A simple Paint App

In the following you will be guided through the creation of your first widget. This provides very powerful and important knowledge when programming Kivy applications, as it lets you create completely new user interfaces with custom elements for your specific purpose.

Basic Considerations

When creating an application, you have to ask yourself three main questions:

  • What data does my application process?
  • How do I visually represent that data?
  • How does the user interact with that data?

If you want to write a very simple line drawing application for example, you most likely want the user to just draw on the screen with his fingers. That’s how the user interacts with your application. While doing so, your application would memorize the positions where the user’s finger was, so that you can later draw lines between those positions. So the points where the fingers were would be your data and the lines that you draw between these would be your visual representation.

In Kivy, an applications user interface is composed of Widgets. Everything that you see on the screen is somehow drawn by a widget. Often you would like to be able to reuse code that you already wrote in a different context, which is why widgets typically represent one specific instance that answers the three questions above. A widget encapsulates data, defines the user’s interaction with that data and draws its visual representation. You can then build anything from simple to complex user interfaces by nesting widgets. There are many widgets built in, such as buttons, sliders and other common stuff. In many cases, however, you need a custom widget that is beyond the scope of what is shipped with Kivy (e.g. a medical visualization widget).

So keep these three questions in mind when you design your widgets. Try to write them in a minimal and reusable manner (I.e. a widget does exactly what its supposed to do and nothing more. If you need more, write more widgets or compose other widgets of smaller widgets).

Paint Widget

We’re sure one of your childhood dreams has always been creating your own multitouch paint program. Allow us to help you achieve that. In the following sections you will successively learn how to write a program like that using Kivy. Make sure that you have read and understood Create an application. You have? Great! Let’s get started!

Initial Structure

Let’s start by writing the very basic code structure that we need. By the way, all the different pieces of code that are used in this section are also available in the examples/guide/firstwidget directory that comes with Kivy, so you don’t need to copy & paste it all the time. Here is the basic code skeleton that we will need:

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from kivy.app import App
from kivy.uix.widget import Widget


class MyPaintWidget(Widget):
    pass


class MyPaintApp(App):
    def build(self):
        return MyPaintWidget()


if __name__ == '__main__':
    MyPaintApp().run()

This is actually really simple. Save it as paint.py. If you run it, you should only see a black screen. As you can see, instead of using a built-in widget such as Button (see Create an application), we are going to write our own widget to do the drawing. We do that by creating a class that inherits from Widget (line 5-6) and although that class does nothing yet, we can still treat it like a normal Kivy widget (line 11). The if __name__ ... construct (line 14) is a Python mechanism that prevents you from executing the code in the if-statement when importing from that file, i.e. if you write import paint, it won’t do something unexpected but just nicely provide the classes defined in the file.

Note

You may be wondering why you have to import App and Widget separately, instead of doing something like from kivy import *. While shorter, this would have the disadvantage of cluttering your namespace and making the start of the application potentially much slower. It’s also not as clear what your application uses. The way we do it is faster and cleaner.

Adding Behaviour

Let’s now add some actual behaviour to the widget, i.e. make it react to user input. Change the code like so:

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from kivy.app import App
from kivy.uix.widget import Widget


class MyPaintWidget(Widget):
    def on_touch_down(self, touch):
        print touch

class MyPaintApp(App):
    def build(self):
        return MyPaintWidget()


if __name__ == '__main__':
    MyPaintApp().run()

This is just to show you how easy it is to react to user input. When a MotionEvent (i.e. a touch, click, etc.) occurs, we simply print the information about the touch object to the console. You won’t see anything on the screen, but if you observe the command-line from which you are running the program, you will see a message for every touch (initially). This also demonstrates that a widget does not always have to have a visual representation.

Now that’s not really an overwhelming user experience. Let’s add some code that actually draws something into our window:

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from kivy.app import App
from kivy.uix.widget import Widget
from kivy.graphics import Color, Ellipse


class MyPaintWidget(Widget):

    def on_touch_down(self, touch):
        with self.canvas:
            Color(1, 1, 0)
            d = 30.
            Ellipse(pos=(touch.x - d / 2, touch.y - d / 2), size=(d, d))


class MyPaintApp(App):

    def build(self):
        return MyPaintWidget()


if __name__ == '__main__':
    MyPaintApp().run()
../_images/guide-3.jpg

If you run your code with these modifications, you will see that every time you touch, there will be a small yellow circle drawn where you touched. How does it work?

