一 系统内置对象读写至文件

- If your objects are of type NSString,NSDictionary,NSArray,NSDate,NSData, or NSNumber, you can use the writeToFile:atomically: method implemented in these classes to write your data to a file. In the case of writing out a dictionary or an array, this method writes the data to the file in the format of an XML property list. Program 19.1 shows how the dictionary you created as a simple glossary in Chapter 15,“Numbers, Strings, and Collections,” can be written to a file as a property list.

 Program 19.1

#import <Foundation/NSObject.h>

#import <Foundation/NSString.h>

#import <Foundation/NSDictionary.h>

#import <Foundation/NSAutoreleasePool.h>

int main (int argc, char *argv[])

int main (int argc, char *argv[])

   {

                NSAutoreleasePool            * pool = [[NSAutoreleasePool alloc] init];

                NSDictionary       *glossary =

                [NSDictionary        dictionaryWithObjectsAndKeys:

                    @”A class defined so other classes can inherit from it.”, @”abstract class”,

                    @”To implement all the methods defined in a protocol”, @”adopt”,

                    @”Storing an object for later use. “,  @”archiving”,

                    nil

                ];

                if ([glossary writeToFile: @”glossary”                  atomically: YES] == NO)

                    NSLog (@”Save to file failed!”);

                [pool drain];

                return 0;

           }

- To read an XML property list from a file into your program, you use the dictionaryWithContentsOfFile: or arrayWithContentsOfFile: methods.To read back data, use the dataWithContentsOfFile: method; to read back string objects, use the stringWithContentsOfFile:method. Program 19.2 reads back the glossary written in Program 19.1 and then displays its contents.  

#import <Foundation/NSObject.h> 

#import <Foundation/NSString.h> 

#import <Foundation/NSDictionary.h> 

#import <Foundation/NSEnumerator.h> 

#import <Foundation/NSAutoreleasePool.h> 

int main (int argc, char *argv[]) 

{ 

    NSAutoreleasePool  * pool = [[NSAutoreleasePool alloc] init]; 

    NSDictionary *glossary; 

    glossary = [NSDictionary dictionaryWithContentsOfFile: @”glossary”]; 

    for ( NSString *key in glossary ) 

        NSLog (@”%@: %@”,          key, [glossary objectForKey: key]); 

     [pool drain]; 

    return 0; 

} 

 二 自定义对象读写至文件

 -   A more flexible approach enables you to save any type of objects to a file, not just strings, arrays, and dictionaries.This is done by creating a keyed archive using the NSKeyedArchiver class.
This implies that you can’t directly archive your AddressBook using this technique because the Objective-C system doesn’t know how to archive an AddressBook object. If you tried to archive it by inserting a line such as NSKeyedArchiver archiveRootObject: myAddressBook toFile: @”addrbook.arch”]; into your program, you’d get the following message displayed if you ran the program:

*** -[AddressBook encodeWithCoder:]: selector not recognized
*** Uncaught exception: <NSInvalidArgumentException>
*** -[AddressBook encodeWithCoder:]: selector not recognized
archiveTest: received signal: Trace/BPT trap
To archive objects other than those listed, you must tell the system how to archive, or encode, your objects, and also how to unarchive, or decode, them.This is done by adding encodeWithCoder: and initWithCoder: methods to your class definitions, according to the <NSCoding> protocol. For our address book example, you’d have to add these methods to both the AddressBook and AddressCard classes.

The encodeWithCoder: method is invoked each time the archiver wants to encode an object from the specified class, and the method tells it how to do so. In a similar manner, the initWithCoder: method is invoked each time an object from the specified class is to be decoded.

Program 19.5 Addresscard.h Interface File
#import <Foundation/NSObject.h>
#import <Foundation/NSString.h>
#import <Foundation/NSKeyedArchiver.h>
@interface AddressCard: NSObject <NSCoding, NSCopying>
{
NSString  *name;
NSString  *email;
}
@property (copy, nonatomic) NSString *name, *email;
-(void) setName: (NSString *) theName andEmail: (NSString *) theEmail;
-(NSComparisonResult) compareNames: (id) element;
-(void) print;
// Additional methods for NSCopying protocol
-(AddressCard *) copyWithZone: (NSZone *) zone;
-(void) retainName: (NSString *) theName andEmail: (NSString *) theEmail;

Ios 4.0 多任务不是传统意义上的多任务。只是把程序的状态保存起来，程序挂起。因为Apple还没准备好多任务同时运行，

主要是因为battery和memory这两个问题还没有解决。

现在IOS 4多任务支持的类型（官网）：  

§  Background audio

§  Voice over IP

§  Background location 

§  Push notifications

§  Local notifications

§  Task finishing - If your app is in mid-task when your customer leaves it, the app can now keep running to finish the task.

