合并字典 - 不兼容的类型错误

Question

我一直试图将两个NSDictionaries合并几个小时.搜索并发现我可以使用[NSMutableDictionary addEntriesFromDictionary:].

    NSDictionary *areaAttributes = [[area entity] attributesByName];
    NSDictionary *gpsAttributes = [[gps entity] attributesByName];

    NSMutableDictionary *areaAttributesM = [areaAttributes mutableCopy];
    NSMutableDictionary *gpsAttributesM = [gpsAttributes mutableCopy];

    NSMutableDictionary *combinedAttributes =  [areaAttributesM addEntriesFromDictionary:gpsAttributesM];

但我得到错误:

Initializing 'NSMutableDictionary *_strong' with an expression of incompatible type 'void'

所以这就是[areaAttributesM addEntriesFromDictionary:gpsAttributesM]回报void？我的理解是否正确？为什么它回归无效？

Answer 1

是的,你是对的.来自文档:

- (void)addEntriesFromDictionary:(NSDictionary *)otherDictionary

至于为什么,这很简单:在Cocoa中改变对象的函数通常返回void,因此您可以轻松地将它们与返回不同对象的函数区分开来.

此外,还有没有理由mutableCopy的gpsAttributes字典; 它只是被用作参数-[addEntriesFromDictionary:],它不需要是可变的.

所以,正确的方法是:

NSDictionary *areaAttributes = [[area entity] attributesByName];
NSDictionary *gpsAttributes = [[gps entity] attributesByName];

NSMutableDictionary *combinedAttributes = [areaAttributes mutableCopy];
[combinedAttributes addEntriesFromDictionary:gpsAttributes];

NSDictionary如果您经常这样做,您可能希望将其包装在函数(或类别中的方法)中:

NSDictionary *mergeDictionaries(NSDictionary *lhs, NSDictionary *rhs) {
    NSMutableDictionary *ret = [lhs mutableCopy];
    [ret addEntriesFromDictionary:rhs];
    return ret;
}