Develop Reference

Is it possible to do Android development using the R programming language?

I'm doing digital signal processing work in R using R Studio and think it would be a neat idea to expand this into an Android app. I know people have used Kivy to develop Android Apps in Python and I know it's possible to wrap R code in Python, so is this feasible? I read that it's possible to run R code on Android, just not sure if adding it to my project is a possibility. I also don't need to have the entire app to be written in R. Are there any examples/documentation related to this?

Turns out you can! I've built a more complex app using Kivy and the rpy2 package, but I'll demonstrate a simple example here.
You can define your R code like so:
r_code.py
from rpy2.robjects.packages import SignatureTranslatedAnonymousPackage
r_code = """
add <- function(x, y) {
return(x + y)
}
"""
r_lib = SignatureTranslatedAnonymousPackage(r_code, "r_lib")
Your Kivy App will look something like this:
main.py
from kivy.app import App
from r_code import r_lib
class MainApp(App):
def build(self):
window = BoxLayout(orientation='vertical')
label = Label(text=str(r_lib.add(3, 4)[0]))
b.add_widget(label)
return b
if __name__ == "__main__":
MainApp().run()
And if you want to package this into an Android App, you can follow the directions from the official doc. If you run into an error with rpy2, you might have to install the correct version for your system from here.
Happy coding!

Buildozer compiles apk, but it crashes on android

I am able to build an .apk, but after I install it on my android phone it simply crashes at startup. My thoughts for failing is that I am using 3rd party libraries e.g(beautifulsoup).
This is how my imports look in main.py:
from kivy.app import App
from kivy.properties import ListProperty, StringProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.gridlayout import GridLayout
from kivy.uix.button import Button
from kivy.uix.label import Label
from kivy.uix.scrollview import ScrollView
from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition
import time, os, random, urllib2, re, cookielib as cj
from bs4 import BeautifulSoup as bs
from functools import partial
I'm running mavericks 10.9.3
Does it have something to do with buildozer.spec file?
I've tried adding BeautifulSoup to app requirements, but it doesn't change a thing.
Any help would be appreciated.

I ran into this problem as well, but I was (apparently) able to get everything working fine with a workaround. Since it doesn't look like you posted a logcat, I'll assume you ran into the same issue I did.
Yes, you do need to list beautifulsoup4 as a requirement in your spec. From looking into bs4's code, it looks like bs4 is willing to use any of several "builders." It supports HTMLParser, html5lib, or lxml. I have no idea why we can't load HTMLParser, but it's actually the least preferred library of the three, and if it weren't for the fact that there's no try block around the import, it seems that everything would work fine (as long as one of the other parsing libraries was available).
With this in mind, I included one of the other libraries, and I decided to hack the import process so that Python would pretend _htmlparser loaded okay :)
This article was instructive: http://xion.org.pl/2012/05/06/hacking-python-imports/
The end result was something like this:
import imp
import sys
class ImportBlocker(object):
def __init__(self, *args):
self.black_list = args
def find_module(self, name, path=None):
if name in self.black_list:
return self
return None
def load_module(self, name):
module = imp.new_module(name)
module.__all__ = [] # Necessary because of how bs4 inspects the module
return module
sys.meta_path = [ImportBlocker('bs4.builder._htmlparser')]
from bs4 import BeautifulSoup
I also added html5lib to the requirements in buildozer.spec.
Now, is this the right way to solve the problem? I don't know. The best approach would probably be to request that the author fix it. It might be as simple as to put the import in a try block. Nevertheless, this is the approach I've gone with for the moment, and it is at least an interesting exercise, and a way to test your app until a better fix comes along.
Additionally, I should warn you that I only did this recently, so I can't 100% guarantee that it won't cause any problems, but at a minimum, it's worked well enough to get my app running and scraping the particular website I was interested in. Good luck!

