背景

传统的activity和fragment之间、fragment和fragment之间的数据传递有以下方式
1、直接使用类似eventbus、livedatabus这类事件总线的方式
2、使用接口回调，找到实现实现该接口的fragment或activity进行强制类型转换再进行方法调用和参数传递
第一种方式如果项目大了，没法调试，代码可读性差，管理维护麻烦
第二种方式显示的持有具有生命周期的fragment或activity理论上都需要套一层WeakReference防止内存泄露，麻烦。
今天我们使用livedata+viewmodel的方式解决数据传递和点击事件回调，总之两个字：优雅！

实现方式

第一步：依赖引入
app的build.gradle下引入livedata和viewmodel依赖

android {
    //...
    compileOptions {
        sourceCompatibility = 1.8
        targetCompatibility = 1.8
    }
    kotlinOptions {
        jvmTarget = "1.8"
    }
}
dependencies {
    ///...
    // ViewModel
    implementation "androidx.lifecycle:lifecycle-viewmodel- ktx:$lifecycle_version"
    // LiveData
    implementation "androidx.lifecycle:lifecycle-livedata- ktx:$lifecycle_version"
    // For using KTX Delegates extensions for fragments
    implementation "androidx.fragment:fragment-ktx:$fragment_version"
}

第二步，viewmodel里声明事件

import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel

class MainViewModel(val data: String) : ViewModel() {

    val showSharedVMToast = MutableLiveData<Unit>()
    
    // Shows a Toast using SharedViewModel implementation
    fun onShowVMToastClick() {
        showSharedVMToast.value = Unit
    }
}

第三步，activity里响应事件

import android.os.Bundle
import android.widget.Toast
import androidx.activity.viewModels
import androidx.appcompat.app.AppCompatActivity
import androidx.lifecycle.Observer

// Adding the interface that communicates with the fragment
class MainActivity : AppCompatActivity() {

    // Initializing the viewModel on call using KTX-Fragments extension.
    private val viewModel: MainViewModel by viewModels()

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

        // Observing viewModel's LiveData
        viewModel.showSharedVMToast.observe(this, Observer {
            Toast.makeText(this, "Hello from sharedVM", Toast.LENGTH_SHORT).show()
        })

        // Adding the DemoFragment to a container FrameLayout
        supportFragmentManager.beginTransaction().add(R.id.demoFragmentContainer, DemoFragment()).commit()
    }
}

第四步，fragment里调用

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import androidx.fragment.app.activityViewModels

class DemoFragment : Fragment() {

    // We create an instance of MainViewModel using specific KTX-Fragments extension to
    // tell the program that we are willing to use the instance of the Activity this fragment is attached to.
    private val sharedVM: MainViewModel by activityViewModels()

    // We don't need Constructor to pass any extra to this fragment because we have access to the data from SharedVM
    // already.
    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_demo, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        //We can tap directly to the viewModel without using Interface callbacks!
        sharedVMButton.setOnClickListener {
            sharedVM.onShowVMToastClick()
        }
    }
}

这样就完成了fragment里点击调用activity事件或数据传递（反之亦如此），核心就是这两个东西

    val showSharedVMToast = MutableLiveData<Unit>()
    
    private val sharedVM: MainViewModel by activityViewModels()

如果要传递普通的内容，如String，Unit换成String即可

扩展

上述点击事件和数据传递使用的是liveData，liveData有个特性就是存储的内容会回显，简单点说就是fragment被销毁后，如果对应的viewmodel没有被销毁，所持有的livedata对象在fragment/activity被重新创建好后会主动回调一次，如果业务逻辑是只想消费一次，如点击事件，不希望数据回显或数据回流，这个时候需要改造liveData成SingleEvent类型，核心思路就是内部给一个标记位或Event Wrapper，代码如下：

方式一：内部标记位

    class SingleLiveEvent<T> : MutableLiveData<T>() {
    
    private val mPending = AtomicBoolean(false)
    
    @MainThread
    override fun observe(owner: LifecycleOwner, observer: Observer<in T>) {
        if (hasActiveObservers()) {
            Log.w("SingleLiveEvent", "Multiple observers registered but only one will be notified of changes.")
        }
        // Observe the internal MutableLiveData
        super.observe(owner) { t ->
            if (mPending.compareAndSet(true, false)) {
                observer.onChanged(t)
            }
        }
    }
    
    @MainThread
    override fun setValue(t: T?) {
        mPending.set(true)
        super.setValue(t)
    }
}
    

使用方式

    //MutableLiveData换成SingleLiveEvent即可
    val mediatorLiveData = SingleLiveEvent<String>()

方式二：Event Wrappter

    /**
 * Used as a wrapper for data that is exposed via a LiveData that represents an event.
 */
open class Event<out T>(private val content: T) {

    var hasBeenHandled = false
        private set // Allow external read but not write

    /**
     * Returns the content and prevents its use again.
     */
    fun getContentIfNotHandled(): T? {
        return if (hasBeenHandled) {
            null
        } else {
            hasBeenHandled = true
            content
        }
    }

    /**
     * Returns the content, even if it's already been handled.
     */
    fun peekContent(): T = content
}

使用方式

class ListViewModel : ViewModel {
    private val _navigateToDetails = MutableLiveData<Event<String>>()

    val navigateToDetails : LiveData<Event<String>>
        get() = _navigateToDetails


    fun userClicksOnButton(itemId: String) {
        _navigateToDetails.value = Event(itemId)  // Trigger the event by setting a new Event as a new value
    }
}

观察的地方

myViewModel.navigateToDetails.observe(this, Observer {
    it.getContentIfNotHandled()?.let { // Only proceed if the event has never been handled
        startActivity(DetailsActivity...)
    }
})