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Code Issues Projects Releases Wiki Activity

Added lifecycle-livedata inline

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pull/157/head
M66B 6 years ago
parent 3fadf58d6a
commit 3669b9ef2e
7 changed files with 1018 additions and 1 deletions
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5
app/build.gradle
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@ -119,6 +119,10 @@ repositories {
maven { url "https://jitpack.io" }
}
configurations.all {
exclude group: "androidx.lifecycle", module: "lifecycle-livedata"
}
dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
@ -178,7 +182,6 @@ dependencies {
// https://mvnrepository.com/artifact/androidx.paging/paging-runtime
implementation "androidx.paging:paging-runtime:$paging_version"
// https://mvnrepository.com/artifact/androidx.preference/preference
implementation "androidx.preference:preference:$preference_version"

150
app/src/main/java/androidx/lifecycle/ComputableLiveData.java
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@ -0,0 +1,150 @@
/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.lifecycle;
import androidx.annotation.MainThread;
import androidx.annotation.NonNull;
import androidx.annotation.RestrictTo;
import androidx.annotation.VisibleForTesting;
import androidx.annotation.WorkerThread;
import androidx.arch.core.executor.ArchTaskExecutor;
import java.util.concurrent.Executor;
import java.util.concurrent.atomic.AtomicBoolean;
/**
* A LiveData class that can be invalidated & computed when there are active observers.
* <p>
* It can be invalidated via {@link #invalidate()}, which will result in a call to
* {@link #compute()} if there are active observers (or when they start observing)
* <p>
* This is an internal class for now, might be public if we see the necessity.
*
* @param <T> The type of the live data
* @hide internal
*/
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
public abstract class ComputableLiveData<T> {
@SuppressWarnings("WeakerAccess") /* synthetic access */
final Executor mExecutor;
@SuppressWarnings("WeakerAccess") /* synthetic access */
final LiveData<T> mLiveData;
@SuppressWarnings("WeakerAccess") /* synthetic access */
final AtomicBoolean mInvalid = new AtomicBoolean(true);
@SuppressWarnings("WeakerAccess") /* synthetic access */
final AtomicBoolean mComputing = new AtomicBoolean(false);
/**
* Creates a computable live data that computes values on the arch IO thread executor.
*/
@SuppressWarnings("WeakerAccess")
public ComputableLiveData() {
this(ArchTaskExecutor.getIOThreadExecutor());
}
/**
* Creates a computable live data that computes values on the specified executor.
*
* @param executor Executor that is used to compute new LiveData values.
*/
@SuppressWarnings("WeakerAccess")
public ComputableLiveData(@NonNull Executor executor) {
mExecutor = executor;
mLiveData = new LiveData<T>() {
@Override
protected void onActive() {
mExecutor.execute(mRefreshRunnable);
}
};
}
/**
* Returns the LiveData managed by this class.
*
* @return A LiveData that is controlled by ComputableLiveData.
*/
@SuppressWarnings("WeakerAccess")
@NonNull
public LiveData<T> getLiveData() {
return mLiveData;
}
@VisibleForTesting
final Runnable mRefreshRunnable = new Runnable() {
@WorkerThread
@Override
public void run() {
boolean computed;
do {
computed = false;
// compute can happen only in 1 thread but no reason to lock others.
if (mComputing.compareAndSet(false, true)) {
// as long as it is invalid, keep computing.
try {
T value = null;
while (mInvalid.compareAndSet(true, false)) {
computed = true;
value = compute();
}
if (computed) {
mLiveData.postValue(value);
}
} finally {
// release compute lock
mComputing.set(false);
}
}
// check invalid after releasing compute lock to avoid the following scenario.
// Thread A runs compute()
// Thread A checks invalid, it is false
// Main thread sets invalid to true
// Thread B runs, fails to acquire compute lock and skips
// Thread A releases compute lock
// We've left invalid in set state. The check below recovers.
