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Introducing the Canvas7:19 with Ben Deitch
In this video we'll get acquainted with the app and learn how to use the Canvas object!
Before we can use our custom view, we're going to need an app to put it in. 0:00 So pause me and go ahead and download it from the project files. 0:03 Cool, now that we've got the app, let's open up MainActivity and 0:07 see how the app works. 0:10 We start by getting an input stream for 0:13 our CSV file, which right now is named goog.csv. 0:15 And it exists inside our resources inside the raw folder. 0:20 If you like to use your own data, just paste the csv file into this folder and 0:25 then rename it to not have any capital letters. 0:31 Then, open up your csv file and delete the headers row. 0:33 Also, if your first row of data has a time instead of a date, 0:38 delete that row as well, okay. 0:43 So back in MainActivity, we start by getting our CSV file as an input stream. 0:45 Then we pass that input stream to our CSV parser 0:50 which gives us back a list of stock data objects. 0:55 And if we open up the stock data class, it's just an object for 0:58 holding all the data from the CSV file. 1:02 And it also has a nice toString function. 1:04 Back in the onCreate method, 1:09 we loop through all the stock data and print it out. 1:10 Seems simple enough. 1:14 Let's run the app and see what happens. 1:15 So we haven't really done anything with the UI yet. 1:21 It's just one empty relative layout. 1:24 But if we check out the log, We can see all of our data and 1:26 now we've just got to turn this data into a custom view. 1:32 Let's start by creating a new class named ChartView. 1:39 And let's make it extend to the view class. 1:46 Then, let's use Alt+Enter to create a constructor which takes in a context. 1:53 But before we get too far with our view, 2:00 let's make sure we know how to add it to the screen. 2:02 Inside the constructor, 2:05 let's make our view stand out by adding a call to setBackgroundColor and 2:06 passing in Color.RED. 2:12 Then, over in MainActivity, let's delete the for loop. 2:17 And create a new chart view variable. 2:23 ChartView, which will call ChartView equals new ChartView and 2:27 let's pass in this for the context. 2:31 Then let's create a relative layout variable to store our root layout. 2:35 And let's call it relative layout and set it equal to 2:42 casting to a relative layout findViewById(R.id.).activity_main. 2:47 And finally, let's add our ChartView to our relative layout, 2:57 relativeLayout.addview and pass in our chartView. 3:00 Now if we run the app. 3:06 We get a completely red screen. 3:09 Awesome, okay, now that we've seen our view, let's start turning it into a chart. 3:12 The first thing we'll need to do is give it some data. 3:16 So let's press in our CSV resource as the second parameter to our chart view. 3:19 And that's gonna be R.raw.goog. 3:27 At least for me. 3:29 Then lets use Alt+Enter to add this parameter to the constructor. 3:31 Next, over in the trophy class, let's create a new field to hold our data. 3:36 List, StockData and we'll need to import list, 3:40 and let's name this data. 3:46 Then let's populate this field in the constructor. 3:51 First, let's change the parameter name from goog to resId. 3:54 Then let's cut out these two lines from MainActivity, 3:58 And paste them into our ChartView's constructor. 4:06 Finally, let's replace R.raw..goog with resId. 4:11 And let's make this second line refer to our new data field. 4:16 Nice. 4:23 Also, it looks like we've got a warning on ChartView. 4:24 Custom view ChartView is missing constructor used by tools. 4:27 If you want your custom view to play nicely with Android's layout tools, 4:31 then you'll need to specify one of these constructors. 4:35 We're not using the tools right now, so we're fine ignoring this. 4:37 Next up is the onDraw function, which is where we're going to draw our chart. 4:41 Let's add a couple lines after our constructor, and 4:45 then use Ctrl+O to bring up the override dialog, and then pick onDraw. 4:47 Which has a canvas object as parameter. 4:53 The canvas is just where we'll be doing our drawing. 4:55 It has a certain width and height and 4:58 has a bunch of functions to help us draw pretty much anything. 5:00 Also, we can get rid of this, called a super. 5:03 Let's start this adventure by creating a couple fields to hold the width and 5:05 height of the canvas. 5:08 Float width and height. 5:13 Then let's populate those fields and the onDraw call, 5:18 width = canvas.getWidth and height = canvas.getHeight. 5:24 Now that we've got that before we try to do anything with our data. 5:30 Let's just try and draw a rectangle on the bottom half of the screen. 5:34 On the next line, let's type canvas.drawRect, and 5:37 then we need to pass in values for each edge of our rectangle. 5:42 So let's pass in 0 for the left edge, height / 2 for 5:46 the top, width for the right, and 0 for the bottom. 5:52 Finally, we just need to pass in a paint object. 5:58 So let's create a new paint field. 6:01 And let's name it paint and set it equal to a new Paint object. 6:05 Then at the bottom of our constructor, 6:13 let's type paint.setColor and pass in Color.Green. 6:17 Then let's pass on our new paint object to our drawRect call. 6:23 And then let's run the app. 6:28 And now we've got a green rectangle on the top half of the screen, 6:35 but we wanted it on the bottom half. 6:38 So what's going on here? 6:41 Well, an Android and a large part of computers in general, the origin is 6:42 actually the upper left corner, and the positive y direction is down. 6:47 So when we've got a top of height divided by two and 6:53 a bottom of 0, we've actually got our bottom above our top. 6:56 To fix this, let's change our bottom from 0 to the height of the screen. 7:01 And if we run the app again, the bottom of our screen is now a green rectangle. 7:08 That's enough learning about the canvas for now. 7:14 In the next video, we'll start drawing our chart. 7:16
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