在應用開發中, 我們需要使用後台任務更新前台界面, 不因頁面切換而導致重新開始, 或因某些任務阻塞界面刷新, 比如顯示下載或播放進度等. 為了追求更優質的用戶體驗, 需要大量使用後台任務, 常見的就是異步任務(AsyncTask)和後台服務(Service), 當然還有RxJava. 我寫了一個示例, 來講講如何使用這些常用的後台方式.
主要
(1) 使用異步任務和後台服務更新頁面, 避免內存洩露.
(2) 使用RxJava的時間間隔\延遲發送\定制迭代, 處理後台任務, 保存發送狀態.
示例: 旋轉屏幕更新進度條, 在摧毀頁面和新建頁面時, 保存和獲取頁面狀態.
![效果](https://www.aspphp.online/bianchen/UploadFiles_4619/201701/2017011815414734.png "\")
Gradle配置: Lambda表達式 + Butterknife + RxJava + LeakCanary.
頁面布局: Spinner選擇使用模式, ProgressBar顯示更新狀態, 可選擇啟動LeakCanary.
主邏輯:
(1) 使用Fragment存儲頁面信息, 包括異步任務, RxJava的觀察者和主題.
// 設置存儲的Fragment
FragmentManager fm = getFragmentManager();
mRetainedFragment = (RetainedFragment) fm.findFragmentByTag(RETAINED_FRAGMENT);
if (mRetainedFragment == null) {
mRetainedFragment = new RetainedFragment();
fm.beginTransaction().add(mRetainedFragment, RETAINED_FRAGMENT).commit();
}
(2) 在頁面重建時, 在onResume中恢復狀態, 繼續更新進度條.
@Override protected void onResume() {
super.onResume();
// 是否包含內存洩露
if (mSTrackLeaks.isChecked()) {
LeakCanary.install(getApplication());
}
mMode = mRetainedFragment.getMode();
mCustomAsyncTask = mRetainedFragment.getCustomAsyncTask();
mObservable = mRetainedFragment.getObservable();
mSubject = mRetainedFragment.getSubject();
mSubscriber = createSubscriber();
switch (mMode) {
case ASYNC_TASK:
if (mCustomAsyncTask != null) {
if (!mCustomAsyncTask.isCompleted()) {
mCustomAsyncTask.setActivity(this);
} else {
mRetainedFragment.setCustomAsyncTask(null);
}
}
break;
case TIME_INTERVAL:
if (mObservable != null) {
mObservable.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.take(MAX_PROGRESS)
.map(x -> x + 1)
.subscribe(mSubscriber);
}
break;
case DELAY_EMIT:
if (mObservable != null) {
mObservable.subscribeOn(Schedulers.io())
.delay(1, TimeUnit.SECONDS)
.observeOn(AndroidSchedulers.mainThread())
.subscribe(mSubscriber);
}
break;
case CUSTOM_ITERATOR:
if (mSubject != null) {
mSubject.subscribe(mSubscriber);
}
default:
break;
}
setBusy(mRetainedFragment.isBusy());
}
生命周期: onCreate -> onRestoreInstanceState -> onResume.
在onResume中設置setActivity: 因為在旋轉頁面時, 會執行onRestoreInstanceState方法, 恢復旋轉屏幕之前保存的數據, 即mPbProgressBar的值, 此時再恢復狀態. 如果移到在onCreate時設置, 則會導致Progress值為0, 因為Activity並沒有開始恢復之前的數據.
啟動異常任務AsyncTask, 在doInBackground中, 調用publishProgress顯示進度, 觸發onProgressUpdate回調, 從而更新進度條.
public class CustomAsyncTask extends AsyncTask {
private WeakReference mActivity; // 弱引用Activity, 防止內存洩露
private boolean mCompleted = false; // 是否完成
// 設置Activity控制ProgressBar
public void setActivity(MainActivity activity) {
mActivity = new WeakReference<>(activity);
}
// 判斷是否完成
public boolean isCompleted() {
return mCompleted;
}
@Override
protected Void doInBackground(Void... params) {
for (int i = 1; i < MainActivity.MAX_PROGRESS + 1; i++) {
SystemClock.sleep(MainActivity.EMIT_DELAY_MS); // 暫停時間
publishProgress(i); // AsyncTask的方法, 調用onProgressUpdate, 表示完成狀態
}
return null;
}
@Override
protected void onProgressUpdate(Integer... progress) {
mActivity.get().setProgressValue(progress[0]); // 更新ProgressBar的值
mActivity.get().setProgressPercentText(progress[0]); // 設置文字
}
@Override
protected void onPreExecute() {
mActivity.get().setProgressText("開始異步任務..."); // 准備開始
mCompleted = false;
}
@Override
protected void onPostExecute(Void result) {
mCompleted = true; // 結束
mActivity.get().setBusy(false);
mActivity.get().setProgressValue(0);
}
}
注意使用WeakReference弱引用Activity, 因為線程的回收不太穩定, 如果持有Activity, 會導致長時間無法釋放, 導致內存洩露.
使用方式
// 處理異步線程的點擊
private void handleAsyncClick() {
// 獲得異步線程
mCustomAsyncTask = new CustomAsyncTask();
mCustomAsyncTask.setActivity(this);
// 存儲異步線程
mRetainedFragment.setCustomAsyncTask(mCustomAsyncTask);
// 執行異步線程
mCustomAsyncTask.execute();
}
存儲異步任務, 在旋轉屏幕時, 頁面重建, 可以讀取當前進度, 繼續更新.
