How touch events are handled
How the view system in Android handles touch events? Let’s try to understand it by designing it from scratch ourselves! (This is not my original but a summary of this https://juejin.cn/post/6844903761052188679) Let’s do this by coming up a series of requirements (from naive to sophisticated) and see how we can design the logic to fulfill them. Requirement 1 In a nested view hierachy, only the most inner view can handle events.
Some unexpected behavior of LiveData(Kotlin really)
LiveData is convenient, powerful and super easy to use. But, there are still some quirks that could possibly leads to some heads banging against walls. How many times can I observe? In theory, we can observe livedata with multiple observers, just like the good old pub-sub pattern, right? Well, let’s test this out. Let’s create a super simple livedata. class MainViewModel : ViewModel() { private val _data = MutableLiveData<Int>() val data: LiveData<Int> = _data fun update(count: Int) { _data.
Help you understander better about measure and layout
Let’s understand more about Android view’s measure and layout process, and have some fun along the way. Let’s say we have this: <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:app="http://schemas.android.com/apk/res-auto" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" tools:context=".MainActivity"> <com.example.stubornview.StubbornView android:layout_width="match_parent" android:layout_height="match_parent" android:background="@color/design_default_color_primary" /> </LinearLayout> class StubbornView @JvmOverloads constructor( context: Context, attrs: AttributeSet? = null, defStyleAttr: Int = 0 ) : View(context, attrs, defStyleAttr) { /** * 1. Parent LinearLayout combines developer's requirements * and the available space of itself into MeasureSpecs and * pass them here.
A bottom up approach
Yes, there is documentation and tons of sample apps out there. But they are all in a top down approach, meaning they give you the final result of how to use it first, and never tells you why and what is behind the scenes. I will fix that in this article, and present a bottom up approach which leads to much better and deeper understanding. First, at the bottom of things, we need to define what is ViewModel.
A sketch
It is quite common in Android that we need some expandable widget to show and hide information. This is my first attempt, note that this is only a “sketch”, and I intentionally leave some room for improvement. One interesting detail worth mentioning is how the animation is done. I used the reverse() function to play animation backwards in order to achieve a smooth and continuous feel.
Here is the code:
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