Tips for Building Custom Views on Android with Canvas APIs @riggaroo Rebecca Franks

Hi Android Developer @ Over Google Developer Expert in Android @riggaroo riggaroo.co.za I’m Rebecca Franks %

What is a Custom View?

What? UI Component not part of the “stock standard” controls. When? Standard Android Framework Views don’t have what you need Want a grouping of views for easier use

Compound View (Group of Views) Extend a ViewGroup (ie LinearLayout, ConstraintLayout etc) Override one or two things / group a bunch of existing components together Difficulty Level: Intermediate

Compound View class CompoundView @JvmOverloads constructor( context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0 ) : LinearLayout(context, attrs, defStyleAttr) { init { inflate(context, R.layout.compound_view_example, this) } }

Fully Custom Component Manually draw the component on screen Handle your own Gestures/Touch events etc Difficulty Level: PRO

Fully Custom Component class CustomView @JvmOverloads constructor( context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0 ) : View(context, attrs, defStyleAttr) { // Called when the view should render its content. override fun onDraw(canvas: Canvas?) { super.onDraw(canvas) // DRAW STUFF HERE } }

What is a Canvas?

Blank Space to draw onto

Canvas Based on top of SkCanvas - SKIA https://skia.org/ Most methods are “similar” on other platforms - ie Web has a element

Canvas - Co-ordinate System Top left is [0,0]- the starting point of that Canvas. All elements are placed relative to [0,0] Uses pixel values - not dp New Canvas commands draw over previously drawn items 0 X Y 0 y x width height

How do I use a Canvas? You need 4 things: 1. Bitmap - to hold the pixels 2. Canvas - host drawing calls 3. Draw Commands - describe what to draw 4. Paint - describe how to draw

What can I draw on a Canvas?

Canvas#drawBitmap(…) canvas.drawBitmap(bitmap, null, destRect, paint)

Canvas#drawBitmap(…) Can be quite confusing & easy to stretch a bitmap - Make sure RectF is correct - Normalise your canvas values

Other Canvas APIs drawRect(pixelPointRect, whitePaint) drawCircle(x, y, SIZE_MAGNIFIER, paint)

Paint

Paint Holds style and colour information for drawing text, bitmaps, paths etc. private val textPaint = Paint().apply { isAntiAlias = true color = Color.RED style = Paint.Style.STROKE }

Paint Typeface information, shadowLayer information private val textPaint = Paint().apply { isAntiAlias = true textSize = fontSize letterSpacing = letterSpace this.typeface = newTypeface setShadowLayer(blurValue, x, y, Color.BLACK) }

Paint with BitmapShader

Paint with BitmapShader Draw parts of a bitmap val bitmapShader = BitmapShader(backingBitmap, TileMode.CLAMP, TileMode.CLAMP) val dropperPaint = Paint().apply { shader = bitmapShader } drawingMatrix.postScale(SCALE, SCALE, point.x, point.y) dropperPaint.shader?.setLocalMatrix(drawingMatrix) //.. in onDraw drawCircle(point.x, point.y, SIZE_OF_MAGNIFIER, dropperPaint)

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Tip: Use KTX for cleaner code

implementation "androidx.core:core-ktx:1.1.0-alpha05" Core KTX is a set of common extensions provided by the Android Team at Google. What is KTX?

canvas.save() canvas.translate(200f, 300f) canvas.drawCircle(...) // drawn on the translated canvas canvas.restore() Without KTX

canvas.withTranslate(200f, 300f) { drawCircle(...) } With KTX

val translateCheckpoint = canvas.save() canvas.translate(200f, 300f) canvas.drawCircle(150f, 150f, RADIUS, circlePaint) val rotateCheckpoint = canvas.save() canvas.rotate(45f) canvas.drawRect(rect, rectPaint) canvas.restoreToCount(rotateCheckpoint) canvas.restoreToCount(translateCheckpoint) Without KTX

canvas.withTranslation(200f, 300f) { drawCircle(150f, 150f, RADIUS, circlePaint) withRotation(45f) { drawRect(rect, rectPaint) } } With KTX

