字典枚举

枚举和总览
过滤字典有几个不同的方法:

// 使用 keysOfEntriesWithOptions:passingTest:，可并行
NSSet *matchingKeys = [randomDict keysOfEntriesWithOptions:NSEnumerationConcurrent
                                               passingTest:^BOOL(id key, id obj, BOOL *stop)
{
    return testObj(obj);
}];
NSArray *keys = matchingKeys.allObjects;
NSArray *values = [randomDict objectsForKeys:keys notFoundMarker:NSNull.null];
__unused NSDictionary *filteredDictionary = [NSDictionary dictionaryWithObjects:values  forKeys:keys];
// 基于 block 的枚举
NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionary];
[randomDict enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
    if (testObj(obj)) {
        mutableDictionary[key] = obj;
    }
}];

// NSFastEnumeration
NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionary];
for (id key in randomDict) {
    id obj = randomDict[key];
    if (testObj(obj)) {
        mutableDictionary[key] = obj;
    }
}

 // NSEnumeration
 NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionary];
 NSEnumerator *enumerator = [randomDict keyEnumerator];
 id key = nil;
 while ((key = [enumerator nextObject]) != nil) {
       id obj = randomDict[key];
       if (testObj(obj)) {
           mutableDictionary[key] = obj;
       }
 }

// 基于 C 数组，通过 getObjects:andKeys: 枚举
NSMutableDictionary *mutableDictionary = [NSMutableDictionary dictionary];
id __unsafe_unretained objects[numberOfEntries];
id __unsafe_unretained keys[numberOfEntries];
[randomDict getObjects:objects andKeys:keys];
for (int i = 0; i < numberOfEntries; i++) {
    id obj = objects[i];
    id key = keys[i];
    if (testObj(obj)) {
       mutableDictionary[key] = obj;
    }
 }

排序
关于字典排序没有太多可说的。你只能将键数组排序为一个新对象，因此你可以使用任何正规的 NSArray 的排序方法:

- (NSArray *)keysSortedByValueUsingSelector:(SEL)comparator;
- (NSArray *)keysSortedByValueUsingComparator:(NSComparator)cmptr;
- (NSArray *)keysSortedByValueWithOptions:(NSSortOptions)opts
                      usingComparator:(NSComparator)cmptr;

摘自：http://objccn.io/issue-7-1/