} while (computed && mInvalid.get());
}
};
// invalidation check always happens on the main thread
@VisibleForTesting
final Runnable mInvalidationRunnable = new Runnable() {
@MainThread
@Override
public void run() {
boolean isActive = mLiveData.hasActiveObservers();
if (mInvalid.compareAndSet(false, true)) {
if (isActive) {
mExecutor.execute(mRefreshRunnable);
}
}
}
};
/**
* Invalidates the LiveData.
* <p>
* When there are active observers, this will trigger a call to {@link #compute()}.
*/
public void invalidate() {
ArchTaskExecutor.getInstance().executeOnMainThread(mInvalidationRunnable);
}
// TODO https://issuetracker.google.com/issues/112197238
@SuppressWarnings({"WeakerAccess", "UnknownNullness"})
@WorkerThread
protected abstract T compute();
}

447
app/src/main/java/androidx/lifecycle/LiveData.java
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@ -0,0 +1,447 @@
/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.lifecycle;
import static androidx.lifecycle.Lifecycle.State.DESTROYED;
import static androidx.lifecycle.Lifecycle.State.STARTED;
import androidx.annotation.MainThread;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.arch.core.executor.ArchTaskExecutor;
import androidx.arch.core.internal.SafeIterableMap;
import java.util.Iterator;
import java.util.Map;
/**
* LiveData is a data holder class that can be observed within a given lifecycle.
* This means that an {@link Observer} can be added in a pair with a {@link LifecycleOwner}, and
* this observer will be notified about modifications of the wrapped data only if the paired
* LifecycleOwner is in active state. LifecycleOwner is considered as active, if its state is
* {@link Lifecycle.State#STARTED} or {@link Lifecycle.State#RESUMED}. An observer added via
* {@link #observeForever(Observer)} is considered as always active and thus will be always notified
* about modifications. For those observers, you should manually call
* {@link #removeObserver(Observer)}.
*
* <p> An observer added with a Lifecycle will be automatically removed if the corresponding
* Lifecycle moves to {@link Lifecycle.State#DESTROYED} state. This is especially useful for
* activities and fragments where they can safely observe LiveData and not worry about leaks:
* they will be instantly unsubscribed when they are destroyed.
*
* <p>
* In addition, LiveData has {@link LiveData#onActive()} and {@link LiveData#onInactive()} methods
* to get notified when number of active {@link Observer}s change between 0 and 1.
* This allows LiveData to release any heavy resources when it does not have any Observers that
* are actively observing.
* <p>
* This class is designed to hold individual data fields of {@link ViewModel},
* but can also be used for sharing data between different modules in your application
* in a decoupled fashion.
*
* @param <T> The type of data held by this instance
* @see ViewModel
*/
public abstract class LiveData<T> {
@SuppressWarnings("WeakerAccess") /* synthetic access */
final Object mDataLock = new Object();
static final int START_VERSION = -1;
@SuppressWarnings("WeakerAccess") /* synthetic access */
static final Object NOT_SET = new Object();
private SafeIterableMap<Observer<? super T>, ObserverWrapper> mObservers =
new SafeIterableMap<>();
// how many observers are in active state
@SuppressWarnings("WeakerAccess") /* synthetic access */
int mActiveCount = 0;
private volatile Object mData = NOT_SET;
// when setData is called, we set the pending data and actual data swap happens on the main
// thread
@SuppressWarnings("WeakerAccess") /* synthetic access */
volatile Object mPendingData = NOT_SET;
private int mVersion = START_VERSION;
private boolean mDispatchingValue;
@SuppressWarnings("FieldCanBeLocal")
private boolean mDispatchInvalidated;
private final Runnable mPostValueRunnable = new Runnable() {
@Override
public void run() {
Object newValue;
synchronized (mDataLock) {
newValue = mPendingData;
mPendingData = NOT_SET;
}
//noinspection unchecked
setValue((T) newValue);
}
};
private void considerNotify(ObserverWrapper observer) {
if (!observer.mActive) {
return;
}
// Check latest state b4 dispatch. Maybe it changed state but we didn't get the event yet.
