Develop Reference

Flutter Device Preview equivalent in React Native App?

recently i've found this plugin can preview many device of Android or iOS device in one installed apps,
gif of the plugin is at https://github.com/aloisdeniel/flutter_device_preview
does react native has something like this?
I know there's some similar plugin like Expo and Appetize but i need something like this,
i mean i can preview of many device in my apps, not installing my app on many devices.

I think Apptize seems to be the most similar option available.
flutter_device_preview is certainly a more practical/simple solution. You change the parameters or device, the app is rendered instantaneously. However, it provides a first-order approximation, as said in flutter_device_preview repo:
Think of Device Preview as a first-order approximation of how your app
looks and feels on a mobile device. With Device Mode you don't
actually run your code on a mobile device. You simulate the mobile
user experience from your laptop, desktop or tablet.
But with Apptize you will be able to run a native app, with no approximations, using a the true mobile-OS on any PC/Mac/Linux OS via web. But of course, has some limitations (currently 7 devices only), and has another purpose.
Here you can generate an iframe to start the testing:
https://appetize.io/docs#embed-your-app

Where can I run into pitfalls when using an Android emulator over a real device for app development?

I want to get into developing apps for Android because of its large userbase. However, at the moment I do not have a smartphone. I'm also not too keen on getting an Android phone in the future.
What problems could I encounter when developing apps for Android solely using an emulator? I could use my parents' Android phones for the occasional testing, but will an emulator prove sufficient otherwise? Furthermore, is it possible to set up an Android Virtual Device with low-end hardware on my computer to test out apps with low-end phones?
note0: I do not plan on developing apps which require use of the phone's sensors (such as gyros, GPS, etc.) I only plan on developing apps which require internet access for API calls and perhaps microphone usage (I can't give away too many details right now ;) )
note1: I know this question has been asked before on stackoverflow (albeit 4 years ago). However, I feel it is important to revisit the question because emulators, hardware, and Android itself have changed enormously in the last few years.
edit: My main issues are problems like the app looking weird on the phone despite it looking fine on the emulator or working slowly on the phone but working fine on the emulator. Will every app which works fine on the emulator work fine on the phone as well with a similar hardware configuration as the emulator? I'm in particular targetting devices with Android 4.x/5.x.

Without a doubt, the biggest problem is that many functions like push
notifications, maps, geo-location, in-app vending etc. cannot be
tested with an emulator. The reason is that all these functions are
present in the Google Play Services library which is not present on
emulators.
Another important reason for testing on hardware devices is that the
OS implementation is changed by manufacturers like Samsung, HTC and
others, and therefore, certain problems & exceptions are not observed
till a real device is used.
But the most important reason has to be that running apps on an
actual phone is a hell of a lot more fun that it is with the painfully
slow and retarded emulators! One of the most important things about
Android is that anybody with an Android phone can easily explore
computer science concepts by running simple programs on their phone.
It is incredibly easy, even for a novice, to just fire up their
Android device and start exploring the world of programming! Till just a few years back, it was incredibly difficult to debug an app on a Symbian or Palm or WinCE device. iOS is still as retarded when it comes to running the simplest apps, you have to create a provisioning profile to run a Hello World app on an iPhone!
EDIT:
If your processor supports Intel virtualization, then the emulator can be made to run as fast as a real device. You need to check that out. If your PC is low-end in the sense that it does not support such virtualization, then running and testing Android apps will be a painful, tedious experience that will suck all the fun out of the development process.
Emulators do have one significant advantage over real devices - an emulator is able to access web services on a localhost. If you are still in development phase on your local server and have not yet shifted to a publicly hosted environment, the emulator can easily access web services on a local server. All you have to do is put your computer on the same network as the local server and you're good to go. A real device cannot access a localhost; its impossible, as far as I know.
The Android emulator is a true emulator - it emulates sensors as well, so in fact you can test sensor programming (with some limitations) on an emulator. It cannot do things like GPS, maps etc. because those features have been migrated to the Google Play Services library, and they require a valid Google account to be used, which is why that library is not present on an emulator. If you only plan to work with web services, APIs', GUIs' then I suppose an emulator is more than sufficient. Unlike the iOS simulator, an Android emulator generally exhibits the WYSIWYG behavior, so your GUI will not differ on actual devices, although you do need to test your GUI on multiple AVDs' with different screen form factors.

