一. 代码及原理



[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];


此段代码的含义就是,暂停当前处理流程,转而去处理其他input source,因limitDate设为了[NSDate distantFuture],则除非处理完毕一个其他input source,否则永不返回。

二.简单示例

可以使用此方法等待异步任务的完成,如下​​NSRunloop​​中代码所示,当shouldKeepRunning为YES时,程序一直循环处理其他消息,当前流程不继续处理,除非其他异步任务或者其他消息处理改变了shouldKeepRunning的值,当前流程能继续。


BOOL shouldKeepRunning = YES;        // global

NSRunLoop *theRL = [NSRunLoop currentRunLoop];

while (shouldKeepRunning && [theRL runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]]);


三.实际使用

一个更实际一点的例子,​​Björn Sållarp​​的获取UIWebView的user-agent字符串的代码。

BSWebViewUserAgent.h


#import 

@interface BSWebViewUserAgent : NSObject {
NSString *userAgent;
UIWebView *webView;
}
@property (nonatomic, retain) NSString *userAgent;
-(NSString*)userAgentString;
@end


BSWebViewUserAgent.m


#import "BSWebViewUserAgent.h"

@implementation BSWebViewUserAgent
@synthesize userAgent;

-(NSString*)userAgentString
{
webView = [[UIWebView alloc] init];
webView.delegate = self;
[webView loadRequest:[NSURLRequest requestWithURL:[NSURL URLWithString:@"www.google.com"]]];

// Wait for the web view to load our bogus request and give us the secret user agent.
while (self.userAgent == nil)
{
// This executes another run loop.
[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
}

return self.userAgent;
}

-(BOOL)webView:(UIWebView *)webView shouldStartLoadWithRequest:(NSURLRequest *)request navigationType:(UIWebViewNavigationType)navigationType
{
self.userAgent = [request valueForHTTPHeaderField:@"User-Agent"];

// Return no, we don't care about executing an actual request.
return NO;
}

- (void)dealloc
{
[webView release];
[userAgent release];
[super dealloc];
}
@end


在​​UIWebView​​中load一个URL,然后使用NSRunloop在等待,在UIWebView的​​shouldStartLoadWithRequest​​回调函数中获取到了user Agent字符串,使得主流程可以继续。而shouldStartLoadWithRequest返回的是NO,所以网页实际上未加载。

此代码的使用方式如下,看起来时同步处理,其中隐含着异步请求以及循环等待。


BSWebViewUserAgent *agent = [[BSWebViewUserAgent alloc] init];
NSLog(@"User-agent: %@", [agent userAgentString]);
[agent release];


四.谨慎使用

除非你确切知道你在做什么,否则别这么用。

手工操作NSRunloop可能会造成意想不到的结果,如​​Calling -[NSRunLoop runMode:beforeDate:] on main thread causes queued NSOperations to run on main thread​ ​所示。

参考:

​iPhone/iPad – Get User-Agent for UIWebView​

​iPhone/iPad – Wait for asynchronous tasks to complete​

​NSRunLoop Class Reference​

​Calling -[NSRunLoop runMode:beforeDate:] on main thread causes queued NSOperations to run on main thread​

​best-way-to-make-nsrunloop-wait-for-a-flag-to-be-set​