// widget_dropdown.ysl2

template "widget[@type='DropDown']", mode="widget_defs" {
    param "hmi_element";
    labels("text box button");
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    dispatch: function(value) {
        if(!this.opened) this.set_selection(value);
    },
    init: function() {
        this.button_elt.setAttribute("onclick", "hmi_widgets['«$hmi_element/@id»'].on_button_click()");
        // Save original size of rectangle
        this.box_bbox = this.box_elt.getBBox()

        // Compute margins
        text_bbox = this.text_elt.getBBox()
        lmargin = text_bbox.x - this.box_bbox.x;
        tmargin = text_bbox.y - this.box_bbox.y;
        this.margins = [lmargin, tmargin].map(x => Math.max(x,0));

        // It is assumed that list content conforms to Array interface.
        this.content = [
        ``foreach "arg" | "«@value»",
        ];

        // Index of first visible element in the menu, when opened
        this.menu_offset = 0;

        // How mutch to lift the menu vertically so that it does not cross bottom border
        this.lift = 0;

        // Event handlers cannot be object method ('this' is unknown)
        // as a workaround, handler given to addEventListener is bound in advance.
        this.bound_close_on_click_elsewhere = this.close_on_click_elsewhere.bind(this);

        this.opened = false;
    },
    // Called when a menu entry is clicked
    on_selection_click: function(selection) {
        this.close();
        this.apply_hmi_value(0, selection);
    },
    on_button_click: function() {
        this.open();
    },
    on_backward_click: function(){
        this.scroll(false);
    },
    on_forward_click:function(){
        this.scroll(true);
    },
    set_selection: function(value) {
        let display_str;
        if(value >= 0 && value < this.content.length){
            // if valid selection resolve content
            display_str = this.content[value];
            this.last_selection = value;
        } else {
            // otherwise show problem
            display_str = "?"+String(value)+"?";
        }
        // It is assumed that first span always stays,
        // and contains selection when menu is closed
        this.text_elt.firstElementChild.textContent = display_str;
    },
    grow_text: function(up_to) {
        let count = 1;
        let txt = this.text_elt; 
        let first = txt.firstElementChild;
        // Real world (pixels) boundaries of current page
        let bounds = svg_root.getBoundingClientRect(); 
        this.lift = 0;
        while(count < up_to) {
            let next = first.cloneNode();
            // relative line by line text flow instead of absolute y coordinate
            next.removeAttribute("y");
            next.setAttribute("dy", "1.1em");
            // default content to allow computing text element bbox
            next.textContent = "...";
            // append new span to text element
            txt.appendChild(next);
            // now check if text extended by one row fits to page
            // FIXME : exclude margins to be more accurate on box size
            let rect = txt.getBoundingClientRect();
            if(rect.bottom > bounds.bottom){
                // in case of overflow at the bottom, lift up one row
                let backup = first.getAttribute("dy");
                // apply lift asr a dy added too first span (y attrib stays)
                first.setAttribute("dy", "-"+String((this.lift+1)*1.1)+"em");
                rect = txt.getBoundingClientRect();
                if(rect.top > bounds.top){
                    this.lift += 1;
                } else {
                    // if it goes over the top, then backtrack
                    // restore dy attribute on first span
                    if(backup)
                        first.setAttribute("dy", backup);
                    else
                        first.removeAttribute("dy");
                    // remove unwanted child
                    txt.removeChild(next);
                    return count;
                }
            }
            count++;
        }
        return count;
    },
    close_on_click_elsewhere: function(e) {
        // inhibit events not targetting spans (menu items)
        if(e.target.parentNode !== this.text_elt){
            e.stopPropagation();
            // close menu in case click is outside box
            if(e.target !== this.box_elt)
                this.close();
        }
    },
    close: function(){
        // Stop hogging all click events
        svg_root.removeEventListener("click", this.bound_close_on_click_elsewhere, true);
        // Restore position and sixe of widget elements
        this.reset_text();
        this.reset_box();
        // Put the button back in place
        this.element.appendChild(this.button_elt);
        // Mark as closed (to allow dispatch)
        this.opened = false;
        // Dispatch last cached value
        this.apply_cache();