//
// we still first check observer.active to keep it as the entrance for events. So even if
// the observer moved to an active state, if we've not received that event, we better not
// notify for a more predictable notification order.
if (!observer.shouldBeActive()) {
observer.activeStateChanged(false);
return;
}
if (observer.mLastVersion >= mVersion) {
return;
}
observer.mLastVersion = mVersion;
//noinspection unchecked
observer.mObserver.onChanged((T) mData);
}
@SuppressWarnings("WeakerAccess") /* synthetic access */
void dispatchingValue(@Nullable ObserverWrapper initiator) {
if (mDispatchingValue) {
mDispatchInvalidated = true;
return;
}
mDispatchingValue = true;
do {
mDispatchInvalidated = false;
if (initiator != null) {
considerNotify(initiator);
initiator = null;
} else {
for (Iterator<Map.Entry<Observer<? super T>, ObserverWrapper>> iterator =
mObservers.iteratorWithAdditions(); iterator.hasNext(); ) {
considerNotify(iterator.next().getValue());
if (mDispatchInvalidated) {
break;
}
}
}
} while (mDispatchInvalidated);
mDispatchingValue = false;
}
/**
* Adds the given observer to the observers list within the lifespan of the given
* owner. The events are dispatched on the main thread. If LiveData already has data
* set, it will be delivered to the observer.
* <p>
* The observer will only receive events if the owner is in {@link Lifecycle.State#STARTED}
* or {@link Lifecycle.State#RESUMED} state (active).
* <p>
* If the owner moves to the {@link Lifecycle.State#DESTROYED} state, the observer will
* automatically be removed.
* <p>
* When data changes while the {@code owner} is not active, it will not receive any updates.
* If it becomes active again, it will receive the last available data automatically.
* <p>
* LiveData keeps a strong reference to the observer and the owner as long as the
* given LifecycleOwner is not destroyed. When it is destroyed, LiveData removes references to
* the observer &amp; the owner.
* <p>
* If the given owner is already in {@link Lifecycle.State#DESTROYED} state, LiveData
* ignores the call.
* <p>
* If the given owner, observer tuple is already in the list, the call is ignored.
* If the observer is already in the list with another owner, LiveData throws an
* {@link IllegalArgumentException}.
*
* @param owner The LifecycleOwner which controls the observer
* @param observer The observer that will receive the events
*/
@MainThread
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer) {
assertMainThread("observe");
if (owner.getLifecycle().getCurrentState() == DESTROYED) {
// ignore
return;
}
LifecycleBoundObserver wrapper = new LifecycleBoundObserver(owner, observer);
ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
if (existing != null && !existing.isAttachedTo(owner)) {
throw new IllegalArgumentException("Cannot add the same observer"
+ " with different lifecycles");
}
if (existing != null) {
return;
}
owner.getLifecycle().addObserver(wrapper);
}
/**
* Adds the given observer to the observers list. This call is similar to
* {@link LiveData#observe(LifecycleOwner, Observer)} with a LifecycleOwner, which
* is always active. This means that the given observer will receive all events and will never
* be automatically removed. You should manually call {@link #removeObserver(Observer)} to stop
* observing this LiveData.
* While LiveData has one of such observers, it will be considered
* as active.
* <p>
* If the observer was already added with an owner to this LiveData, LiveData throws an
* {@link IllegalArgumentException}.
*
* @param observer The observer that will receive the events
*/
@MainThread
public void observeForever(@NonNull Observer<? super T> observer) {
assertMainThread("observeForever");
AlwaysActiveObserver wrapper = new AlwaysActiveObserver(observer);
ObserverWrapper existing = mObservers.putIfAbsent(observer, wrapper);
if (existing != null && existing instanceof LiveData.LifecycleBoundObserver) {
throw new IllegalArgumentException("Cannot add the same observer"
+ " with different lifecycles");
}
if (existing != null) {
return;
}
wrapper.activeStateChanged(true);
}
/**
* Removes the given observer from the observers list.
*
* @param observer The Observer to receive events.
