android Livedata最详尽的使用场景分析，让你爱上Livedata

class CountDownModel : ViewModel() {
    val countDownLivedata = MutableLiveData<String>()
    private var remainSecond = 2000//剩余秒数
    init {
        val countDown = object : CountDownTimer(2000 * 1000, 1000) {
            override fun onTick(millisUntilFinished: Long) {
                remainSecond--
                countDownLivedata.postValue("剩余：${remainSecond} 秒")
            }
            override fun onFinish() {
                countDownLivedata.postValue("倒计时结束")
            }
        }
        countDown.start()
    }
}
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val countDownModel: CountDownModel by viewModels<CountDownModel> {
        ViewModelProvider.NewInstanceFactory()
    }
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_count_down)
        countDownModel.countDownLivedata.observe(this, object : Observer<String> {
            override fun onChanged(value: String?) {
                value?.let {
                    tv_countdown_remainsecond.text = it
                }
            }
        })
    }
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class GlobalLivedata : LiveData<String>() {
    val coundManager = CountDownManager()
    val listener = object : OnDataChangeListener {
        override fun change(data: String) {
           postValue(data)
        }
    }
    override fun onActive() {
        super.onActive()
        coundManager.setListener(listener)
    }
    override fun onInactive() {
        super.onInactive()
        coundManager.removeListener(listener)
    }
    companion object {
        private lateinit var globalData: GlobalLivedata
        fun getInstance(): GlobalLivedata {
            globalData = if (::globalData.isInitialized) globalData else GlobalLivedata()
            return globalData
        }
    }
}
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private val listeners = mutableListOf<OnDataChangeListener>()
    init {
        val countDown = object : CountDownTimer(2000 * 1000, 1000) {
            override fun onTick(millisUntilFinished: Long) {
                remainSecond--
                callback("剩余：${remainSecond} 秒")
            }
            override fun onFinish() {
                callback("倒计时结束")
            }
        }
        countDown.start()
    }
    /**
     * 循环遍历回调消息
     */
    private fun callback(msg:String) {
        for (listener in listeners){
            listener.change(msg)
        }
    }
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GlobalLivedata.getInstance().observe(viewLifecycleOwner,
            { t ->
                inflate.findViewById<TextView>(R.id.tv_fragmentA).text = "fragmenta：${t}"
            })
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GlobalLivedata.getInstance().observe(viewLifecycleOwner,
            { t ->
                inflate.findViewById<TextView>(R.id.tv_fragmentB).text = "fragmentb：${t}"
            })
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class TransMapViewModel: ViewModel() {
    fun sendData() {
        userLivedata.value=User("李白",1200)//对userLivedata进行复制
    }
    val userLivedata =MutableLiveData<User>()
    val mapLiveData = Transformations.map(userLivedata){
        "${it.name} : ${it.age}"//这里可以返回任意类型的数据
    }
}
data class User(var name:String,var age:Int)
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mapViewModel.mapLiveData.observe(this,{
            logEE(it)
            tv_map.text=it
        })
        btn_map.setOnClickListener {
            mapViewModel.sendData()
        }
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class SwitchMapViewModel : ViewModel() {
    fun sendData() {
        userLivedata.value = SwitchUser("李白", 1200)
    }
    private val userLivedata = MutableLiveData<SwitchUser>()
    val mapLiveData = Transformations.switchMap(userLivedata) {
        changeUser(it!!)
    }
    private fun changeUser(it: SwitchUser): LiveData<String> {
        return MutableLiveData("${it.name}  的名字杜甫知道")
    }
}
data class SwitchUser(var name: String, var age: Int)
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model.mapLiveData.observe(this, {
            logEE(it)
        })
        btn_switchmap.setOnClickListener {
            model.sendData()
        }
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class MeditorLiveViewModel : ViewModel() {
    var count =0//计数字段
    fun setData1(name: String) {
        liveData1.value = name
    }
    fun setData2(age: Int) {
        liveData2.value = age
    }
    private val liveData1 = MutableLiveData<String>()
    private val liveData2 = MutableLiveData<Int>()
    val liveCombind = MediatorLiveData<String>()
    init {
        liveCombind.addSource(liveData1) {
            increase()
        }
        liveCombind.addSource(liveData2) {
           increase()
        }
    }
    private fun increase() {
        count++
        if(count==10){
            liveCombind.value="安安安安卓同学，您已经点击 ${count}次，再点我也不跟你玩了，收手吧。。。"
        }
    }
}
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model.liveCombind.observe(this){
            logEE(it)
            tv_count.text=it
        }
        btn_livedata1.setOnClickListener {
            model.setData1("李白")
        }
        btn_livedata2.setOnClickListener {
            model.setData2(1000)
        }
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implementation 'androidx.lifecycle:lifecycle-livedata-ktx:2.2.0'
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/**
     * 开启异步任务
     */
    fun startAsyncWithSecond(second: Int): LiveData<String> = liveData<String> {
        delay(second * 1000L)
        emit("倒计时结束")//用来触发数据回调
    }
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model.startAsyncWithSecond(3).observe(this){
                logEE(it)//model中delay  3s后会返回数据到这里
            }
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fun startAsyncEmitSource(second: Int)= liveData<String> {
        delay(second * 1000L)
        emit("${second} 秒阻塞完成,再阻塞三秒后通知你")
        val emitSourceLivedata = MutableLiveData<String>()
        emitSource(
            emitSourceLivedata
        )
        delay(second*1000L)
        emitSourceLivedata.value="再次阻塞 ${second}秒完成"
    }
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model.startAsyncEmitSource(3).observe(this){
                logEE(it)
            }
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