    },
    // Set text content when content is smaller than menu (no scrolling)
    set_complete_text: function(){
        let spans = this.text_elt.children; 
        let c = 0;
        for(let item of this.content){
            let span=spans[c];
            span.textContent = item;
            span.setAttribute("onclick", "hmi_widgets['«$hmi_element/@id»'].on_selection_click("+c+")");
            c++;
        }
    },
    // Move partial view :
    // false : upward, lower value
    // true  : downward, higher value
    scroll: function(forward){
        let contentlength = this.content.length;
        let spans = this.text_elt.children; 
        let spanslength = spans.length;
        // reduce accounted menu size according to jumps
        if(this.menu_offset != 0) spanslength--;
        if(this.menu_offset < contentlength - 1) spanslength--;
        if(forward){
            this.menu_offset = Math.min(
                contentlength - spans.length + 1, 
                this.menu_offset + spanslength);
        }else{
            this.menu_offset = Math.max(
                0, 
                this.menu_offset - spanslength);
        }
        this.set_partial_text();
    },
    // Setup partial view text content
    // with jumps at first and last entry when appropriate
    set_partial_text: function(){
        let spans = this.text_elt.children; 
        let contentlength = this.content.length;
        let spanslength = spans.length;
        let i = this.menu_offset, c = 0;
        while(c < spanslength){
            let span=spans[c];
            // backward jump only present if not exactly at start
            if(c == 0 && i != 0){
                span.textContent = "↑  ↑  ↑";
                span.setAttribute("onclick", "hmi_widgets['«$hmi_element/@id»'].on_backward_click()");
            // presence of forward jump when not right at the end
            }else if(c == spanslength-1 && i < contentlength - 1){
                span.textContent = "↓  ↓  ↓";
                span.setAttribute("onclick", "hmi_widgets['«$hmi_element/@id»'].on_forward_click()");
            // otherwise normal content
            }else{
                span.textContent = this.content[i];
                span.setAttribute("onclick", "hmi_widgets['«$hmi_element/@id»'].on_selection_click("+i+")");
                i++;
            }
            c++;
        }
    },
    open: function(){
        let length = this.content.length;
        // systematically reset text, to strip eventual whitespace spans
        this.reset_text();
        // grow as much as needed or possible
        let slots = this.grow_text(length);
        // Depending on final size
        if(slots == length) {
            // show all at once
            this.set_complete_text();
        } else {
            // eventualy align menu to current selection, compensating for lift
            let offset = this.last_selection - this.lift;
            if(offset > 0)
                this.menu_offset = Math.min(offset + 1, length - slots + 1);
            else
                this.menu_offset = 0;
            // show surrounding values
            this.set_partial_text();
        }
        // Now that text size is known, we can set the box around it
        this.adjust_box_to_text();
        // Take button out until menu closed
        this.element.removeChild(this.button_elt);
        // Rise widget to top by moving it to last position among siblings
        this.element.parentNode.appendChild(this.element.parentNode.removeChild(this.element));
        // disable interaction with background
        svg_root.addEventListener("click", this.bound_close_on_click_elsewhere, true);
        // mark as open
        this.opened = true;
    },
    // Put text element in normalized state
    reset_text: function(){
        let txt = this.text_elt; 
        let first = txt.firstElementChild;
        // remove attribute eventually added to first text line while opening
        first.removeAttribute("onclick");
        first.removeAttribute("dy");
        // keep only the first line of text
        for(let span of Array.from(txt.children).slice(1)){
            txt.removeChild(span)
        }
    },
    // Put rectangle element in saved original state
    reset_box: function(){
        let m = this.box_bbox;
        let b = this.box_elt;
        b.x.baseVal.value = m.x;
        b.y.baseVal.value = m.y;
        b.width.baseVal.value = m.width;
        b.height.baseVal.value = m.height;
    },
    // Use margin and text size to compute box size
    adjust_box_to_text: function(){
        let [lmargin, tmargin] = this.margins;
        let m = this.text_elt.getBBox();
        let b = this.box_elt;
        b.x.baseVal.value = m.x - lmargin;
        b.y.baseVal.value = m.y - tmargin;
        b.width.baseVal.value = 2 * lmargin + m.width;
        b.height.baseVal.value = 2 * tmargin + m.height;
    },
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}