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 COLLECTED…