make_model: be lazy and correct on multiple entrances

src/util/make_model.js

@@ -40,8 +40,9 @@ export default (options={}) => {
 		upstream_limit: null
 	}
 
-	// some random places to make things work
+	// some random variables to make things work
 	let _xhr = null
+	let _promise = null
 
 	// construct the model
 	return {
@@ -58,14 +59,20 @@ export default (options={}) => {
 			if (limit == Infinity)
 				limit = _cache.upstream_limit || _cache.count || Infinity
 
-			// be lazy: if the data is presented, return the value immediately
+			// be lazy 1: if the data is presented, return the value immediately
 			//
 			// note that Array.every(list) == true if list is empty
-			if (_cache.data.slice(offset, offset+limit).length > 0 && _cache.data.slice(offset, offset+limit).every(data => data != undefined))
-				return Promise.resolve(_cache.data.slice(offset, offset+limit))
+			let _portion = _cache.data.slice(offset, offset+limit)
+			if (_portion.length > 0 && _portion.every(data => data != undefined))
+				return Promise.resolve(_portion)
+
+			// be lazy 2: if there is a promise, return it if ambient matches or
+			// cancel it
+			if (_promise != null)
+				return _promise
 
 			// now the hard work
-			return _configs.api.request({
+			_promise = _configs.api.request({
 				method: 'GET',
 				url: _configs.endpoint,
 				headers: _cache.count == null ? {
@@ -76,34 +83,39 @@ export default (options={}) => {
 					// transform model state to postgest queries
 					//...ambient_queries,
 					offset == 0 ? undefined : {label: 'offset', value: offset},
-					limit == Infinity ? undefined : {label: 'limit', value: limit}
-				]
-			}).then(response => {
-				// gather begin/end/count
-				let [_range, _count] = _xhr.getResponseHeader('content-range').split('/')
-				let [_begin, _end] = _range.split('-').map(v => ~~v)
-
-				// update count if presented
-				if (_count != '*') _cache.count = _count
-
-				// see if an upstream limit is exposed
-				if (_end - _begin + 1 < limit) _cache.upstream_limit = _end - _begin + 1
-
-				// fill the data cache
-				response.forEach((data, i) => {
-					//					_value_filters.forEach(([label, config]) => data[config.alias || label] = config.value_filter(data[config.alias || label]))
-					_cache.data[_begin + i] = data
+						limit == Infinity ? undefined : {label: 'limit', value: limit}
+					]
+				}).then(response => {
+					// gather begin/end/count
+					let [_range, _count] = _xhr.getResponseHeader('content-range').split('/')
+					let [_begin, _end] = _range.split('-').map(v => ~~v)
+
+					// update count if presented
+					if (_count != '*') _cache.count = _count
+
+					// see if an upstream limit is exposed
+					if (_end - _begin + 1 < limit) _cache.upstream_limit = _end - _begin + 1
+
+					// fill the data cache
+					response.forEach((data, i) => {
+						//					_value_filters.forEach(([label, config]) => data[config.alias || label] = config.value_filter(data[config.alias || label]))
+						_cache.data[_begin + i] = data
+					})
+
+					// assert offset/limit and returned range
+					if (offset != _begin || _end - _begin + 1 > limit)
+						throw 'The request and response data range mismatches!'
+					if (_end - _begin + 1 < limit)
+						console.warn('The response range is narrower than requested, probably due to an upstream hard limit.')
+
+					// clean model state
+					_promise = null
+
+					// return data
+					return _cache.data.slice(_begin, _end + 1)
 				})
 
-				// assert offset/limit and returned range
-				if (offset != _begin || _end - _begin + 1 > limit)
-					throw 'The request and response data range mismatches!'
-				if (_end - _begin + 1 < limit)
-					console.warn('The response range is narrower than requested, probably due to an upstream hard limit.')
-
-				// return
-				return _cache.data.slice(_begin, _end + 1)
-			})
+			return _promise
 		}
 	}
 }