Object-C 進階學習[ 異常情況（Exceptions） ]

• 異常處理只有 Mac OS X 10.3 UP 才支援。

• CupWarningException.h

  #import <Foundation/NSException.h>
  
  @interface CupWarningException: NSException
  @end

• CupWarningException.m

  #import "CupWarningException.h"
  
  @implementation CupWarningException
  @end

• CupOverflowException.h

  #import <Foundation/NSException.h>
  
  @interface CupOverflowException: NSException
  @end

• CupOverflowException.m

  #import "CupOverflowException.h"
  
  @implementation CupOverflowException
  @end

• Cup.h

  #import <Foundation/NSObject.h>
  
  @interface Cup: NSObject {
      int level;
  }
  
  -(int) level;
  -(void) setLevel: (int) l;
  -(void) fill;
  -(void) empty;
  -(void) print;
  @end

• Cup.m

  #import "Cup.h"
  #import "CupOverflowException.h"
  #import "CupWarningException.h"
  #import <Foundation/NSException.h>
  #import <Foundation/NSString.h>
  
  @implementation Cup
  -(id) init {
      self = [super init];
  
      if ( self ) {
          [self setLevel: 0];
      }
  
      return self;
  }
  
  -(int) level {
      return level;
  }
  
  -(void) setLevel: (int) l {
      level = l;
  
      if ( level > 100 ) {
          // throw overflow
          NSException *e = [CupOverflowException
              exceptionWithName: @"CupOverflowException"
              reason: @"The level is above 100"
              userInfo: nil];
          @throw e;
      } else if ( level >= 50 ) {
          // throw warning
          NSException *e = [CupWarningException
              exceptionWithName: @"CupWarningException"
              reason: @"The level is above or at 50"
              userInfo: nil];
          @throw e;
      } else if ( level < 0 ) {
          // throw exception
          NSException *e = [NSException
              exceptionWithName: @"CupUnderflowException"
              reason: @"The level is below 0"
              userInfo: nil];
          @throw e;
      }
  }
  
  -(void) fill {
      [self setLevel: level + 10];
  }
  
  -(void) empty {
      [self setLevel: level - 10];
  }
  
  -(void) print {
      printf( "Cup level is: %i\n", level );
  }
  @end

• main.m

  #import "Cup.h"
  #import "CupOverflowException.h"
  #import "CupWarningException.h"
  #import <Foundation/NSString.h>
  #import <Foundation/NSException.h>
  #import <Foundation/NSAutoreleasePool.h>
  #import <stdio.h>
  
  int main( int argc, const char *argv[] ) {
      NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
      Cup *cup = [[Cup alloc] init];
      int i;
  
      // this will work
      for ( i = 0; i < 4; i++ ) {
          [cup fill];
          [cup print];
      }
  
      // this will throw exceptions
      for ( i = 0; i < 7; i++ ) {
          @try {
              [cup fill];
          } @catch ( CupWarningException *e ) {
              printf( "%s: ", [[e name] cString] );
          } @catch ( CupOverflowException *e ) {
              printf( "%s: ", [[e name] cString] );
          } @finally {
              [cup print];
          }
      }
  
      // throw a generic exception
      @try {
          [cup setLevel: -1];
      } @catch ( NSException *e ) {
          printf( "%s: %s\n", [[e name] cString], [[e reason] cString] );
      }
  
      // free memory 
      [cup release];
      [pool release];
  }

• output

  Cup level is: 10
  Cup level is: 20
  Cup level is: 30
  Cup level is: 40
  CupWarningException: Cup level is: 50
  CupWarningException: Cup level is: 60
  CupWarningException: Cup level is: 70
  CupWarningException: Cup level is: 80
  CupWarningException: Cup level is: 90
  CupWarningException: Cup level is: 100
  CupOverflowException: Cup level is: 110
  CupUnderflowException: The level is below 0

• NSAutoreleasePool 是一個記憶體管理類別。
• Exceptions（異常情況）的丟出不需要擴充（extend）NSException 物件，你可簡單的用 id 來代表它： @catch ( id e ) { ... }
• 還有一個 finally 區塊，它的行為就像 Java 的異常處理方式，finally 區塊的內容保證會被呼叫。
• Cup.m 裡的 @"CupOverflowException" 是一個 NSString 常數物件。在 Objective-C 中，@ 符號通常用來代表這是語言的衍生部分。C 語言形式的字串（C string）就像 C/C++ 一樣是 "String constant" 的形式，型別為 char *。