trimStroke.jsx

/**
  * @file trim strokes  
  * https://community.adobe.com/t5/illustrator-discussions/creating-a-sliced-path-dashed-dotted-path/m-p/12736845
  * @version 0.1.0
  * @author sttk3.com
  * @copyright © 2022 sttk3.com
*/

//@target 'illustrator'
//@targetengine 'com.sttk3.trimStroke'
$.localize = true ;

(function() {
  if(app.documents.length <= 0) {return ;}
  var doc = app.documents[0] ;
  var sel = doc.selection ;
  if(sel.length < 2) {return ;}

  // reject when selected item is not a path, not a dashed stroke, or has a fill
  var patternPathItem = /^PathItem$/ ;
  var targetItems = filterItems(sel, function(aItem) {
    return (
      patternPathItem.test(aItem.constructor.name)
      && !aItem.filled
    ) ;
  }) ;
  if(targetItems.length != 2) {
    var message = {
      en: 'There was no target. \nPlease select two strokes with no fill and try again.', 
      ja: '対象がありませんでした。\n塗りのない線を2つ選択してから実行してください。', 
    }
    alert(message) ;
    return ;
  }

  // generate a unique color
  var tempSpot = doc.spots.add() ;
  var tempColor = new SpotColor() ;
  tempColor.spot = tempSpot ;
  tempColor.tint = 100 ;

  // subtract target strokes
  var newSelection = [] ;
  try {
    var newItem1 = subtractStroke(doc, targetItems[0], targetItems[1], tempColor) ;
    if(newItem1) {newSelection.push(newItem1) ;}

    var newItem2 = expandDashedStroke(doc, targetItems[0], tempColor) ;
    if(newItem2) {newSelection.push(newItem2) ;}
    newItem2.move(newItem1, ElementPlacement.PLACEBEFORE) ;
  } catch(e) {
    alert(e) ;
    return ;
  } finally {
    tempSpot.remove() ;
  }

  if(newSelection.length > 0) {
    doc.selection = newSelection ;
    app.executeMenuCommand('group') ;
  }
})() ;

/**
  * something like Array.filter
  * @param {Array} targetItems Array or collection of targets, anything with length and index
  * @param {Function} callback test function
  * @return {Array}
*/
function filterItems(targetItems, callback) {
  var res = [] ;
  for(var i = 0, len = targetItems.length ; i < len ; i++) {
    if(i in targetItems) {
      var val = targetItems[i] ;
      if(callback.call(targetItems, val, i)) {res.push(val) ;}
    }
  }
  return res ;
}

/**
  * subtract mainItem from subItem
  * @param {Document} doc target document
  * @param {PathItem} mainItem dashed stroke to deduct
  * @param {PathItem} subItem stroke to be subtracted
  * @param {SpotColor} tempColor unique color. a marker to identify what is unnecessary
  * @return {GroupItem}
*/
function subtractStroke(doc, mainItem, subItem, tempColor) {
  var tempSpot = tempColor.spot ;

  // object to fill dash
  var dashPath = mainItem.duplicate(mainItem, ElementPlacement.PLACEBEFORE) ;
  dashPath.strokeColor = tempColor ;
  dashPath.strokeCap = StrokeCap.BUTTENDCAP ; // make the dash length unchangeable

  var backupPath ;
  var oldUserInteractionLevel = app.userInteractionLevel ;
  try {
    app.userInteractionLevel = UserInteractionLevel.DONTDISPLAYALERTS ;
    backupPath = subItem.duplicate(subItem, ElementPlacement.PLACEAFTER) ;
      
    doc.selection = [dashPath] ;
    app.executeMenuCommand('OffsetPath v22') ; // Outline Stroke
    subItem.selected = true ;
    app.executeMenuCommand('Make Planet X') ; //  Live Paint Make
    app.executeMenuCommand('Expand Planet X') ; //  Live Paint Expand
    app.redraw() ;

    // reject unexpected structures
    var tempGroups = doc.selection[0].groupItems ;
    if(tempGroups.length <= 0) {
      doc.selection[0].remove() ;
      var message = {
        en: 'Paths with multiple strokes are not supported.', 
        ja: '複数の線を持つパスには対応していません。'
      }
      
