多线程编程
多线程在各种编程语言中都是难点,很多语言中实现起来很麻烦,objective-c虽然源于c,但其多线程编程却相当简单,可以与java相媲美。这篇文章主要从线程创建与启动、线程的同步与锁、线程的交互、线程池等等四个方面简单的讲解一下iphone中的多线程编程。

一、线程创建与启动
线程创建主要有二种方式:

- (id)init;	// designated initializer
- (id)initWithTarget:(id)target selector:(SEL)selector object:(id)argument;

当然,还有一种比较特殊,就是使用所谓的convenient method,这个方法可以直接生成一个线程并启动它,而且无需为线程的清理负责。这个方法的接口是:

+ (void)detachNewThreadSelector:(SEL)aSelector toTarget:(id)aTarget withObject:(id)anArgument

前两种方法创建后,需要手机启动,启动的方法是:

- (void)start;

二、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

//  SellTicketsAppDelegate.h
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

//  SellTicketsAppDelegate.m
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

三、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 ….

四、关于线程池,大家可以查看NSOperation的相关资料。