implementing scanner using phonegap

I was using Android SDK for my application development. Now I am in a situation to move on cross platform development so, I started learning Phonegap as it is well suitable for cross platform.
I want to use external scanner device and card swipper with android device. I googled it but i didnt find any useful ideas.
I explored phonegap.com/ but it explains only about using device sensors,contacts and media information..etc. But It didnt say about implementing external hardware devices.
Please help me.

For a barcode scanner, you probably want to get a bluetooth capable device, as USB devices can be problematic to connect to different devices. Start by using it in HID mode, which emulates a keyboard (nb, doesnt play nice on iPad and other apple devices, popup keyboard disappears, but android ok). Later you can try using SPP mode. When using HID mode with phonegap, you dont need to do anything special, just read as keystrokes in html pages.
There is also a phonegap plugin for zxing to use the camera, but you asked about external physical scanners, which are faster for bulk scanning.
Haven't tried connecting card swipe readers to android yet

Analog video capture to Android phone

I am looking for a way of displaying an analog video stream on an android phone. On a pc/mac/etc you can achieve this using a cheap usb analog-digital converter such as a grabby: http://www.terratec.net/en/products/Grabby_82248.html, and then view on VLC, for example.
Would such a thing work (in theory) on android if the proper drivers were available? (ie. are there any hardware issues which make this impossible?)
Does anyone know if such a device with android drivers is available?
Ultimately I want to make an app which interfaces with the grabby (or similar device) and allows the user to view video on the android and capture and send short clips.

First of all the Android device needs to support USB Host. This limits your userbase significantly.
Then there is the problem with power. Some USB Host devices will be incompatible simply because an Android phone will not be able to push enough power through to port to get it running properly.
I'm not sure about the drivers, but I'm 99% sure it won't work "out of the box".
You should certainly take a look at THIS project. It is pretty similar to what you are trying to do. Maybe you should consider getting in touch with that person.
EDIT:
Based on what it took to get that DVB-T dongle running in the project i mentioned above the chances of creating an app that everyone will be able to simply download and use are EXTREMELY slim. Getting that dongle running required using a modified kernel and special scripts. Of course I could be wrong. You can continue the research yourself or wait for someone with more experience than me to reply.

iOS and Android Installation (direct testing)

Is is possible to directly deploy apps on ios or android devices just for testing?
My Background:
I am currently developing iOS and Android apps but using only emulators.
Maybe you can consider me as an intermediate mobile developer but newbie in direct installation of application created.
Also, I'm afraid my iPhone or my android devices get destroyed if I will try some of the blogs tutorials found on the net.

You can try your application on actual mobile devices on both platforms.
With Android is pretty straightforward: just plugin your device to your computer's USB port. If you happen to have proper drivers for it, whenever you run your application from Eclipse IDE, you will be asked if you want to run your application directly on the device. Make sure you enable debugging on your device.
With iOS, it's also fairly easy. Just plug your iPhone/iPad/iPodTouch to your Mac, launch XCode and select device before pressing run. You might need to create a provisioning profile for it (you will need a Apple iOS Developer Account for this).
I don't think you will ruin any device just by following (and installing) tutorials from the net.

For Android: Yes, you can run the apps directly from Eclipse or Netbeans on your device, works exactly as with the emulator. To make real apps you have to test them on real devices!
You wont destroy your device. Read the Android SDK "getting started" stuff.
Can't really say about iOS though.

You can always buy a second hand cheap phone and use it.
I started developing apps in the Android emulator and I was surprised about the big difference between the emulator en a real device, which really made it worth to buy one of these terminals.