/** * Look up the value of [key], or add a new value if it isn't there. * * Returns the value associated to [key], if there is one. * Otherwise calls [ifAbsent] to get a new value, associates [key] to * that value, and then returns the new value. * * Map<String, int> scores = {'Bob': 36}; * for (var key in ['Bob', 'Rohan', 'Sophena']) { * scores.putIfAbsent(key, () => key.length); * } * scores['Bob']; // 36 * scores['Rohan']; // 5 * scores['Sophena']; // 7 * * Calling [ifAbsent] must not add or remove keys from the map. */ V putIfAbsent(K key, V ifAbsent());
Dart Map<> 添加 元素
Map<String, WidgetBuilder> routesList() {
Map<String, WidgetBuilder> re = new Map<String, WidgetBuilder>();
for (var i = 0; i < RoutesData.key.length; i++) {
re.putIfAbsent(RoutesData.key[i], () => RoutesData.val[i]);
}
return re;
}
Map<String,String> map1 = {'a':'dart','b':'java'};
map1.putIfAbsent('c',()=>'C++');
putIfAbsent 用法：
/** * Look up the value of [key], or add a new value if it isn't there. * * Returns the value associated to [key], if there is one. * Otherwise calls [ifAbsent] to get a new value, associates [key] to * that value, and then returns the new value. * * Map<String, int> scores = {'Bob': 36}; * for (var key in ['Bob', 'Rohan', 'Sophena']) { * scores.putIfAbsent(key, () => key.length); * } * scores['Bob']; // 36 * scores['Rohan']; // 5 * scores['Sophena']; // 7 * * Calling [ifAbsent] must not add or remove keys from the map. */ V putIfAbsent(K key, V ifAbsent());
/** * Look up the value of [key], or add a new value if it isn't there. * * Returns the value associated to [key], if there is one. * Otherwise calls [ifAbsent] to get a new value, associates [key] to * that value, and then returns the new value. * * Map<String, int> scores = {'Bob': 36}; * for (var key in ['Bob', 'Rohan', 'Sophena']) { * scores.putIfAbsent(key, () => key.length); * } * scores['Bob']; // 36 * scores['Rohan']; // 5 * scores['Sophena']; // 7 * * Calling [ifAbsent] must not add or remove keys from the map. */ V putIfAbsent(K key, V ifAbsent());