I understand that this question is some years old. I think the answer #Will gave was fantastic but unfortunately, in my case, I couldn't use it because html5lib was too slow for what I was making. So this is for reference; for anyone who absolutely must use the built-in parser. It's not pretty but it's fairly manageable.
The issue
After much investigation, I nailed down the cause of the problem. In the buildozer log, I noticed that there was a problem compiling the _htmlparser file with the log reading as follows (with my project path replaced with <project-path>):
Compiling <project-path>/.buildozer/android/platform/build/dists/mypackage/private/lib/python2.7/site-packages/bs4/builder/_htmlparser.py ...
SyntaxError: ("(unicode error) \\N escapes not supported (can't load unicodedata module)", ('<project-path>/.buildozer/android/platform/build/dists/mypackage/private/lib/python2.7/site-packages/bs4/builder/_htmlparser.py', 135, None, 'data = u"\\N{REPLACEMENT CHARACTER}"\n'))
Because it was failing to compile, it wasn't being included in the built apk file. So I looked at the file and at the line that was causing the problem: data = u"\N{REPLACEMENT CHARACTER}" which should actually be replaced by data = u"\ufffd"
Quick-fix
Now the easy way out would be to modify the file right in the package. You could just edit the file mentioned in the error above which should, in theory, work but it's not recommended because every time the package gets re-installed or the code gets built from another machine the problem will be back.
Marginally better fix
It would be nice if the fix was all packaged into our code, so drawing inspiration from #Will's answer, here is the code you would need to put in before your bs4 import:
import sys
class ImportFixer(object):
def __init__(self, mname):
self.mname = mname
def find_module(self, name, path=None):
if name == self.mname:
return self
return None
def load_module(self, name):
import _htmlparser as module
module.__name__ = name
return module
sys.meta_path = [ImportFixer('bs4.builder._htmlparser')]
from bs4 import BeautifulSoup
The main difference is that you need to copy the _htmlparser.py file from the bs4 package to your current directory and fix the before mentioned line data = u"\N{REPLACEMENT CHARACTER}" with data = u"\ufffd"
Then, every time the bs4 module is imported, the import is magically intercepted and the local file is used.
Warning: if you ever update Beautiful Soup, you may need to use a more recent _htmlparser.py file in your project with the same fix made as needed.
Final comment
If I made some typos, grammar mistakes or didn't make sense at all, keep in mind that I worked most of the weekend to fix this and quite frankly I'm not thinking straight. Comment if you have any questions and I'll reply and/or edit my answer when I get a chance.

Creation of an input dialog using Kivy

I would like to create a function which only returns its value once the user input text into a TextInput and click on an ok button. For example
n = PopupInput("What number should I add?")
print 5+n
I can't figure out how to write a kivy dialog which will pause execution and wait until the user closes it. In other GUI toolkits, I would have used something like
while True:
if dialog.visable == False:
return int(textbox.text)
else:
wx.Yield()
To allow my code to just sit in one spot while allowing the GUI framework to do its thing. However, I can find no equivalent method for Kivy.
EDIT:
Here's my unsuccessful attempt (its messy)
def PopupOk(text, title='', btn_text='Continue'):
btnclose = Button(text=btn_text, size_hint_y=None, height='50sp')
content = BoxLayout(orientation='vertical')
p = Popup(title=title, content=content, size=('300dp', '300dp'),
size_hint=(None, None))
content.add_widget(Label(text=text))
ti = TextInput(height='50sp', font_size='50sp', input_type='number')
content.add_widget(ti)
def _on_d(*args):
p.is_visable = False
p.bind(on_dismiss=_on_d)
p.is_visable = True
content.add_widget(btnclose)
btnclose.bind(on_release=p.dismiss)
p.open()
while not p.is_visable:
EventLoop.idle()
return ti.text

I would think about this the other way around - what you really want to do is print the number when the popup is closed.
Assuming you have a popup with a textinput for the user to write in, you can do popup.bind(on_dismiss=some_function) to run that some_function when the popup is closed. That means all you need to do is write a function that takes a popup, retrieves the textbox text, and prints whatever answer you want.
I'm not sure how directly this fits with whatever you're really trying to do, but it's a more natural way to work with Kivy's event system. I can maybe answer differently if you have some strongly different requirement.
Edit: Seeing your edit, this is almost what you do, but I think it is a bad idea to try and beat the eventloop into submission this way rather than going with the flow. I would create a new function (as I said above) that takes a textinput and does whatever you really want. By binding on_dismiss to this function, you let kivy take care of starting your computation later whenever the user gets around to dismissing the popup.

