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