基础知识

支持系统和机型

iOS系统的指纹识别功能最低支持的机型为iPhone 5s，最低支持系统为iOS 8，虽然安装iOS 7系统的5s机型可以使用系统提供的指纹解锁功能，但由于API并未开放，所以理论上第三方软件不可使用。

依赖框架

LocalAuthentication.framework

#import <LocalAuthentication/LocalAuthentication.h>

注意事项

做iOS 8以下版本适配时，务必进行API验证，避免调用相关API引起崩溃。

使用类

LAContext 指纹验证操作对象

代码

- (void)authenticateUser
{
    //初始化上下文对象
    LAContext* context = [[LAContext alloc] init];
    //错误对象
    NSError* error = nil;
    NSString* result = @"Authentication is needed to access your notes.";
    
    //首先使用canEvaluatePolicy 判断设备支持状态
    if ([context canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics error:&error]) {
        //支持指纹验证
        [context evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics localizedReason:result reply:^(BOOL success, NSError *error) {
            if (success) {
                //验证成功，主线程处理UI
            }
            else
            {
                NSLog(@"%@",error.localizedDescription);
                switch (error.code) {
                    case LAErrorSystemCancel:
                    {
                        NSLog(@"Authentication was cancelled by the system");
                        //切换到其他APP，系统取消验证Touch ID
                        break;
                    }
                    case LAErrorUserCancel:
                    {
                        NSLog(@"Authentication was cancelled by the user");
                        //用户取消验证Touch ID
                        break;
                    }
                    case LAErrorUserFallback:
                    {
                        NSLog(@"User selected to enter custom password");
                        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                            //用户选择输入密码，切换主线程处理
                        }];
                        break;
                    }
                    default:
                    {
                        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                           //其他情况，切换主线程处理 
                        }];
                        break;
                    }
                }
            }
        }];
    }
    else
    {
        //不支持指纹识别，LOG出错误详情
        
        switch (error.code) {
            case LAErrorTouchIDNotEnrolled:
            {
                NSLog(@"TouchID is not enrolled");
                break;
            }
            case LAErrorPasscodeNotSet:
            {
                NSLog(@"A passcode has not been set");
                break;
            }
            default:
            {
                NSLog(@"TouchID not available");
                break;
            }
        }
        
        NSLog(@"%@",error.localizedDescription);
    }
}

typedef NS_ENUM(NSInteger, LAError)
{
    //授权失败
    LAErrorAuthenticationFailed = kLAErrorAuthenticationFailed,
    
    //用户取消Touch ID授权
    LAErrorUserCancel           = kLAErrorUserCancel,
    
    //用户选择输入密码
    LAErrorUserFallback         = kLAErrorUserFallback,
    
    //系统取消授权(例如其他APP切入)
    LAErrorSystemCancel         = kLAErrorSystemCancel,
    
    //系统未设置密码
    LAErrorPasscodeNotSet       = kLAErrorPasscodeNotSet,

    //设备Touch ID不可用，例如未打开
    LAErrorTouchIDNotAvailable  = kLAErrorTouchIDNotAvailable,
    
    //设备Touch ID不可用，用户未录入
    LAErrorTouchIDNotEnrolled   = kLAErrorTouchIDNotEnrolled,
} NS_ENUM_AVAILABLE(10_10, 8_0);

操作流程

首先判断系统版本，iOS 8及以上版本执行-(void)authenticateUser方法，方法自动判断设备是否支持和开启Touch ID。

iOS 9

感谢秋儿指出iOS 9加入了三种新的错误类型。

/// Authentication was not successful, because there were too many failed Touch ID attempts and
    /// Touch ID is now locked. Passcode is required to unlock Touch ID, e.g. evaluating
    /// LAPolicyDeviceOwnerAuthenticationWithBiometrics will ask for passcode as a prerequisite.
    LAErrorTouchIDLockout   NS_ENUM_AVAILABLE(10_11, 9_0) = kLAErrorTouchIDLockout,

    /// Authentication was canceled by application (e.g. invalidate was called while
    /// authentication was in progress).
    LAErrorAppCancel        NS_ENUM_AVAILABLE(10_11, 9_0) = kLAErrorAppCancel,

    /// LAContext passed to this call has been previously invalidated.
    LAErrorInvalidContext   NS_ENUM_AVAILABLE(10_11, 9_0) = kLAErrorInvalidContext

其中，LAErrorTouchIDLockout是在8.0中也会出现的情况，但并未归为单独的错误类型，这个错误出现，源自用户多次连续使用Touch ID失败，Touch ID被锁，需要用户输入密码解锁，这个错误的交互LocalAuthentication.framework已经封装好了，不需要开发者关心。

LAErrorAppCancel和LAErrorSystemCancel相似，都是当前软件被挂起取消了授权，但是前者是用户不能控制的挂起，例如突然来了电话，电话应用进入前台，APP被挂起。后者是用户自己切到了别的应用，例如按home键挂起。

LAErrorInvalidContext很好理解，就是授权过程中,LAContext对象被释放掉了，造成的授权失败。