android中,对异步编程进行对比,进而选择最合适的实现方式
分别从宜家、家乐福获取商品桌子的信息,获取到后两者进行比较,挑选出最合意的桌子。依赖关系如下图所示。
UI效果图如下所示:
大概业务逻辑是这样:
- 在UI上分别显示:"正在请求宜家数据...","正在请求家乐福数据...","等待任务1和任务2...";
- 开启异步任务,并发请求宜家数据和家乐福数据;
- 当宜家数据请求成功后,把商品信息显示在UI上;
- 当家乐福数据请求成功后,同时也把商品信息显示在UI上;
- 第3步和第4步没有先后顺序;
- 当第3步中的宜家数据和第4步中家乐福数据都请求下来后,开启新的异步任务,比较两家商品谁更好。同时在UI上显示:"开始比较"
- 有了比较结果后,把比较结果(哪家商品更好)显示到UI上。
以下分别用基础的线程、线程池、java8支持的completefuture、rxjava、协程来实现。 需要注意的是,当用户取消任务(从当前activity返回)时,我们要去调用cancel方法。ikea表示宜家,carrefour表示家乐福,goods表示商品(这里我们用买桌子来做比喻)。
用最基础的线程Thread来实现。需要注意如下几点:
- cancel时,需要调用Thread的interrupt方法,同时设置标记量canceled为true
- 线程中捕获InterruptedException,检查标记量canceled
- uiTask中判断ikeaGoods和carrefourGoods是否都具备了,否则不可以去调用betterGoods去做商品比较
- canceled没有被标记为@Volatile,因为目前只在主线程中访问了canceled
override fun requestServer() {
view.displayIKEAGoods(Resource(Resource.LOADING))
view.displayCarrefourGoods(Resource(Resource.LOADING))
view.displayBetterGoods(Resource(Resource.WAITING))
var ikeaGoods: Goods? = null
var carrefourGoods: Goods? = null
val uiTask = { action: () -> Unit ->
mainThreadHandler.post {
if (canceled) return@post
action()
safeLet(ikeaGoods, carrefourGoods) { it0, it1 ->
betterGoods(it0, it1)
}
}
}
threads += Thread {
try {
val goods = backendWork.getGoodsFromIKEA()
uiTask {
ikeaGoods = goods
view.displayIKEAGoods(Resource(Resource.FINISH, goods))
}
} catch (e: InterruptedException) {
e.printStackTrace()
} finally {
val currentThread = Thread.currentThread()
mainThreadHandler.post {
threads -= currentThread
}
}
}.apply { start() }
threads += Thread {
try {
val goods = backendWork.getGoodsFromCarrefour()
uiTask {
carrefourGoods = goods
view.displayCarrefourGoods(Resource(Resource.FINISH, goods))
}
} catch (e: InterruptedException) {
e.printStackTrace()
} finally {
val currentThread = Thread.currentThread()
mainThreadHandler.post {
threads -= currentThread
}
}
}.apply { start() }
}
private fun betterGoods(ikeaGoods: Goods, carrefourGoods: Goods) {
view.displayBetterGoods(Resource(Resource.LOADING))
threads += Thread {
try {
val goods = backendWork.selectBetterOne(ikeaGoods, carrefourGoods)
mainThreadHandler.post {
if (canceled) return@post
view.displayBetterGoods(Resource(Resource.FINISH, goods))
}
} catch (e: InterruptedException) {
e.printStackTrace()
} finally {
val currentThread = Thread.currentThread()
mainThreadHandler.post {
threads -= currentThread
}
}
}.apply { start() }
}
override fun cancel() {
canceled = true
threads.forEach { it.interrupt() }
}
不过,android直接用Thread做异步任务的实现,已经很少了。它本身更容易出错,而且难度大一些,要去理清同步互斥,锁的操作等。
android开发,经常要直接接触线程池的使用和设计。针对上边的业务需求,这个是使用线程池的实现版本。
我们用到了ikeaFuture.get()
和carrefourFuture.get()
,用于模拟等待任务1和任务2。
override fun requestServer() {
view.displayIKEAGoods(Resource(Resource.LOADING))
view.displayCarrefourGoods(Resource(Resource.LOADING))
view.displayBetterGoods(Resource(Resource.WAITING))
val ikeaFuture = backendExecutor.submit<Goods> {
backendWork.getGoodsFromIKEA().alsoPostToUI {
view.displayIKEAGoods(Resource(Resource.FINISH, it))
}
}
val carrefourFuture = backendExecutor.submit<Goods> {
backendWork.getGoodsFromCarrefour().alsoPostToUI {
view.displayCarrefourGoods(Resource(Resource.FINISH, it))
}
}
backendExecutor.submit<Goods> {
val ikeaGoods = ikeaFuture.get()
val carrefourGoods = carrefourFuture.get()
mainHandler.post {
view.displayBetterGoods(
Resource(
Resource.LOADING,
"start compare which one is better"
)
)
}
backendWork.selectBetterOne(ikeaGoods, carrefourGoods).alsoPostToUI {
view.displayBetterGoods(Resource(Resource.FINISH, it))
}
}
}
private fun Goods.alsoPostToUI(task: (Goods) -> Unit): Goods {
mainHandler.post {
task(this)
}
return this
}
