LiveData 非粘性消息的探索和尝试

moneyReceivedViewModel.billLiveData.observe(this, Observer {
	if (it != null) {
		Toast.makeText(this, "到账$it元", Toast.LENGTH_SHORT).show()
	}
})
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class MoneyReceivedViewModel : ViewModel {
	private val _billLiveData = MutableLiveData<String>()
	val billLiveData: LiveData<String> = _billLiveData
	
	// 在 observe 之前和 show Toast 之后重置一下 LiveData
	fun reset() {
		_billLiveData.value = null
	}
}
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class SingleLiveEvent<T> : MutableLiveData<T>() {
    private val mPending: AtomicBoolean = AtomicBoolean(false)

    @MainThread
    override fun observe(owner: LifecycleOwner, observer: Observer<in T>) {
        if (hasActiveObservers()) {
            Log.w(
                TAG,
                "Multiple observers registered but only one will be notified of changes."
            )
        }

        // Observe the internal MutableLiveData
        super.observe(owner, object : Observer<T?> {
            override fun onChanged(t: T?) {
                if (mPending.compareAndSet(true, false)) {
                    observer.onChanged(t)
                }
            }
        })
    }

    @MainThread
    override fun setValue(t: T?) {
        mPending.set(true)
        super.setValue(t)
    }

    /**
     * Used for cases where T is Void, to make calls cleaner.
     */
    @MainThread
    fun call() {
        setValue(null)
    }

    companion object {
        private const val TAG = "SingleLiveEvent"
    }
}
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class MoneyReceivedViewModel : ViewModel() {
    val billLiveEvent = SingleLiveEvent<String>()

    fun payForLiveEvent(money: String) {
        billLiveEvent.value = money
    }
}
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viewModel.payForLiveEvent("100")

viewModel.billLiveEvent.observe(this, Observer {
    Log.d("sample", "到账 ${it} 元")
})

btn.setOnClickListener {
    viewModel.payForLiveEvent("200")
}

btn_wait.setOnClickListener {
    viewModel.billLiveEvent.observe(this, Observer {
        Log.d("sample", "到账 ${it} 元")
    })
}

// 以上代码在 Activity 的 onCreate() 中
// 当 Activity 启动的时候就会输出日志 -> 到账 100 元（没有实现不接收 observe() 之前的事件）
// 点击 btn 之后输出 -> 到账 200 元
// 点击 btn_wait 无输出（实现了事件只被消费一次）
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open class Event<out T>(private val content: T) {

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

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

    /**
     * Returns the content, even if it's already been handled.
     */
    fun peek(): T = content
}
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class MoneyReceivedViewModel : ViewModel() {
    private val _billEvent = MutableLiveData<Event<String>>()

    val billEvent: LiveData<Event<String>>
        get() = _billEvent

    fun payForEvent(msg: String) {
        _billEvent.value = Event(msg)
    }
}
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viewModel.payForEvent("100")

viewModel.billEvent.observe(this, Observer {
		it.consumed()?.let {
			Log.d("sample", "到账 ${it} 元")
		}
})

btn.setOnClickListener {
    viewModel.payForEvent("200")
}

btn_wait.setOnClickListener {
	  viewModel.billEvent.observe(this, Observer {
			it.consumed()?.let {
				Log.d("sample", "到账 ${it} 元")
			}
		})
}

// 以上代码在 Activity 的 onCreate() 中
// 当 Activity 启动的时候就会输出日志 -> 到账 100 元（没有实现不接收 observe() 之前的事件）
// 点击 btn 之后输出 -> 到账 200 元
// 点击 btn_wait 无输出（实现了事件只被消费一次）
复制代码