      // mark it as a special error
      var newError = new Error(message) ;
      newError.number = -128 ;
      throw newError ;
    }

    // remove dashPath
    var itemColor ;
    for(var i = tempGroups.length - 1 ; i >= 0 ; i--) {
      itemColor = tempGroups[i].pathItems[0].fillColor ;
      if((itemColor.constructor.name == 'SpotColor') && (itemColor.spot == tempSpot)) {
        tempGroups[i].remove() ;
        break ;
      }
    }

    backupPath.remove() ;
    res = doc.selection[0] ;
  } catch(e) {
    alert(e) ;
    if(e.number == -128) {res = backupPath ;}
    return res ;
  } finally {
    app.userInteractionLevel = oldUserInteractionLevel ;
  }

  return res ;
}

/**
  * expand a dashed stroke
  * @param {Document} doc target document
  * @param {PathItem} targetItem target dashed stroke
  * @param {SpotColor} tempColor unique color. a marker to identify what is unnecessary
  * @return {GroupItem}
*/
function expandDashedStroke(doc, targetItem, tempColor) {
  var originalPath = targetItem ;
  var tempSpot = tempColor.spot ;

  var pathfinderGroup = originalPath.layer.groupItems.add() ;
  pathfinderGroup.move(originalPath, ElementPlacement.PLACEBEFORE) ;
  var maskWidth = originalPath.strokeWidth + 1 ;

  // object to fill dash
  var dashPath = originalPath.duplicate(pathfinderGroup, ElementPlacement.PLACEATEND) ;
  dashPath.strokeWidth = maskWidth ;
  dashPath.strokeCap = StrokeCap.BUTTENDCAP ; // make the dash length unchangeable

  // object to fill gap
  var gapPath = originalPath.duplicate(pathfinderGroup, ElementPlacement.PLACEATEND) ;
  gapPath.strokeDashes = [] ;
  gapPath.strokeColor = tempColor ;

  var backupPath ;
  var oldUserInteractionLevel = app.userInteractionLevel ;
  try {
    app.userInteractionLevel = UserInteractionLevel.DONTDISPLAYALERTS ;
    backupPath = originalPath.duplicate(originalPath, ElementPlacement.PLACEAFTER) ;

    // leave fill only at gaps
    doc.selection = [pathfinderGroup] ;
    app.executeMenuCommand('OffsetPath v22') ; // Outline Stroke
    app.executeMenuCommand('Live Pathfinder Subtract') ;
    app.executeMenuCommand('expandStyle') ; // Expand Appearance
    app.executeMenuCommand('compoundPath') ; // Compound Path Make
      
    // remove gaps of originalPath by live paint
    originalPath.strokeDashes = [] ;
    originalPath.selected = true ;
    app.executeMenuCommand('Make Planet X') ; //  Live Paint Make
    app.executeMenuCommand('Expand Planet X') ; //  Live Paint Expand
    app.redraw() ;

    // reject unexpected structures
    var tempGroups = doc.selection[0].groupItems ;
    if(tempGroups.length <= 0) {
      doc.selection[0].remove() ;
      var message = {
        en: 'Paths with multiple strokes are not supported.', 
        ja: '複数の線を持つパスには対応していません。'
      }
      
      // mark it as a special error
      var newError = new Error(message) ;
      newError.number = -128 ;
      throw newError ;
    }

    // remove gapPath
    var itemColor ;
    for(var i = tempGroups.length - 1 ; i >= 0 ; i--) {
      itemColor = tempGroups[i].pathItems[0].fillColor ;
      if((itemColor.constructor.name == 'SpotColor') && (itemColor.spot == tempSpot)) {
        tempGroups[i].remove() ;
        break ;
      }
    }

    backupPath.remove() ;
    res = doc.selection[0] ;
  } catch(e) {
    alert(e) ;
    if(e.number == -128) {res = backupPath ;}
    return res ;
  } finally {
    app.userInteractionLevel = oldUserInteractionLevel ;
  }

  return res ;
}