fix(getTemplate): Updated custom templates as promises condition

[Edifear]

now custom promise templates will handled properly, e.g.:
headerCellTemplate: $http.get('./template.html').then(function(res) {return res.data;})

fixes issue #4514

[david-hanger]

The referenced fix for issues #4514 and #5717 works for the above example (explicitly assigning a promise), but it is not quite a complete fix. You are correct that headerCellTemplate now handles a promise value, but it still does not correctly handle a URL as the value. The problem is that the uiGridHeaderCell.compile.pre presumes that $scope.col.headerCellTemplate is already set, but gridClassFactory.defaultColumnBuilder leaves col[templateType] undefined until after resolving the GET request on the URL. The reason the above example works is because headerCellTemplate is immediately assigned the promise as a value. If you change it to this:
headerCellTemplate: './template.html'
... then the headerCellTemplate is treated as empty because the underlying col value is left undefined when read (unless it happens to get assigned before being read, which is unlikely).

Please consider updating this fix to include something akin to the following fix I made for the same issue with filterHeaderTemplate (which can trivially be applied to headerCellTemplate and any other templates whose value is read without dealing with promises). I'm basing this off the current master, commit 84ea8fa.
(sorry; my fixes are on a network without internet access, so I can't provide a proper diff; I'm also attempting to format similarly to the adjacent code).

src/js/core/services/gridClassFactory.js

replace line 101:

             templateGetPromises.push(gridUtil.getTemplate(col[providedType])

... with:

             var promise;
             // reuse existing promise
             if ( col[templateType] && angular.isFunction(col[templateType].then) ){
                 promise = col[templateType];
             }
             // likewise if there is a related promise, e.g. cellTemplatePromise
             if ( col[templateType + 'Promise'] ){
                 promise = col[templateType + 'Promise'];
             }
             else {
                 promise = gridUtil.getTemplate(col[providedType])

Add this after line 118 (following the } after col[templateType] = template;):

                   // resolve the final value for this promise chain
                   return col[templateType];

replace lines 122-123:

                }).catch(angular.noop)
            );

... with:

                }).catch(angular.noop);
             }
             // add the promise to the list (either new or reused prior promise)
             templateGetPromises.push(promise);
             // assign the promise as an intermediate template value, until resolved
             if ( !col[templateType] ){
               col[templateType] = promise;
             }

src/js/core/directives/ui-grid-filter.js

replace line 4:

  angular.module('ui.grid').directive('uiGridFilter', ['$compile', '$templateCache', 'i18nService', 'gridUtil', function ($compile, $templateCache, i18nService, gridUtil) {

... with (adding $q):

  angular.module('ui.grid').directive('uiGridFilter', ['$compile', '$templateCache', '$q', 'i18nService', 'gridUtil', function ($compile, $templateCache, $q, i18nService, gridUtil) {

replace lines 13-15:

                var template = $scope.col.filterHeaderTemplate;

                $elm.append($compile(template)($scope));

... with:

                // handle promise values
                $q.when( $scope.col.filterHeaderTemplate ).then( function( template ){
                  $elm.append($compile(template)($scope));
                });

Note that this also fixes a minor bug where a URL in the cellTemplate gets fetched multiples times (twice, in my observation) because it isn't added to the $templateCache until after the GET response has been handled. The above fix reuses pending promises to avoid these duplicate fetches.