§  Fast app switching - All developers should take advantage of fast app switching, which allows users to leave your app and come right back to where they were when they left - no more having to reload the app.

我使用的是Task finishing， 既当用户挂起程序时，如果还有task没完成，可以把改task完成。

但这个是有限制的，时间的限制，就是说你的后台程序不能执行超过某个时间。

我刚才打log看了，系统返回500s，既是8分钟，8分钟如果还没执行完，就会自动把我们程序结束。

代码如下

#pragma mark -

#pragma mark Background Task Handle

- (void)applicationDidEnterBackground:(UIApplication *)application {

// Request permission to run in the background. Provide an 

// expiration handler in case the task runs long.

NSAssert(self->bgTask == UIBackgroundTaskInvalid, nil);

self->bgTask = [application beginBackgroundTaskWithExpirationHandler: ^{

// Synchronize the cleanup call on the main thread in case

// the task catully finished at around the same time.

dispatch_async(dispatch_get_main_queue(), ^{

if (UIBackgroundTaskInvalid != self->bgTask) {

[application endBackgroundTask:self->bgTask];

self->bgTask = UIBackgroundTaskInvalid;

}

});

}];

// Start the long-running task and return immediately.

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), 

  ^{

  // Do the work assoicated with the task.

  for(int i = 0; i  < 1000; i++) {

                                                  //request network.

                                                  NSLog(@"hahah %d, Time Remain = %f", i, [application backgroundTimeRemaining]);

                                           }

  // Synchronize the cleanup all on the main thread in case

  // the task catully finished at around the same time. 

  dispatch_async(dispatch_get_main_queue(), ^{

  if (UIBackgroundTaskInvalid != self->bgTask) {

  [application endBackgroundTask:self->bgTask];

  self->bgTask = UIBackgroundTaskInvalid;

  }

  });

  });

}

#pragma mark -

#pragma mark Local Notifications

- (void)scheduleAlarmForDate:(NSDate *)theDate {

UIApplication *app = [UIApplication sharedApplication];

NSArray *oldNotifications = [app scheduledLocalNotifications];

// Clear out the old notification before scheduling a new one.

if (0 < [oldNotifications count]) {

[app cancelAllLocalNotifications];

}

// Create a new notification

UILocalNotification *alarm = [[UILocalNotification alloc] init];

if (alarm) {

alarm.fireDate = theDate;

alarm.timeZone = [NSTimeZone defaultTimeZone];

alarm.repeatInterval = 0;

alarm.soundName = @"ping.caf";//@"default";

alarm.alertBody = [NSString stringWithFormat:@"Time to wake up!Now is\n[%@]", 

  [NSDate dateWithTimeIntervalSinceNow:10]];

[app scheduleLocalNotification:alarm];

[alarm release];

}

} 

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plist写到resouce里面去

通过NSBundle把数据取plist读出来

2. 通过NSClassFromString创建类

NSClassFromString

NSSelectorFromString

正常来说，

id myObj = [[NSClassFromString(@"MySpecialClass") alloc] init];

和

id myObj = [[MySpecialClass alloc] init];

是一样的。但是，如果你的程序中并不存在MySpecialClass这个类，下面的写法会出错，而上面的写法只是返回一个空对象而已。

因此，在某些情况下，可以使用NSClassFromString来进行你不确定的类的初始化。

比如在iPhone中，NSTask可能就会出现这种情况，所以在你需要使用NSTask时，最好使用：

[[NSClassFromString(@"NSTask") .....]]

而不要直接使用[NSTask ...]这种写法。

NSClassFromString的好处是：

1 弱化连接，因此并不会把没有的Framework也link到程序中。

for (int c=0; c<[classNames count]; c++) {

NSString *className=[classNames objectAtIndex:c];

id class=[[NSClassFromString(className) alloc] init];

for (int i=0; i<[params count]; i++) {

[class performSelector:NSSelectorFromString([NSString stringWithFormat:@"setA%i",i])];

}

}

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