Objective-C文法之異常處理

Objective-C的異常比較像Java的異常處理，也有@finally的處理，不管異常是否捕獲都都要執行。異常處理捕獲的文法

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

@catch{} 塊 對異常的捕獲應該先細後粗，即是說先捕獲特定的異常，再使用一些泛些的異常類型。我們自訂兩個異常類，看看異常異常處理的使用。1、建立SomethingException，SomeOverException這兩個類，都繼承與NSException類。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、建立Box類，在某些條件下產生異常。

#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

這個類的作用是，初始化Box時，number數字是0，可以用pushIn 方法往Box裡推入數字，每調用一次，number加1.當number數字大於等於6時產生SomethingException異常,告訴你數字達到或超過6了，超過10時產生SomeOverException異常，小於1時產生普通的NSException異常。

這裡寫 [SomeOverException  exceptionWithName:@"BoxOverflowException"  reason:@"The level is above 100"異常的名稱和理由，在捕獲時可以擷取。

3、使用Box，在適當添加下捕獲Box類的異常3.1、在沒超過6時，沒有異常

- (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];    }}

列印結果：

Box number is: 1

Box number is: 2

Box number is: 3

Box number is: 4

Box number is: 5

3.2 超過6，產生異常

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'

這是時，程式拋出異常崩潰了。那怎麼使程式不崩潰呢，做異常處理。3.3、加上異常處理

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];        }    }

運行，程式沒有崩潰，列印結果：

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

超過10時，SomeOverException異常拋出。3.4 、小於0時的異常在Box類的setNumber裡，當number小於0時，我們拋出普通異常。

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

列印結果

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