需要调用线程池的shutdownNow,这里没有调用shutdown。shutdownNow不仅取消了等待队列中的任务,而且对正在执行的任务也会通知interrupt。
override fun cancel() {
backendExecutor.shutdownNow()
}
java8开始支持CompletableFuture,不过要在android N及更高的版本才支持。
override fun requestServer() {
view.displayIKEAGoods(Resource(Resource.LOADING))
view.displayCarrefourGoods(Resource(Resource.LOADING))
view.displayBetterGoods(Resource(Resource.WAITING))
val ikeaFuture = CompletableFuture.supplyAsync(Supplier {
backendWork.getGoodsFromIKEA()
}, backendExecutor).apply {
thenAcceptAsync(Consumer {
view.displayIKEAGoods(Resource(Resource.FINISH, it))
}, mainThreadExecutor)
futures += this
}
val carrefourFuture = CompletableFuture.supplyAsync(Supplier {
backendWork.getGoodsFromCarrefour()
}, backendExecutor).apply {
thenAcceptAsync(Consumer {
view.displayCarrefourGoods(Resource(Resource.FINISH, it))
}, mainThreadExecutor)
futures += this
}
futures += ikeaFuture.thenCombineAsync(
carrefourFuture,
BiFunction<Goods, Goods, Pair<Goods, Goods>> { g0, g1 ->
view.displayBetterGoods(Resource(Resource.LOADING))
Pair(g0, g1)
},
mainThreadExecutor
).thenApplyAsync(java.util.function.Function<Pair<Goods, Goods>, Goods> {
backendWork.selectBetterOne(it.first, it.second)
}, backendExecutor).thenAcceptAsync(Consumer<Goods> {
view.displayBetterGoods(Resource(Resource.FINISH, it))
}, mainThreadExecutor)
}
override fun cancel() {
futures.forEach {
it.cancel(true)
}
}
override fun requestServer() {
view.displayIKEAGoods(Resource(Resource.LOADING))
view.displayCarrefourGoods(Resource(Resource.LOADING))
view.displayBetterGoods(Resource(Resource.WAITING))
val ikeaObservable = goodsModel.getGoodsFromIKEAAsync()
.observeOn(AndroidSchedulers.mainThread())
.doOnNext {
view.displayIKEAGoods(Resource(Resource.FINISH, it))
}
val carrefourObservable = goodsModel.getGoodsFromCarrefourAsync()
.observeOn(AndroidSchedulers.mainThread())
.doOnNext {
view.displayCarrefourGoods(Resource(Resource.FINISH, it))
}
compositeDisposable += Observable.zip(
ikeaObservable,
carrefourObservable,
BiFunction { t1: Goods, t2: Goods ->
view.displayBetterGoods(Resource(Resource.LOADING))
Pair(t1, t2)
}).flatMap {
goodsModel.selectBetterOneAsync(it.first, it.second)
}.observeOn(AndroidSchedulers.mainThread())
.subscribe {
view.displayBetterGoods(Resource(Resource.FINISH, it))
}
}
override fun cancel() {
compositeDisposable.dispose()
}
override fun requestServer() {
uiScope.launch {
val deferredIKEAGoods = goodsModel.getGoodsFromIKEAAsync()
val deferredCarrefourGoods = goodsModel.getGoodsFromCarrefourAsync()
view.displayIKEAGoods(Resource(Resource.LOADING))
view.displayCarrefourGoods(Resource(Resource.LOADING))
launch {
val goods = deferredIKEAGoods.await()
view.displayIKEAGoods(Resource(Resource.FINISH, goods))
}
launch {
val goods = deferredCarrefourGoods.await()
view.displayCarrefourGoods(Resource(Resource.FINISH, goods))
}
view.displayBetterGoods(Resource(Resource.WAITING))
val ikeaGoods = deferredIKEAGoods.await()
val carrefourGoods = deferredCarrefourGoods.await()
view.displayBetterGoods(Resource(Resource.LOADING))
val betterGoods = goodsModel.selectBetterOneAsync(supervisorJob, ikeaGoods, carrefourGoods).await()
view.displayBetterGoods(Resource(Resource.FINISH, betterGoods))
}
}
override fun cancel() {
supervisorJob.cancel()
}
- CompletableFuture虽然早已被java8支持,但是只能在android N及更高版本才可以使用;
- 使用thread做异步的业务逻辑处理,虽然操作的粒度很细,但是要照顾的细节太多,还要理清锁和同步互斥,开发效率并不会提高;
- 若使用线程池,对于任务的依赖关系只能用简单的future的get来处理,实现任务依赖的能力过于单薄;
- rxjava的代码量相对就少很多,对依赖关系处理能力强,可以轻松进行线程切换,若条件允许,建议使用rxjava;
- kotlin通过扩展包的方式支持了协程,可是在android上java还不支持协程。在纯kotlin项目中,相对rxjava,协程更轻量级,异步任务处理能力也挺强;
- 但是若是在java+kotlin混合项目中,协程则不能充分应用项目工程所有地方(仅在kotlin代码中使用)。此时为了工程保持接口统一,也可能需要放弃使用协程了。