Kivy is really built around the principle of events and async callbacks. Because it uses OpenGL and relies upon frames rendered on the GPU, not the CPU, you never want to use blocking code. So kivy uses event binding to circumvent the issue.
Here is one approach.
from kivy.app import App
from kivy.uix.popup import Popup
from kivy.uix.button import Button
from kivy.uix.textinput import TextInput
class MainApp(App):
def build(self):
self.button = Button(text="Click",
on_release=self.get_caption)
return self.button
def get_caption(self, btn):
Popup(title="Enter text here",
content=TextInput(focus=True),
size_hint=(0.6, 0.6),
on_dismiss=self.set_caption).open()
def set_caption(self, popup):
self.button.text = popup.content.text
MainApp().run()
You place content in a popup, when give it a "set_caption" function to call when it's dismissed. There you respond to the change. No blocking. No waiting. Having worked with threading to stop GUI blocking in wxWidgets, I really think this is a better way...;-)
Cheers

What you are looking for can be achieved with the following code. You need to specify routines as methods in the main program:
import kivy
kivy.require('1.5.0') # replace with your current kivy version !
from kivy.uix.popup import Popup
from kivy.uix.button import Button
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.label import Label
from kivy.app import App
from kivy.clock import Clock
class YourApp(App):
def build(self):
return Button(text='Press for popup!', on_press=self.callpopup)
def callpopup(self, event):
dlg = MessageBox(titleheader="Titel Header", message="Any Message", options={"YES": "printyes()", "NO": "printno()"})
print "Messagebox shows as kivy popup and we wait for the \nuser action and callback to go to either routine"
def printyes(self):
# routine for going yes
print "You chose the Yes routine"
def printno(self):
# routine for going no
print "You chose the No routine"
class MessageBox(YourApp):
def __init__(self, titleheader="Title", message="Message", options={"OK": "self.ok()", "NO": "self.no()"}):
def popup_callback(instance):
"callback for button press"
# print('Popup returns:', instance.text)
self.retvalue = instance.text
self.popup.dismiss()
self.retvalue = None
self.options = options
box = BoxLayout(orientation='vertical')
box.add_widget(Label(text=message, font_size=20))
b_list = []
buttonbox = BoxLayout(orientation='horizontal')
for b in options:
b_list.append(Button(text=b, size_hint=(1,.35), font_size=20))
b_list[-1].bind(on_press=popup_callback)
buttonbox.add_widget(b_list[-1])
box.add_widget(buttonbox)
self.popup = Popup(title=titleheader, content=box, size_hint=(None, None), size=(400, 400))
self.popup.open()
self.popup.bind(on_dismiss=self.OnClose)
def OnClose(self, event):
self.popup.unbind(on_dismiss=self.OnClose)
self.popup.dismiss()
if self.retvalue != None:
command = "super(MessageBox, self)."+self.options[self.retvalue]
# print "command", command
exec command
if __name__ == '__main__':
YourApp().run()

back button functionality on android with kivy

in Kivy, when I press the back button on my android device it throws me out of the application. is there a way to return back to the previous screen using the Kivy language and not python? this is what I have written in kivy:
<MyAppClass>:
AnchorLayout:
anchor_x : 'center'
anchor_y : 'top'
ScreenManager:
size_hint : 1, .9
id: _screen_manager
Screen:
name:'screen1'
StackLayout:
# irrelevant code
Screen:
name:'screen2'
StackLayout:
# irrelevant code
I need to manipulate the screen manager and its screens from python... if I can do so I will be ok with python.

