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：

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@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。

1、新建SomethingException，SomeOverException这两个类，都继承与NSException类。

SomethingException.h

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#import <Foundation/Foundation.h>



@interface SomethingException : NSException



@end

SomethingException.m

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#import "SomethingException.h"



@implementation SomethingException



@end

SomeOverException.h

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#import <Foundation/Foundation.h>



@interface SomeOverException : NSException



@end

SomeOverException.m

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#import "SomeOverException.h"



@implementation SomeOverException



@end

2、新建Box类，在某些条件下产生异常。

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#import <Foundation/Foundation.h>



@interface Box : NSObject

{

NSInteger number;

}

-(void) setNumber: (NSInteger) num;

-(void) pushIn;

-(void) pullOut;

-(void) printNumber;

@end

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@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 [SomeOverException exceptionWithName:@"BoxOverflowException" reason:@"The level is above 100"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

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- (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：

Box number is: 1

Box number is: 2

Box number is: 3

Box number is: 4

Box number is: 5

3.2 超过6，产生异常

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for (int i = 0; i < 11; i++) {

[box pushIn];

[box printNumber];

}

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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

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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：

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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。

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@try {

[box setNumber:-10];

}

@catch (NSException *exception) {

NSLog(@"%@",[exception name]);

}

@finally {

[box printNumber];

}

打印结果：

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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

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