  • Line 8: We use Python’s with statement with the widget’s Canvas object. This is like an area in which the widget can draw things to represent itself on the screen. By using the with statement with it, all successive drawing commands that are properly indented will modify this canvas. The with statement also makes sure that after our drawing, internal state can be cleaned up properly.
  • Line 9: You might have guessed it already: This sets the Color for successive drawing operations to yellow (default color format is RGB, so (1, 1, 0) is yellow). This is true until another color is set. Think of this as dipping your brushes in that color which you can then use to draw on a canvas until you dip the brushes into another color.
  • Line 10: We specify the diameter for the circle that we are about to draw. Using a variable for that is preferable since we need to refer to that value multiple times and we don’t want to have to change it in several places if we want the circle bigger or smaller.
  • Line 11: To draw a circle, we simply draw an Ellipse with equal width and height. Since we want the circle to be drawn where the user touches, we pass the touch’s position to the ellipse. Note that we need to shift the ellipse by -d/2 in the x and y directions (i.e. left and downwards) because the position specifies the bottom left corner of the ellipse’s bounding box, and we want it to be centered around our touch.

That was easy, wasn’t it? It gets better! Update the code to look like this:

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from kivy.app import App
from kivy.uix.widget import Widget
from kivy.graphics import Color, Ellipse, Line


class MyPaintWidget(Widget):

    def on_touch_down(self, touch):
        with self.canvas:
            Color(1, 1, 0)
            d = 30.
            Ellipse(pos=(touch.x - d / 2, touch.y - d / 2), size=(d, d))
            touch.ud['line'] = Line(points=(touch.x, touch.y))

    def on_touch_move(self, touch):
        touch.ud['line'].points += [touch.x, touch.y]


class MyPaintApp(App):

    def build(self):
        return MyPaintWidget()


if __name__ == '__main__':
    MyPaintApp().run()
../_images/guide-4.jpg
This is what has changed:
  • Line 3: We now not only import the Ellipse drawing instruction, but also the Line drawing instruction. If you look at the documentation for Line, you will see that it accepts a points argument that has to be a list of 2D point coordinates, like (x1, y1, x2, y2, ..., xN, yN).
  • Line 13: This is where it gets interesting. touch.ud is a Python dictionary (type <dict>) that allows us to store custom attributes for a touch.
  • Line 13: We make use of the Line instruction that we imported and set a Line up for drawing. Since this is done in on_touch_down, there will be a new line for every new touch. By creating the line inside the with block, the canvas automatically knows about the line and will draw it. We just want to modify the line later, so we store a reference to it in the touch.ud dictionary under the arbitrarily chosen but aptly named key ‘line’. We pass the line that we’re creating the initial touch position because that’s where our line will begin.
  • Lines 15: We add a new method to our widget. This is similar to the on_touch_down method, but instead of being called when a new touch occurs, this method is being called when an existing touch (for which on_touch_down was already called) moves, i.e. its position changes. Note that this is the same MotionEvent object with updated attributes. This is something we found incredibly handy and you will shortly see why.
  • Line 16: Remember: This is the same touch object that we got in on_touch_down, so we can simply access the data we stored away in the touch.ud dictionary! To the line we set up for this touch earlier, we now add the current position of the touch as a new point. We know that we need to extend the line because this happens in on_touch_move, which is only called when the touch has moved, which is exactly why we want to update the line. Storing the line in the touch.ud makes it a whole lot easier for us as we don’t have to maintain our own touch-to-line bookkeeping.

So far so good. This isn’t exactly beautiful yet, though. It looks a bit like spaghetti bolognese. What about we give each touch its own color? Great, let’s do it:

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from random import random
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.graphics import Color, Ellipse, Line


class MyPaintWidget(Widget):

    def on_touch_down(self, touch):
        color = (random(), random(), random())
        with self.canvas:
            Color(*color)
            d = 30.
            Ellipse(pos=(touch.x - d / 2, touch.y - d / 2), size=(d, d))
            touch.ud['line'] = Line(points=(touch.x, touch.y))

    def on_touch_move(self, touch):
        touch.ud['line'].points += [touch.x, touch.y]


class MyPaintApp(App):

    def build(self):
        return MyPaintWidget()


if __name__ == '__main__':
    MyPaintApp().run()
../_images/guide-5.jpg

Here are the changes:

  • Line 1: We import Python’s random() function that will give us random values in the range of [0., 1.).
  • Line 10: In this case we simply create a new tuple of 3 random float values that will represent a random RGB color. Since we do this in on_touch_down, every new touch will get its own color. Don’t get confused by the use of two = operators. We’re just binding the tuple to color as well as a shortcut for use within this method because we’re lazy.
  • Line 12: As before, we set the color for the canvas. Only this time we use the random values we generated and feed them to the color class using Python’s tuple unpacking syntax (since the Color class expects three individual color components instead of just 1. If we were to pass the tuple directly, that would be just 1 value being passed, regardless of the fact that the tuple itself contains 3 values).

