Question

我从远程服务器获取文件列表，然后需要下载每个文件，下载完成后，我应该通知UI进行显示，但不会调用livedata观察器。我在下面写一个演示，

class MainActivity : AppCompatActivity() {
    val liveData = MutableLiveData<Test>()

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        liveData.observe(this, Observer {
            Log.d("Test","receive ${it.pos}")
        })


    }

    override fun onResume() {
        super.onResume()

        // example this is a file list 
        for (i in 0..15){
            lifecycleScope.launch {
                withContext(Dispatchers.IO){
                    delay(100)// download file or check file is exits
                    Log.d("Test","send $i")
                    liveData.postValue(Test(i))
                }
            }


        }
    }
}

获取日志

2020-04-17 12:35:21.847 15449-15490 D/Test: send 2
2020-04-17 12:35:21.848 15449-15491 D/Test: send 3
2020-04-17 12:35:21.849 15449-15495 D/Test: send 0
2020-04-17 12:35:21.853 15449-15491 D/Test: send 5
2020-04-17 12:35:21.873 15449-15490 D/Test: send 1
2020-04-17 12:35:21.873 15449-15496 D/Test: send 4
2020-04-17 12:35:21.876 15449-15491 D/Test: send 6
2020-04-17 12:35:21.883 15449-15449 D/Test: receive 6
2020-04-17 12:35:21.898 15449-15490 D/Test: send 10
2020-04-17 12:35:21.899 15449-15492 D/Test: send 9
2020-04-17 12:35:21.899 15449-15497 D/Test: send 7
2020-04-17 12:35:21.900 15449-15499 D/Test: send 11
2020-04-17 12:35:21.900 15449-15499 D/Test: send 13
2020-04-17 12:35:21.901 15449-15499 D/Test: send 14
2020-04-17 12:35:21.901 15449-15490 D/Test: send 12
2020-04-17 12:35:21.902 15449-15494 D/Test: send 8
2020-04-17 12:35:21.903 15449-15494 D/Test: send 15
2020-04-17 12:35:21.964 15449-15449 D/Test: receive 15

为什么只有两次观察者onChanged被调用

Answer 1

将您的onResume()更改为此：

override fun onResume() {
    super.onResume()

    lifecycleScope.launch {
        withContext(Dispatchers.IO) {
            for (i in 0..15) {
                delay(100)// download file or check file is exits
                Log.d("Test", "send $i")
                liveData.postValue(Test(i))
            }
        }
    }
}

您启动1个协程，并在其中迭代文件列表。

Answer 2

liveData.postValue

如果需要从后台线程设置值，则可以使用postValue（Object）

将任务发布到主线程以设置给定值。

如果在主线程执行发布的任务之前多次调用此方法，则只会分派最后一个值。

因此将postValue更改为setValue后一切正常

LiveData Observer未与Kotlin协程调用

2 个答案: