您当前的位置: 问题一:
下面代码中，for循环中循环变量是什么？这是什么用法？为什么不是c语言中的for(;;)形式？
- (void)saveInterfaceState
{
NSURL *currentURL = nil;
NSMutableArray *navigationStack = [NSMutableArray array];
for (UIViewController *vc in rootNavigationController.viewControllers) {
if ([vc isKindOfClass:[DocSetViewController class]]) {
DocSetViewController *docSetViewController = (DocSetViewController *)vc;
NSMutableDictionary *stackItem = [NSMutableDictionary dictionary];
[stackItem setObject:[docSetViewController.docSet.path lastPathComponent]
forKey:@"docSetName"];
NSManagedObject *rootNode = docSetViewController.rootNode;
if (rootNode) {
NSManagedObjectID *rootNodeObjectID = rootNode.objectID;
[stackItem setObject:[rootNodeObjectID URIRepresentation]
forKey:@"rootNodeID"];
}
[navigationStack addObject:stackItem];
} else if ([vc isKindOfClass:[DetailViewController class]]) {
DetailViewController *detailVC = (DetailViewController *)vc;
currentURL = detailVC.currentURL;
}
}
if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPad) {
currentURL = self.detailViewController.currentURL;
}
NSMutableDictionary *interfaceStateInfo = [NSMutableDictionary dictionaryWithObjectsAndKeys:
navigationStack, @"navigationStack", nil];
if (currentURL) {
[interfaceStateInfo setObject:currentURL forKey:@"currentURL"];
}
NSData *interfaceStateData = [NSKeyedArchiver archivedDataWithRootObject:interfaceStateInfo];
[[NSUserDefaults standardUserDefaults] setObject:interfaceStateData forKey:@"InterfaceState"];
}
问题二：
dispatch_once中的^{}是什么用法？
+ (id)sharedDownloadManager
{
static id sharedDownloadManager = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedDownloadManager = [[self alloc] init];
});
return sharedDownloadManager;
}
------解决方案--------------------
1.
for (UIViewController *vc in rootNavigationController.viewControllers) {}这是for(.. in ..) 的用法，在一些高级语言中很常见。效率上也要高一些,看起来更简洁，它等同于
for (int i =0 ;i < rootNavigationController.viewControllers.count; i++) {
UIViewController *vc= rootNavigationController.viewControllers[i];
}2.
^{}为无参无返回值的block用法(相当于其它高级语言中的匿名函数，如C#)。
block的声明：
typedef void(^myBlock)(void);
myBlock firstBlock= //...
myBlock secondBlock= //....