*/
@MainThread
public void removeObserver(@NonNull final Observer<? super T> observer) {
assertMainThread("removeObserver");
ObserverWrapper removed = mObservers.remove(observer);
if (removed == null) {
return;
}
removed.detachObserver();
removed.activeStateChanged(false);
}
/**
* Removes all observers that are tied to the given {@link LifecycleOwner}.
*
* @param owner The {@code LifecycleOwner} scope for the observers to be removed.
*/
@SuppressWarnings("WeakerAccess")
@MainThread
public void removeObservers(@NonNull final LifecycleOwner owner) {
assertMainThread("removeObservers");
for (Map.Entry<Observer<? super T>, ObserverWrapper> entry : mObservers) {
if (entry.getValue().isAttachedTo(owner)) {
removeObserver(entry.getKey());
}
}
}
/**
* Posts a task to a main thread to set the given value. So if you have a following code
* executed in the main thread:
* <pre class="prettyprint">
* liveData.postValue("a");
* liveData.setValue("b");
* </pre>
* The value "b" would be set at first and later the main thread would override it with
* the value "a".
* <p>
* If you called this method multiple times before a main thread executed a posted task, only
* the last value would be dispatched.
*
* @param value The new value
*/
protected void postValue(T value) {
boolean postTask;
synchronized (mDataLock) {
postTask = mPendingData == NOT_SET;
mPendingData = value;
}
if (!postTask) {
return;
}
ArchTaskExecutor.getInstance().postToMainThread(mPostValueRunnable);
}
/**
* Sets the value. If there are active observers, the value will be dispatched to them.
* <p>
* This method must be called from the main thread. If you need set a value from a background
* thread, you can use {@link #postValue(Object)}
*
* @param value The new value
*/
@MainThread
protected void setValue(T value) {
assertMainThread("setValue");
mVersion++;
mData = value;
dispatchingValue(null);
}
/**
* Returns the current value.
* Note that calling this method on a background thread does not guarantee that the latest
* value set will be received.
*
* @return the current value
*/
@Nullable
public T getValue() {
Object data = mData;
if (data != NOT_SET) {
//noinspection unchecked
return (T) data;
}
return null;
}
int getVersion() {
return mVersion;
}
/**
* Called when the number of active observers change to 1 from 0.
* <p>
* This callback can be used to know that this LiveData is being used thus should be kept
* up to date.
*/
protected void onActive() {
}
/**
* Called when the number of active observers change from 1 to 0.
* <p>
* This does not mean that there are no observers left, there may still be observers but their
* lifecycle states aren't {@link Lifecycle.State#STARTED} or {@link Lifecycle.State#RESUMED}
* (like an Activity in the back stack).
* <p>
* You can check if there are observers via {@link #hasObservers()}.
*/
protected void onInactive() {
}
/**
* Returns true if this LiveData has observers.
*
* @return true if this LiveData has observers
*/
@SuppressWarnings("WeakerAccess")
public boolean hasObservers() {
return mObservers.size() > 0;
}
/**
* Returns true if this LiveData has active observers.
