in another tab / window.
 */
public class PlaceHyperlink extends Anchor {

    /**
     * These providers are injected using {@link GinBinder#requestStaticInjection(Class[])}. This lets us use
     * PlaceHyperlinks in UIBinder code without needing the {@code @UiField(provided = true)} to construct them.
     */
    @Inject
    private static Provider<PlaceController> placeControllerProvider;
    @Inject
    private static Provider<PlaceHistoryMapper> placeHistoryMapperProvider;

    @Nonnull
    private final PlaceController placeController;
    @Nonnull
    private final PlaceHistoryMapper placeHistoryMapper;
    @Nullable
    private Place targetPlace;

    public PlaceHyperlink() {
        this(placeControllerProvider.get(), placeHistoryMapperProvider.get(), null);
    }

    public PlaceHyperlink(@Nonnull Place targetPlace) {
        this(placeControllerProvider.get(), placeHistoryMapperProvider.get(), checkNotNull(targetPlace));
    }

    @VisibleForTesting
    public PlaceHyperlink(@Nonnull PlaceController placeController,
                          @Nonnull PlaceHistoryMapper placeHistoryMapper,
                          @Nullable Place targetPlace) {
        this.placeController = checkNotNull(placeController);
        this.placeHistoryMapper = checkNotNull(placeHistoryMapper);
        this.targetPlace = targetPlace;
        if (targetPlace != null) {
            setHref(targetPlace);
        }
        configureClickHandler();
    }

    public void setPlace(@Nonnull Place targetPlace) {
        this.targetPlace = checkNotNull(targetPlace);
        setHref(targetPlace);
    }

    private void setHref(@Nonnull Place targetPlace) {
        String placeToken = placeHistoryMapper.getToken(targetPlace);
        checkArgument(placeToken != null, "No token mapped for place %s", targetPlace);
        setHref("#" + placeToken);
    }

    private void configureClickHandler() {
        addClickHandler(new ClickHandler() {
            @Override
            public void onClick(ClickEvent event) {
                // we only want to intercept simple left clicks, to avoid preventing standard browser behaviors
                // (e.g. we don't want to prevent the user from opening the link in another tab using ctrl + click)
                if (isLeftClick(event) && !isModifierKeyDown(event)) {
                    placeController.goTo(targetPlace);
                    event.preventDefault();
                }
            }
        });
    }


    private static boolean isLeftClick(ClickEvent event) {
        return event.getNativeButton() == NativeEvent.BUTTON_LEFT;
    }

    private static boolean isModifierKeyDown(ClickEvent event) {
        return event.isAltKeyDown()
                || event.isControlKeyDown()
                || event.isMetaKeyDown()
                || event.isShiftKeyDown();
    }

}