如何使用 LiveData 在 ViewModel 中启动 Kotlin 协程

Question

I was trying to get all launchable installed apps asynchronously from ViewModel .

This is my ViewModel class looks like:

class AppInstalledViewModel(application: Application) : AndroidViewModel(application) {
    private var appInstalledLiveData: LiveData<ArrayList<AppInstalled>>? = null

    fun getAppInstalledLiveData(): LiveData<ArrayList<AppInstalled>> {
        if (appInstalledLiveData == null) {
            appInstalledLiveData = liveData {
                val appInstalled = ArrayList<AppInstalled>()
                val pm: PackageManager =
                    getApplication<Application>().applicationContext.packageManager

                val main = Intent(Intent.ACTION_MAIN, null).addCategory(Intent.CATEGORY_LAUNCHER)

                // Get launchable installed apps
                val launchAble = pm.queryIntentActivities(main, 0)

                // Sort the installed app list
                Collections.sort(launchAble, ResolveInfo.DisplayNameComparator(pm))

                // Iterate over each launchable app and
                // add it to AppInstalled ArrayList
                for (l in launchAble) {
                    val activityInfo = l.activityInfo

                    activityInfo.run {
                        appInstalled.add(
                            AppInstalled(
                                loadLabel(pm).toString(),
                                applicationInfo.packageName,
                                loadIcon(pm)
                            )
                        )
                    }
                }

                emit(appInstalled)
            }
        }
        return appInstalledLiveData as LiveData<ArrayList<AppInstalled>>
    }
}

There is no problem with loading the app list. But, it seems that my code still not run asynchronously because it still freezes for a very few seconds before moving to the app installed list activity.

And this is the code I use to observe the LiveData :

appInstalledViewModel.getAppInstalledLiveData().observe(this, Observer { appInstalledItem ->
    if (appInstalledItem.size > 0) {
        appInstalledAdapter.appInstalled.addAll(appInstalledItem)
        app_list_progressbar.visibility = View.GONE
    }
})

I'm so confusing trying to use Kotlin on Android Development. This is my first time developing an app using Kotlin. I don't even know if the code I wrote above is a good code in the context of Kotlin's style.

Please help me! Thank you!

Answer 1

It is because of executing the long-running job in Main dispatcher. I highly recommend taking a look at Coroutines Dispatchers .

So, if you change the dispatcher to Dispatchers.Default , leading to fetch installed apps executing in a background thread, consequently, the UI will never be frozen.

appInstalledLiveData = liveData(Dispatchers.Default) {
    ...
}

---

More:

If you want to achieve a better structure, it could be even better not accessing the LiveData by calling a function:

class AppInstalledViewModel(application: Application) : AndroidViewModel(application) {

    private val reloadLiveData = MutableLiveData<Unit>()

    val installedAppsLiveData: LiveData<ArrayList<AppInstalled>> =
        reloadLiveData.switchMap {
            liveData(Dispatchers.Default) {
                emit(fetchInstalledApps())
            }
        }

    fun reloadApps() {
        reloadLiveData.postValue(Unit)
    }

    private fun fetchInstalledApps(): ArrayList<AppInstalled> {
        val appInstalled = ArrayList<AppInstalled>()
        val pm: PackageManager = getApplication<Application>().applicationContext.packageManager

        val main = Intent(Intent.ACTION_MAIN, null).addCategory(Intent.CATEGORY_LAUNCHER)

        // Get launchable installed apps
        val launchAble = pm.queryIntentActivities(main, 0)

        // Sort the installed app list
        Collections.sort(launchAble, ResolveInfo.DisplayNameComparator(pm))

        // Iterate over each launchable app and
        // add it to AppInstalled ArrayList
        for (l in launchAble) {
            val activityInfo = l.activityInfo

            activityInfo.run {
                appInstalled.add(
                    AppInstalled(
                        loadLabel(pm).toString(),
                        applicationInfo.packageName,
                        loadIcon(pm)
                    )
                )
            }
        }
        return appInstalled
    }
}

Now, in the fragment:

appInstalledViewModel.installedAppsLiveData.observe(this, Observer { appInstalledItem ->
    if (appInstalledItem.size > 0) {
        appInstalledAdapter.appInstalled.addAll(appInstalledItem)
        app_list_progressbar.visibility = View.GONE
    }
})

appInstalledViewModel.reloadApps()

Using this structure, if you want to refresh the loaded data, it's just enough to call appInstalledViewModel.reloadApps() , then the list of apps gets updated.