*
* @return true if this LiveData has active observers
*/
@SuppressWarnings("WeakerAccess")
public boolean hasActiveObservers() {
return mActiveCount > 0;
}
class LifecycleBoundObserver extends ObserverWrapper implements GenericLifecycleObserver {
@NonNull
final LifecycleOwner mOwner;
LifecycleBoundObserver(@NonNull LifecycleOwner owner, Observer<? super T> observer) {
super(observer);
mOwner = owner;
}
@Override
boolean shouldBeActive() {
return mOwner.getLifecycle().getCurrentState().isAtLeast(STARTED);
}
@Override
public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
if (mOwner.getLifecycle().getCurrentState() == DESTROYED) {
removeObserver(mObserver);
return;
}
activeStateChanged(shouldBeActive());
}
@Override
boolean isAttachedTo(LifecycleOwner owner) {
return mOwner == owner;
}
@Override
void detachObserver() {
mOwner.getLifecycle().removeObserver(this);
}
}
private abstract class ObserverWrapper {
final Observer<? super T> mObserver;
boolean mActive;
int mLastVersion = START_VERSION;
ObserverWrapper(Observer<? super T> observer) {
mObserver = observer;
}
abstract boolean shouldBeActive();
boolean isAttachedTo(LifecycleOwner owner) {
return false;
}
void detachObserver() {
}
void activeStateChanged(boolean newActive) {
if (newActive == mActive) {
return;
}
// immediately set active state, so we'd never dispatch anything to inactive
// owner
mActive = newActive;
boolean wasInactive = LiveData.this.mActiveCount == 0;
LiveData.this.mActiveCount += mActive ? 1 : -1;
if (wasInactive && mActive) {
onActive();
}
if (LiveData.this.mActiveCount == 0 && !mActive) {
onInactive();
}
if (mActive) {
dispatchingValue(this);
}
}
}
private class AlwaysActiveObserver extends ObserverWrapper {
AlwaysActiveObserver(Observer<? super T> observer) {
super(observer);
}
@Override
boolean shouldBeActive() {
return true;
}
}
private static void assertMainThread(String methodName) {
if (!ArchTaskExecutor.getInstance().isMainThread()) {
throw new IllegalStateException("Cannot invoke " + methodName + " on a background"
+ " thread");
}
}
}

156
app/src/main/java/androidx/lifecycle/MediatorLiveData.java
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@ -0,0 +1,156 @@
/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.lifecycle;
import androidx.annotation.CallSuper;
import androidx.annotation.MainThread;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.arch.core.internal.SafeIterableMap;
import java.util.Map;
/**
* {@link LiveData} subclass which may observe other {@code LiveData} objects and react on
* {@code OnChanged} events from them.
* <p>
* This class correctly propagates its active/inactive states down to source {@code LiveData}
* objects.
* <p>
* Consider the following scenario: we have 2 instances of {@code LiveData}, let's name them
* {@code liveData1} and {@code liveData2}, and we want to merge their emissions in one object:
* {@code liveDataMerger}. Then, {@code liveData1} and {@code liveData2} will become sources for
* the {@code MediatorLiveData liveDataMerger} and every time {@code onChanged} callback
* is called for either of them, we set a new value in {@code liveDataMerger}.
*
* <pre>
* LiveData<Integer> liveData1 = ...;
* LiveData<Integer> liveData2 = ...;
*
* MediatorLiveData<Integer> liveDataMerger = new MediatorLiveData<>();
* liveDataMerger.addSource(liveData1, value -> liveDataMerger.setValue(value));
* liveDataMerger.addSource(liveData2, value -> liveDataMerger.setValue(value));
* </pre>
* <p>
* Let's consider that we only want 10 values emitted by {@code liveData1}, to be
* merged in the {@code liveDataMerger}. Then, after 10 values, we can stop listening to {@code
* liveData1} and remove it as a source.
* <pre>
* liveDataMerger.addSource(liveData1, new Observer<Integer>() {
* private int count = 1;
*
* {@literal @}Override public void onChanged(@Nullable Integer s) {
* count++;
* liveDataMerger.setValue(s);
* if (count > 10) {
* liveDataMerger.removeSource(liveData1);
* }
* }
* });
* </pre>
*
* @param <T> The type of data hold by this instance
*/
@SuppressWarnings("WeakerAccess")
public class MediatorLiveData<T> extends MutableLiveData<T> {
private SafeIterableMap<LiveData<?>, Source<?>> mSources = new SafeIterableMap<>();
/**
* Starts to listen the given {@code source} LiveData, {@code onChanged} observer will be called
* when {@code source} value was changed.
* <p>
* {@code onChanged} callback will be called only when this {@code MediatorLiveData} is active.
* <p> If the given LiveData is already added as a source but with a different Observer,
* {@link IllegalArgumentException} will be thrown.
*
* @param source the {@code LiveData} to listen to
* @param onChanged The observer that will receive the events
* @param <S> The type of data hold by {@code source} LiveData
*/
@MainThread
public <S> void addSource(@NonNull LiveData<S> source, @NonNull Observer<? super S> onChanged) {
Source<S> e = new Source<>(source, onChanged);
Source<?> existing = mSources.putIfAbsent(source, e);
if (existing != null && existing.mObserver != onChanged) {
throw new IllegalArgumentException(
"This source was already added with the different observer");
}
if (existing != null) {
return;
}
if (hasActiveObservers()) {
e.plug();
}
}
/**
* Stops to listen the given {@code LiveData}.
