diff --git a/QRCode/QRCodeTests/QRCodeTests.swift b/QRCode/QRCodeTests/QRCodeTests.swift
index 31fe592..245b24e 100644
--- a/QRCode/QRCodeTests/QRCodeTests.swift
+++ b/QRCode/QRCodeTests/QRCodeTests.swift
@@ -25,12 +25,4 @@ class QRCodeTests: XCTestCase {
         // This is an example of a functional test case.
         XCTAssert(true, "Pass")
     }
-    
-    func testPerformanceExample() {
-        // This is an example of a performance test case.
-        self.measure() {
-            // Put the code you want to measure the time of here.
-        }
-    }
-    
 }
diff --git a/QRCode/Source/QRCode.swift b/QRCode/Source/QRCode.swift
index d278d12..b9eb138 100644
--- a/QRCode/Source/QRCode.swift
+++ b/QRCode/Source/QRCode.swift
@@ -9,8 +9,18 @@
 import UIKit
 import AVFoundation
 
+
 open class QRCode: NSObject, AVCaptureMetadataOutputObjectsDelegate {
     
+    @objc public enum Error : Int, Swift.Error{
+        public typealias RawValue = Int
+
+        case sessionAlreadyRunning
+        case sessionAlreadyStopped
+        case cannotOpenCamera
+        case cannotSaveData
+    }
+    
     /// corner line width
     var lineWidth: CGFloat
     /// corner stroke color
@@ -135,33 +145,31 @@ open class QRCode: NSObject, AVCaptureMetadataOutputObjectsDelegate {
     ///
     ///  - parameter view:       the scan view, the preview layer and the drawing layer will be insert into this view
     ///  - parameter completion: the completion call back
-    open func prepareScan(_ view: UIView, completion:@escaping (_ stringValue: String)->()) {
+    open func prepareScan(_ view: UIView, completion:@escaping (_ stringValue: String)->()) throws {
         
         scanFrame = view.bounds
         
         completedCallBack = completion
         currentDetectedCount = 0
         
-        setupSession()
+        try setupSession()
         setupLayers(view)
     }
     
     /// start scan
-    open func startScan() {
+    open func startScan() throws {
         if session.isRunning {
             print("the  capture session is running")
-            
-            return
+            throw QRCode.Error.sessionAlreadyRunning
         }
         session.startRunning()
     }
     
     /// stop scan
-    open func stopScan() {
+    open func stopScan() throws {
         if !session.isRunning {
             print("the capture session is not running")
-            
-            return
+            throw QRCode.Error.sessionAlreadyStopped
         }
         session.stopRunning()
     }
@@ -173,20 +181,20 @@ open class QRCode: NSObject, AVCaptureMetadataOutputObjectsDelegate {
         view.layer.insertSublayer(previewLayer, at: 0)
     }
     
-    func setupSession() {
+    func setupSession() throws{
         if session.isRunning {
             print("the capture session is running")
-            return
+            throw QRCode.Error.sessionAlreadyRunning
         }
         
         if !session.canAddInput(videoInput) {
             print("can not add input device")
-            return
+            throw QRCode.Error.cannotOpenCamera
         }
         
         if !session.canAddOutput(dataOutput) {
             print("can not add output device")
-            return
+            throw QRCode.Error.cannotSaveData
         }
         
         session.addInput(videoInput)
diff --git a/QRCodeDemo/QRCodeDemo/ViewController.swift b/QRCodeDemo/QRCodeDemo/ViewController.swift
index 5ea5bc4..2e87f67 100644
--- a/QRCodeDemo/QRCodeDemo/ViewController.swift
+++ b/QRCodeDemo/QRCodeDemo/ViewController.swift
@@ -19,8 +19,12 @@ class ViewController: UIViewController {
     override func viewDidLoad() {
         super.viewDidLoad()
         
-		scanner.prepareScan(view) { (stringValue) -> () in
-            print(stringValue)
+        do{
+            try scanner.prepareScan(view) { (stringValue) -> () in
+                print(stringValue)
+            }
+        }catch(_){
+            print("Error during preparing scan")
         }
         // test scan frame
         scanner.scanFrame = view.bounds
@@ -29,7 +33,11 @@ class ViewController: UIViewController {
     override func viewDidAppear(_ animated: Bool) {
         super.viewDidAppear(animated)
         
-        scanner.startScan()
+        do{
+            try scanner.startScan()
+        }catch(_){
+            print("Error during starting scan")
+        }
     }
     
     override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
diff --git a/QRcodeDemoObjc/QRcodeDemoObjc/ViewController.m b/QRcodeDemoObjc/QRcodeDemoObjc/ViewController.m
index 1c017ee..c4d3210 100644
--- a/QRcodeDemoObjc/QRcodeDemoObjc/ViewController.m
+++ b/QRcodeDemoObjc/QRcodeDemoObjc/ViewController.m
@@ -21,17 +21,25 @@ - (void)viewDidLoad {
     [super viewDidLoad];
     
     // alloc and prepare for scan
+    NSError *error;
     self.scanner = [[QRCode alloc] init];
-    [self.scanner prepareScan:self.view completion:^(NSString * __nonnull stringValue) {
+    [self.scanner prepareScan:self.view error:&error completion:^(NSString * _Nonnull stringValue) {
         NSLog(@"%@", stringValue);
     }];
+    if (error) {
+        NSLog(@"Error during preparing scan");
+    }
 }
 
 - (void)viewDidAppear:(BOOL)animated {
     [super viewDidAppear:animated];
     
     // start scan
-    [self.scanner startScan];
+    NSError *error;
+    [self.scanner startScanAndReturnError:&error];
+    if (error) {
+        NSLog(@"Error during starting scan");
+    }
 }
 
 - (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {