用了 Android Architecture Component ** 也有一段時(shí)間了，期間也遇到過不少問題稚晚，也慢慢的解決了炊昆。map和switchmap**是 LiveData 中Transformations提供的兩個(gè)方法缝其，那具體能干些啥呢盗胀？這是個(gè)好問題。如果知道RxJava,那么可以簡單的對標(biāo) map和flatmap沸手，咱先這么理解著外遇，對不對先不管，起碼契吉，腦子中有個(gè)概念跳仿。
Transformations.map
先上源碼
/**
* Applies the given function on the main thread to each value emitted by {@code source}
* LiveData and returns LiveData, which emits resulting values.
* <p>
* The given function {@code func} will be executed on the main thread.
* <p>
* Suppose that you have a LiveData, named {@code userLiveData}, that contains user data and you
* need to display the user name, created by concatenating the first and the last
* name of the user. You can define a function that handles the name creation, that will be
* applied to every value emitted by {@code useLiveData}.
*
* <pre>
* LiveData<User> userLiveData = ...;
* LiveData<String> userName = Transformations.map(userLiveData, user -> {
* return user.firstName + " " + user.lastName
* });
* </pre>
*
* @param source a {@code LiveData} to listen to
* @param func a function to apply
* @param <X> a type of {@code source} LiveData
* @param <Y> a type of resulting LiveData.
* @return a LiveData which emits resulting values
*/
@MainThread
public static <X, Y> LiveData<Y> map(@NonNull LiveData<X> source,
@NonNull final Function<X, Y> func) {
final MediatorLiveData<Y> result = new MediatorLiveData<>();
result.addSource(source, new Observer<X>() {
@Override
public void onChanged(@Nullable X x) {
result.setValue(func.apply(x));
}
});
return result;
}
就是 傳進(jìn)去一個(gè)Livedata形式的參數(shù)和一個(gè)方法，然后將這個(gè)LiveData捐晶。通過方法中的邏輯再輸出為LiveData,最簡單的用法就是像kotlin集合中的map用法菲语。將集合中每個(gè)元素，做你方法中的操作惑灵，比如加減之類的山上。源碼中的那個(gè)注釋大概就是這個(gè)各操作，
我們來擴(kuò)展一下英支，假設(shè)現(xiàn)在有這么一個(gè)場景：界面上有個(gè)輸入框佩憾，你輸入一些字母 然后添加到數(shù)據(jù)庫中。完了之后彈個(gè)吐司干花。
我們來簡寫一下viewmodel中的代碼：
val strLivedata=MutableLiveData<String>()
fun yourFun(str:String)="新${str}被添加到數(shù)據(jù)庫中"
fun addStrClick(str:String)= { strLivedata.value=str}
val addStr:LiveData=Transformations.map(strLivedata, ::yourFun)
然后Activity中的代碼：
viewModel.addSt.observe(this, Observer{
吐司（it）
})
button.setOnClickListener {
viewModel.addStr(edittext.text.toString())
}
恩妄帘，基本上就是這樣
Transformations.switchMap
switchmap 說實(shí)話，一開始的時(shí)候池凄，我立即起來和用起來抡驼，比較費(fèi)勁，尤其是配合了lamada表達(dá)式肿仑，更是難受致盟。
先上源碼:
/**
* Creates a LiveData, let's name it {@code swLiveData}, which follows next flow:
* it reacts on changes of {@code trigger} LiveData, applies the given function to new value of
* {@code trigger} LiveData and sets resulting LiveData as a "backing" LiveData
* to {@code swLiveData}.
* "Backing" LiveData means, that all events emitted by it will retransmitted
* by {@code swLiveData}.
* <p>
* If the given function returns null, then {@code swLiveData} is not "backed" by any other
* LiveData.
*
* <p>
* The given function {@code func} will be executed on the main thread.
*
* <p>
* Consider the case where you have a LiveData containing a user id. Every time there's a new
* user id emitted, you want to trigger a request to get the user object corresponding to that
* id, from a repository that also returns a LiveData.
* <p>
* The {@code userIdLiveData} is the trigger and the LiveData returned by the {@code
* repository.getUserById} is the "backing" LiveData.
* <p>
* In a scenario where the repository contains User(1, "Jane") and User(2, "John"), when the
* userIdLiveData value is set to "1", the {@code switchMap} will call {@code getUser(1)},
* that will return a LiveData containing the value User(1, "Jane"). So now, the userLiveData
* will emit User(1, "Jane"). When the user in the repository gets updated to User(1, "Sarah"),
* the {@code userLiveData} gets automatically notified and will emit User(1, "Sarah").
* <p>
* When the {@code setUserId} method is called with userId = "2", the value of the {@code
* userIdLiveData} changes and automatically triggers a request for getting the user with id
* "2" from the repository. So, the {@code userLiveData} emits User(2, "John"). The LiveData
* returned by {@code repository.getUserById(1)} is removed as a source.
*
* <pre>
* MutableLiveData<String> userIdLiveData = ...;
* LiveData<User> userLiveData = Transformations.switchMap(userIdLiveData, id ->
* repository.getUserById(id));
*
* void setUserId(String userId) {
* this.userIdLiveData.setValue(userId);
* }
* </pre>
*
* @param trigger a {@code LiveData} to listen to
* @param func a function which creates "backing" LiveData
* @param <X> a type of {@code source} LiveData
* @param <Y> a type of resulting LiveData
*/
@MainThread
public static <X, Y> LiveData<Y> switchMap(@NonNull LiveData<X> trigger,
@NonNull final Function<X, LiveData<Y>> func) {
final MediatorLiveData<Y> result = new MediatorLiveData<>();
result.addSource(trigger, new Observer<X>() {
LiveData<Y> mSource;
@Override
public void onChanged(@Nullable X x) {
LiveData<Y> newLiveData = func.apply(x);
if (mSource == newLiveData) {
return;
}
if (mSource != null) {
result.removeSource(mSource);
}
mSource = newLiveData;
if (mSource != null) {
result.addSource(mSource, new Observer<Y>() {
@Override
public void onChanged(@Nullable Y y) {
result.setValue(y);
}
});
}
}
});
return result;
}
怎么理解呢，先看注釋上的那個(gè)例子尤慰。
定義一個(gè)用戶id字符串的livedata勾邦，一個(gè)用戶的livedata是通過Transformations.switchMap賦值的，就是傳進(jìn)去userid的livedata割择，然后通過這個(gè)id，去獲取這個(gè)用戶信息萎河。再提供一個(gè)設(shè)置userid的方法荔泳，這樣流程就串起來了蕉饼。我們來擴(kuò)展下：輸入框中輸入一個(gè)字符串，然后搜索玛歌，然后調(diào)用接口或者其他昧港，用recycleview展示出來。
先看viewModel中的代碼：
//查詢的關(guān)鍵字livedata
val query = MutableLiveData<String>()
//你點(diǎn)擊搜索是調(diào)用的方法
fun queryByStr(str: String) = apply { query.value = str}
//你的處理方法
fun yourFun(str:String):LiveData<List<String>>{
return 你網(wǎng)絡(luò)或者數(shù)據(jù)庫.query(str)
}
//查詢結(jié)果livedata
val queryResult=LiveData<List<String>>=Transformations.switchMap(
query,
::yourFun
)
再看activity中的代碼：
recycleview初始化支子，設(shè)置adapter就不寫了
搜索.setOnClickListener{
viewModel.queryByStr(搜索框.text.toString())}
viewModel.queryResult.observe(this,Observer {
//搜索結(jié)果變化了
直接給adapter的數(shù)據(jù)源list改變创肥，然后notify一下就完事了
})
大概基本上差不多就這樣了，怎么說呢值朋，光看代碼的話叹侄，其實(shí)也不難理解，主要是本人對lamada表達(dá)是有點(diǎn)怵的慌昨登，尤其是配合kotlin趾代，本來一長串，結(jié)果給省略掉了丰辣，看起來賊費(fèi)事撒强，當(dāng)然了。這個(gè)主要跟我的基礎(chǔ)語法有關(guān)系笙什，如果你深諳lamada表達(dá)式飘哨，那就沒毛病了∷銎荆總結(jié)一下芽隆。這兩個(gè)操作符。map是你將你的函數(shù)用于你傳參的livedata的數(shù)據(jù)通過函數(shù)體中的邏輯改變淘正，然后將結(jié)果傳到下游摆马。而switchmap，轉(zhuǎn)換跟map差不多鸿吆，只不過傳到下游的是livedata類型囤采。rxjava中，map的作用是操作基本函數(shù)類型惩淳，而flatmap操作的是observable類型蕉毯。當(dāng)然了，還有一些能深入去理解的東西思犁，我還沒理解到代虾。
