Adding error handling to the connection flow.

remoting/ios/app/pin_entry_view.mm

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #if !defined(__has_feature) || !__has_feature(objc_arc)
 #error "This file requires ARC support."
 #endif
 
 #import "remoting/ios/app/pin_entry_view.h"
 
 #import "ios/third_party/material_components_ios/src/components/Buttons/src/MaterialButtons.h"
 #import "remoting/ios/app/remoting_theme.h"
 
 static const CGFloat kMargin = 5.f;
-static const CGFloat kPadding = 6.f;
+static const CGFloat kPadding = 8.f;
 static const CGFloat kLineSpace = 12.f;
 
-@interface PinEntryView () {
+@interface PinEntryView ()<UITextFieldDelegate> {
   UISwitch* _pairingSwitch;
   UILabel* _pairingLabel;
   MDCFloatingButton* _pinButton;
   UITextField* _pinEntry;
 }
 @end
 
 @implementation PinEntryView
 
 @synthesize delegate = _delegate;
@@ skipping 16 matching lines @@
     _pairingLabel.text = @"Remember my PIN on this device.";
     [self addSubview:_pairingLabel];
 
     _pinButton =
         [MDCFloatingButton floatingButtonWithShape:MDCFloatingButtonShapeMini];
     [_pinButton setImage:RemotingTheme.arrowIcon forState:UIControlStateNormal];
     [_pinButton addTarget:self
                    action:@selector(didTapPinEntry:)
          forControlEvents:UIControlEventTouchUpInside];
     _pinButton.translatesAutoresizingMaskIntoConstraints = NO;
+    _pinButton.enabled = NO;
     [self addSubview:_pinButton];
 
     _pinEntry = [[UITextField alloc] init];
     _pinEntry.textColor = [UIColor whiteColor];
     _pinEntry.secureTextEntry = YES;
     _pinEntry.keyboardType = UIKeyboardTypeNumberPad;
     // TODO(nicholss): L18N this.
     _pinEntry.attributedPlaceholder = [[NSAttributedString alloc]
         initWithString:@"Enter PIN"
             attributes:@{
               NSForegroundColorAttributeName :
                   [UIColor colorWithRed:1.f green:1.f blue:1.f alpha:0.5]
             }];
+    _pinEntry.delegate = self;
     [self addSubview:_pinEntry];
   }
   return self;
 }
 
 #pragma mark - UIView
 
 - (BOOL)canBecomeFirstResponder {
   return [_pinEntry canBecomeFirstResponder];
 }
@@ skipping 26 matching lines @@
   _pairingSwitch.center = CGPointMake(
       kMargin + _pairingSwitch.frame.size.width / 2.f,
       buttonSize + _pairingSwitch.frame.size.height / 2.f + kLineSpace);
 
   _pairingLabel.frame =
       CGRectMake(kMargin + _pairingSwitch.frame.size.width + kPadding,
                  buttonSize + kLineSpace, 0.f, 0.f);
   [_pairingLabel sizeToFit];
 }
 
+#pragma mark - UITextFieldDelegate
+
+- (BOOL)textField:(UITextField*)textField
+    shouldChangeCharactersInRange:(NSRange)range
+                replacementString:(NSString*)string {
+  if (textField == _pinEntry) {
+    NSUInteger length = _pinEntry.text.length - range.length + string.length;
+    _pinButton.enabled = length >= 6;
+  }
+  return YES;
+}
+
+- (BOOL)textFieldShouldReturn:(UITextField*)textField {
+  NSLog(@"textFieldShouldReturn");
+  if ([_pinButton isEnabled]) {
+    [self didTapPinEntry:textField];
+    return YES;
+  }
+  return NO;
+}
+
+#pragma mark - Public
+
+- (void)clearPinEntry {
+  _pinEntry.text = @"";
+  _pinButton.enabled = NO;
+}
+
 #pragma mark - Private
 
 - (void)didTapPinEntry:(id)sender {
   [_delegate didProvidePin:_pinEntry.text createPairing:_pairingSwitch.isOn];
   [_pinEntry endEditing:YES];
 }
 
 @end