How to become the MacGyver of android custom views

Angus MacGyver: The man that can kill someone with a paperclip

@fernando_cejas!http://www.android10.org/!http://github.com/android10

Who I am…

Software engineer!GDG Barcelona organizer

Android lover!Geek

How android draws view hierarchy

Drawing begins with the root node of the layout.!!

The layout hierarchy is traversed in the order of declaration.!!

Parents are drawn before their children and children are drawn in the order of declaration.

Drawing the layout is a three pass process:!Measure!Layout!Draw

In terms of methods in View class:!requestLayout(): measure + layout!invalidate(): draw

How android draws view hierarchy

View!19.572 lines

ImageView!1.260 lines

TextView!9.220 lines

ViewGroup!6.771 lines

Button!121 lines

LinearLayout!1.898 lines

RelativeLayout!1.812 lines

…!… lines

View framework

View.onAttachedToWindow()

View.onDetachedFromWindow()

View added

Animate View

View.onMeasure()

View.onLayout()

View.onDraw()

View removed

Rendering loop

View lifecycle

Many reasons to go custom:!!

Performance!!

Flexibility!!

Innovation!!

Reusability

To remember: there is no custom view composition :(

Views: going custom…

Views: going custom…

Measuring!!

Layouting!!

Drawing!!

Saving state!!

Handling events

View responsibilities

Encapsulates the layout requirements passed from parent to child.!!

Represents a requirement for either the width or the height. !!

Is comprised of a size and a mode.

There are 3 possible modes:!UNSPECIFIED !EXACTLY !AT_MOST

MeasureSpec

It tells Android how big you want your custom view to be, taking into consideration the layout constraints provided by the parent.

onMeasure()

Called from layout when the view should assign a size and position to each of its children. Used frequently when extending ViewGroup.

onLayout()

Called by Android when the view needs to draw itself. Here is the place for the logic related with drawing the different components or content of our view.

onDraw()

You need to request a new layout if a property changes that might affect the size or shape of the view.

You have to invalidate the view after any change to its properties that might change its appearance.

requestLayout()

invalidate()

There are 3 different ways:!!

Compound Views!!

Custom Compound Views!!

Flat Custom Views

Implementing custom views

These are usually our starting point.!!

Perform pretty well in many situations.!!

Simple to implement.

1. Subclass one of the built-in layouts.!2. Inflate a merge layout in the constructor.!3. Initialize members to point to inner views with findViewById().!4. Add your own APIs to query and update the view state.

Compound views

Compound views

Compound views

Is a compound view which overrides onMeasure(), onLayout() and extends ViewGroup.

Custom compound views

It is a fully custom view that measures, arranges, and draws all its elements. Extends from View.

https://github.com/android10/Android-DonutViews

Flat custom views

To define additional attributes it is a must to create an attrs.xml file in your res/values folder and define them like the following example:

Declaring custom attributes

To use custom attributes already defined, in your layout file you have to declare them in the XML header as following:

Using custom attributes

Reading custom attributes in code

We implement onTouchEvent() to handle touch screen motion events.

Making our view to react…

For persisting view state you want to have a look at View.BasedSavedState class.

The canvas API allows to create complex graphical effects: Canvas, Paint and Shader classes will help with this.

Saving view state

Custom and advance drawing

The View class supports the creation of an image of its current display.

Creating screenshots of views

Enabling “show layout bounds” options on the developer options screen.

Real world examples

Avoid custom views if they are not extremely necessary.!Creating objects ahead of time is an important optimisation. !Initialise your stuff in OnAttachToWindow() method when possible.!If you do not need complex measurement just use onSizeChanged() method.!If you define own views, ensure you review the ViewConfiguration class.!When using custom attributes always recycle your TypedArray.

Tips and tricks

The more custom, the less features for free.!!

Avoid premature optimisation.!!

Go custom only on core components.!!

Start with stock widgets and compound views.!!

Do not reinvent the wheel.

Wrapping up

Questions?

Thanks!!!

@fernando_cejas!http://www.android10.org/!http://github.com/android10

Java Developers never RIP, they just get GARBAGE

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