Kivy on android binds the back button to the esc button so binding and listening to esc button in your app would help you handle how your app behaves when the back button is pressed.
In other words in your app when testing it on your desktop listen to the escape key from the system keyboard, this will be automatically be translated to being the back button on your android device. Something like::
def on_start():
from kivy.base import EventLoop
EventLoop.window.bind(on_keyboard=self.hook_keyboard)
def hook_keyboard(self, window, key, *largs):
if key == 27:
# do what you want, return True for stopping the propagation
return True

i guess that i have solved it but should thank both #inclement and #qua-non! your answers guys led me to the right way! so in kv i assume that i gave an id to my screen manager (please refer to my question where i have written the kv code) , in python i should do the following:
from kivy.core.window import Window
from kivy.properties import ObjectProperty
class MyAppClass(FloatLayout):#its a FloatLayout in my case
_screen_manager=ObjectProperty(None)
def __init__(self,**kwargs):
super(MyAppClass,self).__init__(**kwargs)
#code goes here and add:
Window.bind(on_keyboard=self.Android_back_click)
def Android_back_click(self,window,key,*largs):
if key == 27:
self._scree_manager.current='screen1'#you can create a method here to cache in a list the number of screens and then pop the last visited screen.
return True
class MyApp(App):
def build(self):
return MyAppClass()
if __name__=='__main__':
MyApp().run()

This is certainly possible. Here's a short example app with the method I use to do this:
from kivy.utils import platform
from kivy.core.window import Window
class ExampleApp(App):
manager = ObjectProperty()
def build(self):
sm = MyScreenManager()
self.manager = sm
self.bind(on_start=self.post_build_init)
return sm
def post_build_init(self, *args):
if platform() == 'android':
import android
android.map_key(android.KEYCODE_BACK, 1001)
win = Window
win.bind(on_keyboard=self.my_key_handler)
def my_key_handler(self, window, keycode1, keycode2, text, modifiers):
if keycode1 in [27, 1001]:
self.manager.go_back()
return True
return False
This should give the right basic idea, but a few notes:
ScreenManager doesn't keep track of the previous screens, it's up to you to implement this how you like. My example assumes you defined a class MyScreenManager with a go_back method.
It might not be necessary to bind to on_start and run post_build_init, this is just how the example I originally used did it (see below). It might be important sometimes though, possibly if the window is not initialised when build() is run, and the original mailing list post suggests the author needed it for some reason.
The example listens for keycodes 27 or 1001. As qua-non said while I was writing this, the former listens for esc, so you can get the same behaviour on desktop.
I didn't try without the android.map_key line, but it seems like it may not be necessary.
You mention you want to use kivy language and not python. You need to do some python to get this result, and I don't see a way around that (it's not really the domain of the kv language). I guess you could shift some stuff to kv by defining a 'go_back' event somewhere and triggering this when the key is pressed, along with binding your screenmanager to watch that event, but it seems like a long way around.
I based my code on the mailing list thread at https://groups.google.com/forum/#!topic/kivy-users/7rOZGMMIFXI . There might be a better way, but this is quite functional.

Now all the way in 2020 I'm using:
Clock.schedule_once(lambda x: Window.bind(on_keyboard=self.hook_keyboard))
in combination with a similar hook_keyboard method to the other answers, to delay the bind in my build method. Works fine, but none of these other ways ways seemed to work for me anymore.

Kivy TextInput and Fonts

the code in question
txt = TextInput(text='%s'%default, multiline=False, size_hint=(0.5,1))
txt.font_name = gAssets + "FreeSans.ttf"
Txt.font_size = 14
If I comment out the font_name attribute the text in the input lines up about right. (still sits a little bit high in the box but workable)
(using the normal TextInput with the default font (DroidSans.ttf))
However once I uncomment the line that sets it to FreeSans.ttf (larger character set) It now sits way to high in the text field
(using normal TextInput with FreeSans.ttf)
I am using kivy 1.3 and have been unsuccessful at getting the padding attribute to work(however I would be happy to use it if someone could demonstrate how to use it with a TextInput.)

You can alter padding inside your code using VariableListPropery. Example:
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.textinput import TextInput
from kivy.properties import VariableListProperty
class MyTextInput(TextInput):
padding = VariableListProperty(['24dp', '48dp'])
class MyApp(App):
def build(self):
return MyTextInput(text='This is an example text', multiline=False)
if __name__ == '__main__':
MyApp().run()
This code requires 1.7 version, as noted in documentation of the widget. I recommend uprgrading as I don't even see any API archive anywhere to check how it was setted before.