*
* @param toRemote {@code LiveData} to stop to listen
* @param <S> the type of data hold by {@code source} LiveData
*/
@MainThread
public <S> void removeSource(@NonNull LiveData<S> toRemote) {
Source<?> source = mSources.remove(toRemote);
if (source != null) {
source.unplug();
}
}
@CallSuper
@Override
protected void onActive() {
for (Map.Entry<LiveData<?>, Source<?>> source : mSources) {
source.getValue().plug();
}
}
@CallSuper
@Override
protected void onInactive() {
for (Map.Entry<LiveData<?>, Source<?>> source : mSources) {
source.getValue().unplug();
}
}
private static class Source<V> implements Observer<V> {
final LiveData<V> mLiveData;
final Observer<? super V> mObserver;
int mVersion = START_VERSION;
Source(LiveData<V> liveData, final Observer<? super V> observer) {
mLiveData = liveData;
mObserver = observer;
}
void plug() {
mLiveData.observeForever(this);
}
void unplug() {
mLiveData.removeObserver(this);
}
@Override
public void onChanged(@Nullable V v) {
if (mVersion != mLiveData.getVersion()) {
mVersion = mLiveData.getVersion();
mObserver.onChanged(v);
}
}
}
}

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app/src/main/java/androidx/lifecycle/MutableLiveData.java
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/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.lifecycle;
/**
* {@link LiveData} which publicly exposes {@link #setValue(T)} and {@link #postValue(T)} method.
*
* @param <T> The type of data hold by this instance
*/
@SuppressWarnings("WeakerAccess")
public class MutableLiveData<T> extends LiveData<T> {
@Override
public void postValue(T value) {
super.postValue(value);
}
@Override
public void setValue(T value) {
super.setValue(value);
}
}

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app/src/main/java/androidx/lifecycle/Observer.java
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/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.lifecycle;
/**
* A simple callback that can receive from {@link LiveData}.
*
* @param <T> The type of the parameter
*
* @see LiveData LiveData - for a usage description.
*/
public interface Observer<T> {
/**
* Called when the data is changed.
* @param t The new data
*/
void onChanged(T t);
}

194
app/src/main/java/androidx/lifecycle/Transformations.java
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/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.lifecycle;
import androidx.annotation.MainThread;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.arch.core.util.Function;
/**
* Transformation methods for {@link LiveData}.
* <p>
* These methods permit functional composition and delegation of {@link LiveData} instances. The
* transformations are calculated lazily, and will run only when the returned {@link LiveData} is
* observed. Lifecycle behavior is propagated from the input {@code source} {@link LiveData} to the
* returned one.
*/
@SuppressWarnings("WeakerAccess")
public class Transformations {
private Transformations() {
}
/**
* Returns a {@code LiveData} mapped from the input {@code source} {@code LiveData} by applying
* {@code mapFunction} to each value set on {@code source}.
* <p>
* This method is analogous to {@link io.reactivex.Observable#map}.
* <p>
* {@code transform} will be executed on the main thread.
* <p>
* Here is an example mapping a simple {@code User} struct in a {@code LiveData} to a
* {@code LiveData} containing their full name as a {@code String}.
*
* <pre>
* LiveData<User> userLiveData = ...;
* LiveData<String> userFullNameLiveData =
* Transformations.map(
* userLiveData,
* user -> user.firstName + user.lastName);
* });
* </pre>
*
* @param source the {@code LiveData} to map from
* @param mapFunction a function to apply to each value set on {@code source} in order to set
* it
* on the output {@code LiveData}
* @param <X> the generic type parameter of {@code source}
* @param <Y> the generic type parameter of the returned {@code LiveData}
* @return a LiveData mapped from {@code source} to type {@code <Y>} by applying
* {@code mapFunction} to each value set.
