Objective-C语法之异常处理 .

Objective-C的异常比较像Java的异常处理，也有@finally的处理，不管异常是否捕获都都要执行。

异常处理捕获的语法：

@try {
        <#statements#>
    }
    @catch (NSException *exception) {
        <#handler#>
    }
    @finally {
        <#statements#>
    }

@catch{}块对异常的捕获应该先细后粗，即是说先捕获特定的异常，再使用一些泛些的异常类型。

我们自定义两个异常类，看看异常异常处理的使用。

SomethingException.h

#import <Foundation/Foundation.h>

@interface SomethingException : NSException

@end

SomethingException.m

#import "SomethingException.h"

@implementation SomethingException

@end

SomeOverException.h

#import <Foundation/Foundation.h>

@interface SomeOverException : NSException

@end

SomeOverException.m

#import "SomeOverException.h"

@implementation SomeOverException

@end

2、新建Box类，在某些条件下产生异常。

#import <Foundation/Foundation.h>

@interface Box : NSObject
{
    NSInteger number;
}
-(void) setNumber: (NSInteger) num;
-(void) pushIn;
-(void) pullOut;
-(void) printNumber;
@end

@implementation Box
-(id) init {
    self = [super init];
    
    if ( self ) {
        [self setNumber: 0];
    }
    
    return self;
}

-(void) setNumber: (NSInteger) num {
    number = num;
    
    if ( number > 10 ) {
        NSException *e = [SomeOverException
                          exceptionWithName: @"BoxOverflowException"
                          reason: @"The level is above 100"
                          userInfo: nil];
        @throw e;
    } else if ( number >= 6 ) {
        // throw warning
        NSException *e = [SomethingException
                          exceptionWithName: @"BoxWarningException"
                          reason: @"The level is above or at 60"
                          userInfo: nil];
        @throw e;
    } else if ( number < 0 ) {
        // throw exception
        NSException *e = [NSException
                          exceptionWithName: @"BoxUnderflowException"
                          reason: @"The level is below 0"
                          userInfo: nil];
        @throw e;
    }
}

-(void) pushIn {
    [self setNumber: number + 1];
}

-(void) pullOut {
    [self setNumber: number - 1];
}

-(void) printNumber {
    NSLog(@"Box number is: %d", number);
}
@end

这个类的作用是，初始化Box时，number数字是0，可以用pushIn方法往Box里推入数字，每调用一次，number加1.当number数字大于等于6时产生SomethingException异常,告诉你数字达到或超过6了，超过10时产生SomeOverException异常，小于1时产生普通的NSException异常。

这里写[SomeOverExceptionexceptionWithName:@"BoxOverflowException"reason:@"Thelevelisabove100"异常的名称和理由，在捕获时可以获取。

3、使用Box，在适当添加下捕获Box类的异常

3.1、在没超过6时，没有异常

- (void)viewDidLoad
{
    [super viewDidLoad];    
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
    Box *box = [[Box alloc]init];
    for (int i = 0; i < 5; i++) {
        [box pushIn];
        [box printNumber];
    }
}

打印结果：

Boxnumberis:1

Boxnumberis:2

Boxnumberis:3

Boxnumberis:4

Boxnumberis:5

3.2超过6，产生异常

for (int i = 0; i < 11; i++) {
            [box pushIn];
            [box printNumber];
    }

2012-07-04 09:12:05.889 ObjectiveCTest[648:f803] Box number is: 1
2012-07-04 09:12:05.890 ObjectiveCTest[648:f803] Box number is: 2
2012-07-04 09:12:05.890 ObjectiveCTest[648:f803] Box number is: 3
2012-07-04 09:12:05.890 ObjectiveCTest[648:f803] Box number is: 4
2012-07-04 09:12:05.891 ObjectiveCTest[648:f803] Box number is: 5
2012-07-04 09:12:05.891 ObjectiveCTest[648:f803] *** Terminating app due to uncaught exception 'BoxWarningException', reason: 'The number is above or at 60'

这是时，程序抛出异常崩溃了。那怎么使程序不崩溃呢，做异常处理。

3.3、加上异常处理

for (int i = 0; i < 11; i++) {
        @try {
            [box pushIn];
        }
        @catch (SomethingException *exception) {
            NSLog(@"%@ %@", [exception name], [exception reason]);
        }
        @catch (SomeOverException *exception) {
            NSLog(@"%@", [exception name]);
        }
        @finally {
            [box printNumber];
        }
    }

运行，程序没有崩溃，打印结果：

2012-07-04 09:14:35.165 ObjectiveCTest[688:f803] Box number is: 1
2012-07-04 09:14:35.167 ObjectiveCTest[688:f803] Box number is: 2
2012-07-04 09:14:35.167 ObjectiveCTest[688:f803] Box number is: 3
2012-07-04 09:14:35.167 ObjectiveCTest[688:f803] Box number is: 4
2012-07-04 09:14:35.167 ObjectiveCTest[688:f803] Box number is: 5
2012-07-04 09:14:35.167 ObjectiveCTest[688:f803] BoxWarningException The number is above or at 60
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] Box number is: 6
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] BoxWarningException The number is above or at 60
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] Box number is: 7
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] BoxWarningException The number is above or at 60
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] Box number is: 8
2012-07-04 09:14:35.168 ObjectiveCTest[688:f803] BoxWarningException The number is above or at 60
2012-07-04 09:14:35.169 ObjectiveCTest[688:f803] Box number is: 9
2012-07-04 09:14:35.169 ObjectiveCTest[688:f803] BoxWarningException The number is above or at 60
2012-07-04 09:14:35.169 ObjectiveCTest[688:f803] Box number is: 10
2012-07-04 09:14:35.169 ObjectiveCTest[688:f803] BoxOverflowException
2012-07-04 09:14:35.225 ObjectiveCTest[688:f803] Box number is: 11

超过10时，SomeOverException异常抛出。

3.4、小于0时的异常

在Box类的setNumber里，当number小于0时，我们抛出普通异常。

@try {
        [box setNumber:-10];
    }
    @catch (NSException *exception) {
        NSLog(@"%@",[exception name]);
    }
    @finally {
        [box printNumber];
    }

打印结果：

2012-07-04 09:17:42.405 ObjectiveCTest[753:f803] BoxUnderflowException
2012-07-04 09:17:42.406 ObjectiveCTest[753:f803] Box number is: -10