ToolbarOffsetChangedListener.java

public class ToolbarOffsetChangedListener implements AppBarLayout.OnOffsetChangedListener {
    private static final String TAG = "ToolbarOffsetChangedListener";

    private Toolbar toolbar;
    private Drawable[] icons;

    /**
     * 觸發開始改變顏色的 offset [0, - AppBar's height]
     */
    private int verticalOffsetTrigger = -1;

    /**
     * 改變顏色的作用距離
     */
    private int transformDistance = 100;

    private int backgroundExpandColor = Color.TRANSPARENT;
    private int backgroundCollapseColor = Color.WHITE;
    private int textExpandColor = Color.TRANSPARENT;
    private int textCollapseColor = Color.BLACK;
    private int iconExpandColor = Color.WHITE;
    private int iconCollapseColor = Color.BLACK;

    private int[] backgroundGradient;
    private int[] textGradient;
    private int[] iconGradient;

    public ToolbarOffsetChangedListener(@NonNull Toolbar toolbar, @Nullable Drawable... icons) {
        this.toolbar = toolbar;
        this.icons = icons;

        // Calculate default gradient
        backgroundGradient = ColorUtils.getGradientARGB(backgroundExpandColor, backgroundCollapseColor, transformDistance + 1);
        textGradient = ColorUtils.getGradientARGB(textExpandColor, textCollapseColor, transformDistance + 1);
        iconGradient = ColorUtils.getGradientARGB(iconExpandColor, iconCollapseColor, transformDistance + 1);
    }

    public void setVerticalOffsetTrigger(int verticalOffsetTrigger) {
        this.verticalOffsetTrigger = verticalOffsetTrigger;
    }

    public void setTransformDistance(int transformDistance) {
        this.transformDistance = transformDistance;
    }

    public void setBackgroundColor(@ColorInt int expand, @ColorInt int collapse) {
        backgroundExpandColor = expand;
        backgroundCollapseColor = collapse;
        backgroundGradient = ColorUtils.getGradientARGB(backgroundExpandColor, backgroundCollapseColor, transformDistance + 1);
    }

    public void setTextColor(@ColorInt int expand, @ColorInt int collapse) {
        textExpandColor = expand;
        textCollapseColor = collapse;
        textGradient = ColorUtils.getGradientARGB(textExpandColor, textCollapseColor, transformDistance + 1);
    }

    public void setIconColor(@ColorInt int expand, @ColorInt int collapse) {
        iconExpandColor = expand;
        iconCollapseColor = collapse;
        iconGradient = ColorUtils.getGradientARGB(iconExpandColor, iconCollapseColor, transformDistance + 1);
    }

    @Override
    public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
        int backgroundColor;
        int textColor;
        int iconColor;

        if (verticalOffset > verticalOffsetTrigger) {
            // Full expand
            backgroundColor = backgroundGradient[0];
            textColor = textGradient[0];
            iconColor = iconGradient[0];
        } else if (verticalOffset < verticalOffsetTrigger - transformDistance) {
            // Full collapse
            backgroundColor = backgroundGradient[transformDistance];
            textColor = textGradient[transformDistance];
            iconColor = iconGradient[transformDistance];
        } else {
            // During transform
            int step = Math.abs(verticalOffset - verticalOffsetTrigger);
            backgroundColor = backgroundGradient[step];
            textColor = textGradient[step];
            iconColor = iconGradient[step];
        }

        toolbar.setBackgroundColor(backgroundColor);
        toolbar.setTitleTextColor(textColor);
        for (Drawable icon : icons) {
            icon.mutate();
            icon.setColorFilter(colorColor, PorterDuff.Mode.SRC_ATOP);
        }
    }
}