common/src/main/java/org/saturnclient/ui/elements/Scroll.java

package org.saturnclient.ui.elements;

import java.util.ArrayList;
import java.util.List;

import org.saturnclient.config.Theme;
import org.saturnclient.ui.Element;
import org.saturnclient.ui.ElementContext;
import org.saturnclient.ui.ElementRenderer;
import org.saturnclient.ui.RenderScope;

/**
 * A vertically scrolling container. It scrolls with the mouse wheel or by
 * dragging its scrollbar, which shows along its right edge when the children
 * don't fit.
 */
public class Scroll extends Element {
    /** Units scrolled per mouse wheel notch */
    private static final double SCROLL_SPEED = 24;
    /** Space left below the last child when scrolled to the end */
    private static final int END_SPACE = 10;

    private static final int BAR_WIDTH = 5;
    private static final int BAR_INSET = 5;
    private static final int MIN_THUMB = 20;

    /** Width to keep clear at the right of the content for the scrollbar and its click area */
    public static final int SCROLLBAR_SPACE = BAR_WIDTH + BAR_INSET * 2 + 4;

    int padding = 0;
    boolean background = true;

    protected List<Element> children = new ArrayList<>();

    /** Kept fractional so small trackpad deltas add up instead of being dropped */
    private double scroll = 0;
    private int maxScroll = 0;

    /** Where in the thumb it was grabbed, while it's being dragged */
    private Integer thumbGrab = null;

    public Scroll(int padding) {
        this.padding = padding;
    }

    public void draw(Element element) {
        ElementRenderer.draw(children, element);
    }

    /**
     * Skips the scroll's own background, for when it sits on a {@link Panel}.
     */
    public Scroll noBackground() {
        this.background = false;
        return this;
    }

    /**
     * Removes every child and scrolls back to the top.
     */
    public void clear() {
        children.clear();
        scroll = 0;
        maxScroll = 0;
    }

    @Override
    public void render(RenderScope renderScope, ElementContext ctx) {
        updateMaxScroll();
        int offset = getOffset();

        if (background) {
            renderScope.drawRoundedRectangle(0, 0, width, height,
                    Theme.BG_RADIUS.value, Theme.BACKGROUND.value);
        }

        renderScope.enableScissor(padding, padding, width - padding, height - padding);
        renderScope.getMatrixStack().push();
        renderScope.getMatrixStack().translate(padding, -offset + padding);

        ElementRenderer.render(children, ctx.elapsed, renderScope,
                ctx.mouseX - padding,
                ctx.mouseY - padding + offset,
                ctx.childFrame(padding, padding - offset, padding, height - padding));

        renderScope.getMatrixStack().pop();
        renderScope.disableScissor();

        if (maxScroll > 0) {
            boolean active = thumbGrab != null || ctx.isHovering(getBarX() - BAR_INSET, 0, BAR_WIDTH + BAR_INSET * 2,
                    height);

            renderScope.drawRoundedRectangle(getBarX(), getThumbY(), BAR_WIDTH, getThumbHeight(), BAR_WIDTH / 2,
                    active ? Theme.SCROLL.value : Theme.withAlpha(0.7f, Theme.SCROLL.value));
        }
    }

    @Override
    public void scroll(int mouseX, int mouseY, double horizontalAmount, double verticalAmount) {
        setScroll(scroll - verticalAmount * SCROLL_SPEED);
    }

    @Override
    public void keyPressed(int keyCode, int scanCode, int modifiers) {
        ElementRenderer.keyPressed(children, keyCode, scanCode, modifiers);
    }

    @Override
    public void mouseClicked(double mouseX, double mouseY, int button) {
        // Children scrolled out of view still have positions under the rest of the
        // screen, so only clicks inside the scroll's own bounds may reach them.
        if (mouseX < 0 || mouseY < 0 || mouseX > width || mouseY > height) {
            for (Element child : children) {
                child.focused = false;
            }
            return;
        }

        if (maxScroll > 0 && mouseX >= getBarX() - BAR_INSET) {
            int thumbY = getThumbY();
            int thumbHeight = getThumbHeight();

            // Grab the thumb where it was clicked, or center it on a click in the track.
            boolean onThumb = mouseY >= thumbY && mouseY <= thumbY + thumbHeight;
            thumbGrab = onThumb ? (int) mouseY - thumbY : thumbHeight / 2;
            dragThumb(mouseY);
            return;
        }

        ElementRenderer.mouseClicked(children, mouseX - padding, mouseY - padding + getOffset(), button);
    }

    @Override
    public void mouseDragged(double mouseX, double mouseY, int button, double deltaX, double deltaY) {
        if (thumbGrab != null) {
            dragThumb(mouseY);
            return;
        }

        ElementRenderer.mouseDragged(children, mouseX - padding, mouseY - padding + getOffset(), button, deltaX,
                deltaY);
    }

    @Override
    public void mouseReleased(double mouseX, double mouseY, int button) {
        thumbGrab = null;
        ElementRenderer.mouseReleased(children, mouseX - padding, mouseY - padding + getOffset(), button);
    }

    private void dragThumb(double mouseY) {
        int travel = getTrackHeight() - getThumbHeight();
        if (travel > 0) {
            setScroll((mouseY - thumbGrab - BAR_INSET) / travel * maxScroll);
        }
    }

    private void setScroll(double value) {
        scroll = Math.max(0, Math.min(maxScroll, value));
    }

    private int getOffset() {
        return (int) Math.round(scroll);
    }

    private void updateMaxScroll() {
        int bottom = 0;
        for (Element element : children) {
            bottom = Math.max(bottom, element.y + element.height);
        }

        int overflow = bottom + padding * 2 - height;
        maxScroll = overflow > 0 ? overflow + END_SPACE : 0;

        // The content may have shrunk, such as after filtering.
        setScroll(scroll);
    }

    private int getBarX() {
        return width - BAR_WIDTH - BAR_INSET;
    }

    private int getTrackHeight() {
        return height - BAR_INSET * 2;
    }

    private int getThumbHeight() {
        int track = getTrackHeight();
        return Math.max(MIN_THUMB, (int) ((long) track * height / (height + maxScroll)));
    }

    private int getThumbY() {
        int travel = getTrackHeight() - getThumbHeight();
        return BAR_INSET + (int) (travel * (scroll / maxScroll));
    }
}