Android Weekly Issue #428

Kotlin Flow Retry Operator with Exponential Backoff Delay

这是讲协程Flow系列文章中的一篇.

对于重试的两个操作符:

• retryWhen
• retry

retryWhen的使用:

.retryWhen { cause, attempt ->
    if (cause is IOException && attempt < 3) {
        delay(2000)
        return@retryWhen true
    } else {
        return@retryWhen false
    }
}

retry:

.retry(retries = 3) { cause ->
    if (cause is IOException) {
        delay(2000)
        return@retry true
    } else {
        return@retry false
    }
}

可以把时间指数延长:

viewModelScope.launch {
    var currentDelay = 1000L
    val delayFactor = 2
    doLongRunningTask()
        .flowOn(Dispatchers.Default)
        .retry(retries = 3) { cause ->
            if (cause is IOException) {
                delay(currentDelay)
                currentDelay = (currentDelay * delayFactor)
                return@retry true
            } else {
                return@retry false
            }
        }
        .catch {
            // error
        }
        .collect {
            // success
        }
}

Fragments: Rebuilding the Internals

Fragment在Android 10已经废弃, 现在不在framework中了, 只在AndroidX中有.

这个Fragment 1.3.0-alpha08版本的发布, 有一些关于FragmentManager内部状态的重要更新.
解决了很多issue, 简化了fragment的生命周期, 还提供了一个FragmentManager多个back stacks的支持.

核心就是这个FragmentStateManager类.

这个FragmentStateManager负责:

• 转换Fragment的生命周期状态.
• 跑动画和转场.
• 处理延迟转换.

Postponed fragments

关于状态的确定, 有一个case是一个难点: postponed fragments.
这是一个以前就有的东西, 通常跟shared element transition动画有关系.

postponed fragment有两个特点:

• view创建了, 但是不可见.
• lifecycle顶多到STARTED.

只有调用这个方法: startPostponedEnterTransition()之后, fragment的transition才会跑, view会变成可见, fragment会移动到RESUMED.

所以有这个bug: Postponed Fragments leave the Fragments and FragmentManager in an inconsistent state bug.

这个issue相关联的还有好几个issues.

在容器层面解决问题

用一个SpecialEffectsController(以后名字可能会改)来处理所有动画转场相关的东西.

这样FragmentManager就被解放出来, 不需要处理postponed的逻辑, 而是交给了container, 这样就避免了FragmentManager中状态不一致的问题.

新的StateManager构架

原先: 一个FragmentManager总管所有.

现在: FragmentManager和各个FragmentStateManager的实例交流.

• The FragmentManager only has state that applies to all fragments.
• The FragmentStateManager manages the state at the fragment level.
• The SpecialEffectsController manages the state at the container level.

总体

这个改动新发布, 实验阶段, 总体来说是应该没有行为改变的.

如果有行为改变, 对你的程序造成了影响, 也可以暂时关闭(FragmentManager.enableNewStateManager(false)), 并且报告个issue.

A Framework For Speedy and Scalable Development Of Android UI Tests

讲了一整套的测试实践.

没有用Appium, 用的UI Automator和Espresso.

Basic Coroutine Level 1

Kotlin协程的概念.

Android Lint Framework — An Introduction

Android Lint的介绍.

创建一个Lint规则, 保证每个人都用项目自定义的ImageView, 而不是原生的ImageView.

具体做法:

• 首先从创建一个叫做custom-lint的module. 需要依赖lint-api和lint-checks:

compileOnly "com.android.tools.lint:lint-api:$androidToolsVersion"
compileOnly "com.android.tools.lint:lint-checks:$androidToolsVersion"

这里用了compileOnly是因为不想lint API在runtime available.

