ios 线程锁 与 线程交互

一、线程的同步与锁

要说明线程的同步与锁，最好的例子可能就是多个窗口同时售票的售票系统了。我们知道在java中，使用synchronized来同步，而iphone虽然没有提供类似java下的synchronized关键字，但提供了NSCondition对象接口。查看NSCondition的接口说明可以看出，NSCondition是iphone下的锁对象，所以我们可以使用NSCondition实现iphone中的线程安全。

这是来源于网上的一个例子：
SellTicketsAppDelegate.h 文件

import <UIKit/UIKit.h>
 
@interface SellTicketsAppDelegate : NSObject <UIApplicationDelegate> {
     int tickets;
     int count;
     NSThread* ticketsThreadone;
     NSThread* ticketsThreadtwo;
     NSCondition* ticketsCondition;
     UIWindow *window;
 }
@property (nonatomic, retain) IBOutlet UIWindow *window;
@end

SellTicketsAppDelegate.m 文件

import "SellTicketsAppDelegate.h"
 
@implementation SellTicketsAppDelegate
@synthesize window;
 
- (void)applicationDidFinishLaunching:(UIApplication *)application {
     tickets = 100;
     count = 0;
     // 锁对象
     ticketCondition = [[NSCondition alloc] init];
     ticketsThreadone = [[NSThread alloc] initWithTarget:self selector:@selector(run) object:nil];
     [ticketsThreadone setName:@"Thread-1"];
     [ticketsThreadone start];  
 
 
     ticketsThreadtwo = [[NSThread alloc] initWithTarget:self selector:@selector(run) object:nil];
     [ticketsThreadtwo setName:@"Thread-2"];
     [ticketsThreadtwo start];
     //[NSThread detachNewThreadSelector:@selector(run) toTarget:self withObject:nil];
      // Override point for customization after application launch
     [window makeKeyAndVisible]; 
 
 }
 
- (void)run{
     while (TRUE) {
         // 上锁
         [ticketsCondition lock];
         if(tickets > 0){
             [NSThread sleepForTimeInterval:0.5];
             count = 100 - tickets;
             NSLog(@"当前票数是:%d,售出:%d,线程名:%@",tickets,count,[[NSThread currentThread] name]);
             tickets--;
         }else{
             break;
         }
         [ticketsCondition unlock];
     }
 }
 
- (void)dealloc {
    [ticketsThreadone release];
     [ticketsThreadtwo release];
     [ticketsCondition release]; 
     [window release];
     [super dealloc];
}
@end

二、线程的交互
线程在运行过程中，可能需要与其它线程进行通信，如在主线程中修改界面等等，可以使用如下接口：

- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(id)arg waitUntilDone:(BOOL)wait

由于在本过程中，可能需要释放一些资源，则需要使用NSAutoreleasePool来进行管理,如：

- (void)startTheBackgroundJob {
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    // to do something in your thread job
    ...
    [self performSelectorOnMainThread:@selector(makeMyProgressBarMoving) withObject:nil waitUntilDone:NO];
    [pool release];
}

如果你什么都不考虑，在线程函数内调用 autorelease 、那么会出现下面的错误：
NSAutoReleaseNoPool(): Object 0x********* of class NSConreteData autoreleased with no pool in place ….

三、关于线程池，大家可以查看NSOperation的相关资料。

原帖： http://www.voland.com.cn/iphone-in-the-multi-threaded-programming