how to debug when react native app doesnt start on ios device

Question

when I start the react-native app, it always starts, as long as I have it connected to the mac.

As soon as I disconnect the iphone, the app mostly hangs on startup and crashes, without any further error messages. But, other errors will show up on the screen (red and yellow flagged errors).

When I doubleclick the homebutton, strangely, the app seems to be in a started state, but I am not able to switch to it.

How can you debug best such a behavior and find the the things causing the strange startup behavior?

Update:

I found those errors

2016-08-08 16:25:23.604 [warn][tid:main][RCTEventEmitter.m:54] Sending `websocketFailed` with no listeners registered.
2016-08-08 16:25:23.612113 PhoenixApp[525:64778] Sending `websocketFailed` with no listeners registered.
2016-08-08 16:25:23.742 [warn][tid:com.facebook.react.JavaScript] The regenerator/runtime module is deprecated; please import regenerator-runtime/runtime instead.
2016-08-08 16:25:23.741927 PhoenixApp[525:65027] The regenerator/runtime module is deprecated; please import regenerator-runtime/runtime instead.
2016-08-08 16:25:23.860 [info][tid:com.facebook.react.JavaScript] Running application "PhoenixApp" with appParams: {"rootTag":1,"initialProps":{}}. __DEV__ === true, development-level warning are ON, performance optimizations are OFF
2016-08-08 16:25:23.860251 PhoenixApp[525:65027] Running application "PhoenixApp" with appParams: {"rootTag":1,"initialProps":{}}. __DEV__ === true, development-level warning are ON, performance optimizations are OFF
2016-08-08 16:25:24.139 [info][tid:com.facebook.react.JavaScript] null
2016-08-08 16:25:24.138808 PhoenixApp[525:65027] null

followed by (each second)

2016-08-08 16:31:43.159749 PhoenixApp[525:64956] [] __nw_connection_get_connected_socket_block_invoke 176 Connection has no connected handler
2016-08-08 16:31:45.372329 PhoenixApp[525:65445] [] __nw_connection_get_connected_socket_block_invoke 177 Connection has no connected handler
2016-08-08 16:31:47.575208 PhoenixApp[525:64955] [] __nw_connection_get_connected_socket_block_invoke 178 Connection has no connected handler
2016-08-08 16:31:49.788935 PhoenixApp[525:65445] [] __nw_connection_get_connected_socket_block_invoke 179 Connection has no connected handler
2016-08-08 16:31:51.970877 PhoenixApp[525:64955] [] __nw_connection_get_connected_socket_block_invoke 180 Connection has no connected handler
2016-08-08 16:31:54.173791 PhoenixApp[525:65445] [] __nw_connection_get_connected_socket_block_invoke 181 Connection has no connected handler

AppDelegate.m

/**
 * Copyright (c) 2015-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 */

#import "AppDelegate.h"

#import "RCTBundleURLProvider.h"
#import "RCTRootView.h"

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
  NSURL *jsCodeLocation;

  [[RCTBundleURLProvider sharedSettings] setDefaults];
  jsCodeLocation = [[RCTBundleURLProvider sharedSettings] jsBundleURLForBundleRoot:@"index.ios" fallbackResource:nil];

  RCTRootView *rootView = [[RCTRootView alloc] initWithBundleURL:jsCodeLocation
                                                      moduleName:@"PhoenixTrello"
                                               initialProperties:nil
                                                   launchOptions:launchOptions];
  rootView.backgroundColor = [[UIColor alloc] initWithRed:1.0f green:1.0f blue:1.0f alpha:1];

  self.window = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
  UIViewController *rootViewController = [UIViewController new];
  rootViewController.view = rootView;
  self.window.rootViewController = rootViewController;
  [self.window makeKeyAndVisible];
  return YES;
}

@end

setup:

react-native-cli: 1.0.0 react-native: 0.32.0-rc.0 react: 15.3.0

Xcode 8 beta 4, iOS 10

Answer 1

Try build app in release mode.

Edit scheme

• Select Run tab
• Select Info tab
• Change Build Configuration to Release
• Build and run project

While your device is connected to Xcode, Xcode performs some action/optimizations and attach debugger. Sometimes this actions prevent from showing errors. If you run your app in release mode, this actions are omitted and app crashes.