• 之后创建自定义规则. 每个lint check的实现都叫一个detector. 需要继承Detector, 并且利用Scanners来做扫描. 报告错误需要定义Issue. 还可以创建LintFx, 作为quick fix.

class ImageViewUsageDetector : LayoutDetector() {
  
   // Applicable elements
  
  
    override fun visitElement(context: XmlContext, element: Element) {
        context.report(
            issue = ISSUE, 
            location = context.getElementLocation(element), 
            message = REPORT_MESSAGE, 
            quickfixData = computeQuickFix() 
        )
    }

   
    private fun computeQuickFix(): LintFix {
        return LintFix.create()
            .replace().text(SdkConstants.IMAGE_VIEW)
            .with(TINTED_IMAGE_VIEW)
            .build()
    }

  // Issue, implementation, and other constants 

}

• 然后把定义好的自定义规则注册.

class Registry: IssueRegistry() {

    override val issues: List<Issue>
        get() = listOf(ImageViewUsageDetector.ISSUE)

    override val api: Int = CURRENT_API

}

• 创建入口, 在build.gradle文件中:

// Configure jar to register our lint registry
jar {
    manifest {
        attributes("Lint-Registry-v2": "com.tintedimagelint.lint.Registry")
    }
}

• 加上依赖和一些配置.

android {
    
    // Configurations above
    lintOptions {
        lintConfig file('../analysis/lint/lint.xml')
        htmlOutput file("$project.buildDir/reports/lint/lint-reports.html")
        xmlOutput file("$project.buildDir/reports/lint/lint-reports.xml")
        abortOnError false
    }
    //Configurations below
}

dependencies {
    // Dependencies above

    // Include custom lint module as a lintCheck
    lintChecks project(":custom-lint")

    // Dependencies below
}

Codelabs for new Android game technologies

关于Android Game新技术的Codelabs:

• 资源打包: Play Asset Delivery -> https://codelabs.developers.google.com/codelabs/unity-gamepad/#0
• 帧率和图像: Android Performance Tuner -> https://codelabs.developers.google.com/codelabs/android-performance-tuner-unity/#0

都是Unity的game.

Android Vitals - When did my app start?

系列文章之六, 我的app啥时候启动的?

看个结论吧:

Here's how we can most accurately measure the app start time when monitoring cold start:

• Up to API 24: Use the class load time of a content provider.
• API 24 - API 28: Use Process.getStartUptimeMillis().
• API 28 and beyond: Use Process.getStartUptimeMillis() but filter out weird values (e.g. more than 1 min to get to Application.onCreate()) and fallback to the time ContentProvider.onCreate() is called.

Comparing Three Dependency Injection Solutions

比较三种依赖注入的解决方案.

• 手写方式.
• Koin.
• Dagger Hilt.

Avoiding memory leaks when using Data Binding and View Binding

使用Data Binding和View Binding的时候, 注意内存泄漏问题.

Google建议在Fragment中使用binding时, 要在onDestroyView中置为null:

private var _binding: ResultProfileBinding? = null
// This property is only valid between onCreateView and
// onDestroyView.
private val binding get() = _binding!!

override fun onCreateView(
    inflater: LayoutInflater,
    container: ViewGroup?,
    savedInstanceState: Bundle?
): View? {
    _binding = ResultProfileBinding.inflate(inflater, container, false)
    val view = binding.root
    return view
}

override fun onDestroyView() {
    super.onDestroyView()
    _binding = null
}

有个博客中介绍的方法, 可以简化成这样:

private val binding: FragmentFirstBinding by viewBinding()

Fragment还有一个参数的构造, 可以传入布局id:

class FirstFragment : Fragment(R.layout.fragment_first) {

  private val binding: FragmentFirstBinding by viewBinding()
  
  override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
      super.onViewCreated(view, savedInstanceState)
      // Any code we used to do in onCreateView can go here instead
  }
}

冷知识: DataBinding实现了ViewBinding.

public abstract class ViewDataBinding extends BaseObservable implements ViewBinding

所以ViewBinding和DataBinding方法通用.

Anti-patterns of automated software testing

关于测试的一些anti-patterns.

推荐阅读.

Using bytecode analysis to find unused dependencies

关于这个库: https://github.com/autonomousapps/dependency-analysis-android-gradle-plugin的说明.

Code

• https://github.com/jmfayard/refreshVersions: 一个依赖版本管理的gradle插件.