This looks a lot nicer already! With a lot of skill and patience, you might even be able to create a nice little drawing!

Note

Since by default the Color instructions assume RGB mode and we’re feeding a tuple with three random float values to it, it might very well happen that we end up with a lot of dark or even black colors if we are unlucky. That would be bad because by default the background color is dark as well, so you wouldn’t be able to (easily) see the lines you draw. There is a nice trick to prevent this: Instead of creating a tuple with three random values, create a tuple like this: (random(), 1., 1.). Then, when passing it to the color instruction, set the mode to HSV color space: Color(*color, mode='hsv'). This way you will have a smaller number of possible colors, but the colors that you get will always be equally bright. Only the hue changes.

Bonus Points

At this point, we could say we are done. The widget does what it’s supposed to do: It traces the touches and draws lines. It even still draws circles at the positions where a line begins.

But what if the user wants to start a new drawing? With the current code, the only option to clear the window would be to restart the entire application. Luckily, we can do better. Let us add a Clear button that erases all the lines and circles that have been drawn so far. There are two options now:

  • We could either create the button as a child of our widget. That would imply that if you create more than one widget, every widget gets its own button. If you’re not careful, this will also allow users to draw on top of the button, which might not be what you want.
  • Or we set up the button only once, initially, in our app class and when it’s pressed we clear the widget.

For our simple example, that doesn’t really matter at all. For larger applications you should give some thought to who does what in your code. We’ll go with the second option here so that you see how you can build up your application’s widget tree in your app class’s build() method. We’ll also change to the HSV color space (see preceding note):

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from random import random
from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.button import Button
from kivy.graphics import Color, Ellipse, Line


class MyPaintWidget(Widget):

    def on_touch_down(self, touch):
        color = (random(), 1, 1)
        with self.canvas:
            Color(*color, mode='hsv')
            d = 30.
            Ellipse(pos=(touch.x - d / 2, touch.y - d / 2), size=(d, d))
            touch.ud['line'] = Line(points=(touch.x, touch.y))

    def on_touch_move(self, touch):
        touch.ud['line'].points += [touch.x, touch.y]


class MyPaintApp(App):

    def build(self):
        parent = Widget()
        painter = MyPaintWidget()
        clearbtn = Button(text='Clear')
        parent.add_widget(painter)
        parent.add_widget(clearbtn)

        def clear_canvas(obj):
            painter.canvas.clear()
        clearbtn.bind(on_release=clear_canvas)

        return parent


if __name__ == '__main__':
    MyPaintApp().run()
../_images/guide-6.jpg

Here’s what happens:

  • Line 4: We added an import statement to be able to use the Button class.
  • Line 25: We create a dummy Widget() object as a parent for both our painting widget and the button we’re about to add. This is just a poor-man’s approach to setting up a widget tree hierarchy. We could just as well use a layout or do some other fancy stuff. Again: This widget does absolutely nothing except holding the two widgets we will now add to it as children.
  • Line 26: We create our MyPaintWidget() as usual, only this time we don’t return it directly but bind it to a variable name.
  • Line 27: We create a button widget. It will have a label on it that displays the text ‘Clear’.
  • Line 28 & 29: We set up the widget hierarchy by making both the painter and the clear button children of the dummy parent widget. That means painter and button are now siblings in the usual computer science tree terminology.
  • Lines 31 & 32: Up to now, the button did nothing. It was there, visible, and you could press it, but nothing would happen. We change that here: We create a small throw-away function that is going to be our callback function which is called when the button is pressed. The function just clears the painter’s canvas’ contents, making it black again.
  • Line 33: We bind the button’s on_release event (which is fired when the button is pressed and then released) to the callback we just defined.

Note

The Kivy Widget class, by design, is kept simple. There are no general properties such as background color and border color. Instead, the examples and documentation illustrate how to easily handle such simple things yourself, as we have done here, setting the color for the canvas, and drawing the shape. From a simple start, you can move to more elaborate customization. Higher-level built-in widgets, deriving from Widget, such as Button, do have convenience properties such as background_color, but these vary by widget. Use the API docs to see what is offered by a widget, and subclass if you need to add more functionality.

Congratulations! You’ve written your first Kivy widget. Obviously this was just a quick introduction. There is much more to discover. We suggest taking a short break to let what you just learned sink in. Maybe draw some nice pictures to relax? If you feel like you’ve understood everything and are ready for more, we encourage you to read on.