Tip: Use Matrices to make Canvas Operations Easier

Matrix 3*3 matrix for transforming canvas or co-ordinates - Scale, Skew, Rotate, Translate, Perspective

Matrix val newMatrix = Matrix() // onDraw… newMatrix.postRotate(20f) canvas.withMatrix(newMatrix) { drawBitmap(bitmap, null, rect, paint) }

Perspective drawing matrix.setPolyToPoly(src, 0, dst, 0, points) canvas.withMatrix(newMatrix) { drawBitmap(bitmap, null, rect, paint) }

val src = floatArrayOf( 0f, 0f, // top left point width, 0f, // top right point width, height, // bottom right point 0f, height // bottom left point ) val dst = floatArrayOf( 50f, -200f, // top left point width, -200f, // top right point width, height +200f, // bottom right point 0f, height // bottom left point ) Perspective drawing

Tip: How to work with two Coordinate Systems

Working with sizes and points is difficult Measured Width of View = 1080 Measured Height of View = 2180 My data class sizes are in the range: 0 - 100 or 0 to Custom Size All points inside are also in that range.

Scale/Translate Canvas early on in rendering Measured Width of View = 1080 Measured Height of View = 2180 val (scale, x, y) = size.fitCenter(canvas.size()) canvas.withTranslation(x, y) { withScale(scale, scale) { // Canvas at this point will now // be in your coordinate system } }

Mapping points between two co-ordinate systems fun mapPoint(point: Point): Point { computeMatrix.reset() computeMatrix.postTranslate(20f, 20f) computeMatrix.postRotate(20f, x, y) val arrayPoint = floatArrayOf(point.x, point.y) computeMatrix.mapPoints(arrayPoint) return Point(arrayPoint[0], arrayPoint[1]) }

Tip: Check that the Canvas APIs you are using work across different API versions

Paint#setShadowLayer Works on drawText() Weird results with drawBitmap() and hardware acceleration Doesn’t work 100% with drawPath() paint.setShadowLayer(20f, 200f, 200f, Color.BLACK)

Paint#setShadowLayer API 26 (Hardware acc) API 28 - Hardware acc API 26 - Software acc

Difference in Hardware / Software Rendering Creating a Canvas offscreen - always software rendered On screen Canvas - choose hardware or software To disable HA on a view (you probably don’t want to do this): init { setLayerType(LAYER_TYPE_SOFTWARE,null) }

https://developer.android.com/guide/topics/graphics/hardware-accel.html Supported hardware methods from API level

Test your Canvas code on all different API levels ✅

When should you use a Custom View / Fragment / ?

Fragment Whole logical component in your app that is mostly standalone Don’t need custom drawing Using Built in components Don’t really need callbacks to other parts (workarounds with ViewModels but not great) When built in components don’t cut it - ie need a Canvas drawing, Custom Gesture Handling Encapsulate logic and expose a callback listener to user of a component Don’t want users to be concerned of the internals Custom View Literally just bunch of layouts grouped in a file Need each individual component available in layout No custom logic around touches / drawing etc Manually handle each components logic yourself

Looking to the Future: Jetpack Compose?

Jetpack Compose • Declarative UI toolkit for Android • Inspired by React, Flutter, Vue… • Pre-alpha - ie not production ready • Unbundled from platform

Jetpack Compose import androidx.compose.* import androidx.ui.core.* @Composable fun Greeting(name: String) { Text ("Hello $name!") }

Canvas with Jetpack Compose @Composable fun DrawRectangle(color: Color) { val paint = Paint() paint.color = color Draw { canvas, parentSize -> canvas.drawRect(parentSize.toRect(), paint) } }

Final Thoughts - Normalise your canvas coordinate system - Use Matrix class for easier transformations - Double check that the Canvas features are available - Use KTX - Canvas concepts are not disappearing anytime soon…

Thank you! Rebecca Franks rebecca@overhq.com @riggaroo