*/
@MainThread
@NonNull
public static <X, Y> LiveData<Y> map(
@NonNull LiveData<X> source,
@NonNull final Function<X, Y> mapFunction) {
final MediatorLiveData<Y> result = new MediatorLiveData<>();
result.addSource(source, new Observer<X>() {
@Override
public void onChanged(@Nullable X x) {
result.setValue(mapFunction.apply(x));
}
});
return result;
}
/**
* Returns a {@code LiveData} mapped from the input {@code source} {@code LiveData} by applying
* {@code switchMapFunction} to each value set on {@code source}.
* <p>
* The returned {@code LiveData} delegates to the most recent {@code LiveData} created by
* calling {@code switchMapFunction} with the most recent value set to {@code source}, without
* changing the reference. In this way, {@code switchMapFunction} can change the 'backing'
* {@code LiveData} transparently to any observer registered to the {@code LiveData} returned
* by {@code switchMap()}.
* <p>
* Note that when the backing {@code LiveData} is switched, no further values from the older
* {@code LiveData} will be set to the output {@code LiveData}. In this way, the method is
* analogous to {@link io.reactivex.Observable#switchMap}.
* <p>
* {@code switchMapFunction} will be executed on the main thread.
* <p>
* Here is an example class that holds a typed-in name of a user
* {@code String} (such as from an {@code EditText}) in a {@link MutableLiveData} and
* returns a {@code LiveData} containing a List of {@code User} objects for users that have
* that name. It populates that {@code LiveData} by requerying a repository-pattern object
* each time the typed name changes.
* <p>
* This {@code ViewModel} would permit the observing UI to update "live" as the user ID text
* changes.
*
* <pre>
* class UserViewModel extends AndroidViewModel {
* MutableLiveData<String> nameQueryLiveData = ...
*
* LiveData<List<String>> getUsersWithNameLiveData() {
* return Transformations.switchMap(
* nameQueryLiveData,
* name -> myDataSource.getUsersWithNameLiveData(name));
* }
*
* void setNameQuery(String name) {
* this.nameQueryLiveData.setValue(name);
* }
* }
* </pre>
*
* @param source the {@code LiveData} to map from
* @param switchMapFunction a function to apply to each value set on {@code source} to create a
* new delegate {@code LiveData} for the returned one
* @param <X> the generic type parameter of {@code source}
* @param <Y> the generic type parameter of the returned {@code LiveData}
* @return a LiveData mapped from {@code source} to type {@code <Y>} by delegating
* to the LiveData returned by applying {@code switchMapFunction} to each
* value set
*/
@MainThread
@NonNull
public static <X, Y> LiveData<Y> switchMap(
@NonNull LiveData<X> source,
@NonNull final Function<X, LiveData<Y>> switchMapFunction) {
final MediatorLiveData<Y> result = new MediatorLiveData<>();
result.addSource(source, new Observer<X>() {
LiveData<Y> mSource;
@Override
public void onChanged(@Nullable X x) {
LiveData<Y> newLiveData = switchMapFunction.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;
}
/**
* Creates a new {@link LiveData} object that does not emit a value until the source LiveData
* value has been changed. The value is considered changed if {@code equals()} yields
* {@code false}.
*
* @param source the input {@link LiveData}
* @param <X> the generic type parameter of {@code source}
* @return a new {@link LiveData} of type {@code X}
*/
@MainThread
@NonNull
public static <X> LiveData<X> distinctUntilChanged(@NonNull LiveData<X> source) {
final MediatorLiveData<X> outputLiveData = new MediatorLiveData<>();
outputLiveData.addSource(source, new Observer<X>() {
boolean mFirstTime = true;
@Override
public void onChanged(X currentValue) {
final X previousValue = outputLiveData.getValue();
if (mFirstTime
|| (previousValue == null && currentValue != null)
|| (previousValue != null && !previousValue.equals(currentValue))) {
mFirstTime = false;
outputLiveData.setValue(currentValue);
}
}
});
return outputLiveData;
}
}
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