NSAssert和NSParameterAssert在开发环境中经常被使用，调试和验证代码参数的完整性，断言为真，则表明程序运行正常，而断言为假，则意味着它已经在代码中发现了意料之外的错误。xCode中的断言在Debug模式默认是开启的，Realse版本中是禁用的.
基础断言
基础类库中了两种断言，NSAssert和NSParameterAssert是OC断言，NSCAssert和NSCParameterAssert是C语言断言。先来看一下NSAssert定义:
<pre><code>The NSAssert macro evaluates the condition and serves as a front end to the assertion handler. Each thread has its own assertion handler, which is an object of class NSAssertionHandler. When invoked, an assertion handler prints an error message that includes the method and class names (or the function name). It then raises an NSInternalInconsistencyException exception. If condition evaluates to NO, the macro invokes handleFailureInMethod:object:file:lineNumber:description: on the assertion handler for the current thread, passing desc as the description string. This macro should be used only within Objective-C methods. Assertions are disabled if the preprocessor macro NS_BLOCK_ASSERTIONS is defined. **Important:Important** Do not call functions with side effects in the condition parameter of this macro. The condition parameter is not evaluated when assertions are disabled, so if you call functions with side effects, those functions may never get called when you build the project in a non-debug configuration. **Note:Note** Not all release configurations disable assertions by default.</code></pre>
NSParameterAssert的定义:
<pre>Assertions evaluate a condition and, if the condition evaluates to false, call the assertion handler for the current thread, passing it a format string and a variable number of arguments. Each thread has its own assertion handler, which is an object of class NSAssertionHandler. When invoked, an assertion handler prints an error message that includes method and class names (or the function name). It then raises an NSInternalInconsistencyException exception. This macro validates a parameter for an Objective-C method. Simply provide the parameter as the condition argument. The macro evaluates the parameter and, if it is false, it logs an error message that includes the parameter and then raises an exception. Assertions are disabled if the preprocessor macro NS_BLOCK_ASSERTIONS is defined. All assertion macros return void. **Important:Important** Do not call functions with side effects in the condition parameter of this macro. The condition parameter is not evaluated when assertions are disabled, so if you call functions with side effects, those functions may never get called when you build the project in a non-debug configuration. **Note:Note** Not all release configurations disable assertions by default.</pre>
两者的定义类似，大概意思就是如果是false就会调用当前线程Assertion Hanlder进行处理，非Debug模式下可能所有的断言都不会调用，最后一句很重要，并不是所有的发布配置会禁用断言，如果想看断言是否禁用，需要看一下设置:
简单测试:
<pre><code>`
NSString *result=@"中山郎";
NSInteger count=10;
NSAssert(count>10, @"总数必须大于10");
NSLog(@"断言执行之后");
</code></pre> 崩溃信息： <pre><code>
** FECategory[23811:248235] *** Assertion failure in -[ViewController setupAssert], /ViewController.m:45**
** FECategory[23811:248235] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '****总数必须大于****10'**`</code></pre>
NSAssertionHandler
NSAssert异常处理的时候默认是NSAssertionHandler处理的，不过我们可以自定自己的Handler，实现两个方法:
<pre><code>`
- (void)handleFailureInMethod:(SEL)selector object:(id)object file:(NSString *)fileName lineNumber:(NSInteger)line description:(nullable NSString *)format,... NS_FORMAT_FUNCTION(5,6);
- (void)handleFailureInFunction:(NSString *)functionName file:(NSString *)fileName lineNumber:(NSInteger)line description:(nullable NSString *)format,... NS_FORMAT_FUNCTION(4,5);`</code></pre>
handleFailureInMethod处理OC方法中的断言，handleFailureInFunction处理C函数中的断言
自定义继承自NSAssertionHandler的类FEAssertionHandler：
<pre><code>`
@implementation FEAssertionHandler
-(void)handleFailureInMethod:(SEL)selector object:(id)object file:(NSString *)fileName lineNumber:(NSInteger)line description:(NSString *)format, ...{
NSLog(@"FlyElephant-FEAssertionHandler: Method %@ for object %@ in %@--line:%li", NSStringFromSelector(selector), object, fileName, (long)line);
}
-(void)handleFailureInFunction:(NSString *)functionName file:(NSString *)fileName lineNumber:(NSInteger)line description:(NSString *)format, ...{
NSLog(@"FlyElephant-FEAssertionHandler:Function (%@) in %@--line:%li", functionName, fileName, (long)line);
}
@end</code></pre> AppDelegate中设置断言处理: <pre><code>
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
// Override point for customization after application launch.
FEAssertionHandler *hanlder=[[FEAssertionHandler alloc]init];
[[[NSThread currentThread] threadDictionary] setValue:hanlder forKey:NSAssertionHandlerKey];
return YES;
}