dispatch_once

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dispatch_once的意思是程序在执行期间只执行一次dispatch_once包含的代码：

建一单视图应用程序：‘

//  TestClass.h#import <Foundation/Foundation.h>@interface TestClass : NSObject- (void)test;@end

//  TestClass.m#import "TestClass.h"@implementation TestClass- (void)test{    static dispatch_once_t onceToken;    dispatch_once(&onceToken, ^{        NSLog(@"This is a test.");    });}@end

//  ViewController.h#import <UIKit/UIKit.h>@interface ViewController : UIViewController@end

//  ViewController.m#import "ViewController.h"#import "TestClass.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad{    [super viewDidLoad];    self.view.backgroundColor = [UIColor whiteColor];        UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];    [button setFrame:CGRectMake(20, 20, 280, 30)];    [button setTitle:@"Test" forState:UIControlStateNormal];    [button addTarget:self action:@selector(buttonClick:) forControlEvents:UIControlEventTouchUpInside];    [self.view addSubview:button];}- (void)buttonClick:(id)seneder{    TestClass *testObj = [[TestClass alloc] init];    [testObj test];    [testObj release];}@end

无论按钮按几次，This is a test. 只会打印一次