通過LocalBroadcastManager的Intent傳送當前狀態, 更新頁面.
public class CustomService extends IntentService {
public static final String KEY_EXTRA_BUSY = "busy";
public static final String KEY_EXTRA_PROGRESS = "progress";
private LocalBroadcastManager mLbm;
public CustomService() {
super(CustomService.class.getSimpleName());
}
@Override protected void onHandleIntent(Intent intent) {
mLbm = LocalBroadcastManager.getInstance(getApplicationContext());
Intent broadcastIntent = new Intent(MainActivity.UPDATE_PROGRESS_FILTER);
broadcastIntent.putExtra(KEY_EXTRA_BUSY, true);
mLbm.sendBroadcast(broadcastIntent);
for (int i = 1; i < MainActivity.MAX_PROGRESS + 1; ++i) {
broadcastIntent = new Intent(MainActivity.UPDATE_PROGRESS_FILTER);
broadcastIntent.putExtra(KEY_EXTRA_PROGRESS, i);
mLbm.sendBroadcast(broadcastIntent);
SystemClock.sleep(MainActivity.EMIT_DELAY_MS);
}
broadcastIntent = new Intent(MainActivity.UPDATE_PROGRESS_FILTER);
broadcastIntent.putExtra(KEY_EXTRA_BUSY, false);
broadcastIntent.putExtra(KEY_EXTRA_PROGRESS, 0);
mLbm.sendBroadcast(broadcastIntent);
}
}
使用方式, 先判斷進度, 後判斷狀態.
private void handleIntentServiceClick() {
mTvProgressText.setText("開始消息服務...");
Intent intent = new Intent(this, CustomService.class);
startService(intent);
}
...
private BroadcastReceiver mUpdateProgressReceiver = new BroadcastReceiver() {
@Override public void onReceive(Context context, Intent intent) {
if (intent.hasExtra(CustomService.KEY_EXTRA_PROGRESS)) {
int progress = intent.getIntExtra(CustomService.KEY_EXTRA_PROGRESS, 0);
mPbProgressBar.setProgress(progress);
setProgressPercentText(progress);
}
if (intent.hasExtra(CustomService.KEY_EXTRA_BUSY)) {
setBusy(intent.getBooleanExtra(CustomService.KEY_EXTRA_BUSY, false));
}
}
};
RxJava更新進度條的方法有很多種, 可以使用時間間隔, 延遲發送, 和定制迭代器, 但是如果需要處理頁面重建的連續更新, 需要存儲PublishSubject, 使用定制迭代器即可完成.
時間間隔: 在旋轉頁面時, 會刷新數據, 重新開始.
延遲發送: 在旋轉頁面時, 會完成前一個發送後, 重新開始.
定制迭代: 在旋轉頁面時, 可以正常完成連續更新.
時間間隔
private void handleTimeIntervalClick() {
mTvProgressText.setText("開始時間間隔...");
mSubscriber = createSubscriber();
mObservable = Observable.interval(1, TimeUnit.SECONDS);
mObservable.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.take(MAX_PROGRESS)
.map(x -> x + 1)
.subscribe(mSubscriber);
mRetainedFragment.setObservable(mObservable);
}
Observable.interval觀察者, take終止條件, map數據加工.
延遲發送
private void handleDelayEmitClick() {
mTvProgressText.setText("開始延遲發射...");
mSubscriber = createSubscriber();
mObservable = createObservable();
mObservable.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(mSubscriber);
mRetainedFragment.setObservable(mObservable);
}
觀察者發送數據時, 會延遲一秒, 即SystemClock.sleep.
// 創建延遲觀察者
private Observable createObservable() {
return Observable.create(new Observable.OnSubscribe() {
@Override public void call(Subscriber subscriber) {
for (long i = 1; i < MAX_PROGRESS + 1; i++) {
SystemClock.sleep(EMIT_DELAY_MS);
subscriber.onNext(i);
}
subscriber.onCompleted();
}
});
}
定制迭代器, 在RetainedFragment中存儲PublishSubject.
private void handleCustomIteratorClick() {
mTvProgressText.setText("開始定制迭代器...");
mObservable = Observable.from(new CustomIterator());
mSubscriber = createSubscriber();
mSubject = PublishSubject.create();
mObservable.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(mSubject);
mSubject.subscribe(mSubscriber);
mRetainedFragment.setObservable(mObservable);
mRetainedFragment.setSubject(mSubject);
}
定制迭代器, 重寫next方法, 返回數據.
public class CustomIterator implements Iterable {
private List mNumberList = new ArrayList<>();
public CustomIterator() {
for (long i = 0; i < MainActivity.MAX_PROGRESS; i++) {
mNumberList.add(i + 1);
}
}
@Override public Iterator iterator() {
return new Iterator() {
private int mCurrentIndex = 0;
@Override public boolean hasNext() {
return mCurrentIndex < mNumberList.size() && mNumberList.get(mCurrentIndex) != null;
}
@Override public Long next() {
SystemClock.sleep(MainActivity.EMIT_DELAY_MS);
return mNumberList.get(mCurrentIndex++);
}
// 不允許使用
@Override public void remove() {
throw new UnsupportedOperationException();
}
};
}
}
效果動畫
比較而言, 使用異步任務容易造成內存洩露, 並且可擴展性比較小, 適合簡單的更新; 使用後台服務比較重, 需要另起進程, 適合復雜的數據處理, 不適合更新頁面; 使用RxJava, 容易擴展, 可以控制釋放時機, 是比較不錯的選擇.
