簡介
這是Android官方推薦“正確寫一個Android App"的方法，這個官方推薦的方法由一些非常基本library以及pattern組成，稱為Architecture components，包括以下：Lifecycles: 每個app必考慮的東西
Lifecycle-aware observables: 觀察lifecycles的方法
Lightweight ViewModel: 這是獨立於Activity和Fragment之外，可以單獨測試邏輯之處
Object mapping for sqlite
要解決什麼問題？
通常programmer把所有的邏輯（商業/UI/Data/與OS互動等）都寫在activity或是fragment中，不過activity事實上是設計來操縱UI與OS互動，越簡單越能避免life cycle造成的問題，主要是因為activity / fragment 這類型的class，programmer並沒有絕對掌控權，從instance creation到destruction都是OS決定，所以要盡量減少對這些class的dependency。此外model應該永遠獨立於view之外，永遠不受到OS相關決定（回收記憶體之類）的影響。
Life-cycle Aware Components
我們通常會把某些邏輯或是object寫在一個activity或是fragment裡面，應且在life cycle callback中作出對應的動作。例如以下MyLocationListener instance會在MyActivity的onStart()和onStop() 時也去做start() / stop() 動作：class MyLocationListener {
public MyLocationListener(Context context, Callback callback) {
// ...
}
void start() {
// connect to system location service
}
void stop() {
// disconnect from system location service
}
}
class MyActivity extends AppCompatActivity {
private MyLocationListener myLocationListener;
@Override
public void onCreate(...) {
myLocationListener = new MyLocationListener(this, (location) -> {
// update UI
});
}
@Override
public void onStart() {
super.onStart();
myLocationListener.start();
// manage other components that need to respond
// to the activity lifecycle
}
@Override
public void onStop() {
super.onStop();
myLocationListener.stop();
// manage other components that need to respond
// to the activity lifecycle
}
}但是如果有很多類似的component在這個activity裡面，那他的life cycle callbacks就會充滿這類code，變得複雜難maintain。
此外我們不能保證async operations會在我們要的life cycle callbacks回來：
class MyActivity extends AppCompatActivity {
private MyLocationListener myLocationListener;
public void onCreate(...) {
myLocationListener = new MyLocationListener(this, location -> {
// update UI
});
}
@Override
public void onStart() {
super.onStart();
Util.checkUserStatus(result -> {
// what if this callback is invoked AFTER activity is stopped?
if (result) {
myLocationListener.start();
}
});
}
@Override
public void onStop() {
super.onStop();
myLocationListener.stop();
}
}android.arch.lifecycle提供了一些design pattern來幫助我們免於煩這些（鳥）事。Life Cycle
LifeCycle這個class把某個有life cycle概念的component (e.g. Activity / Fragment / Service) 的life cycle給抽象化了，它有states和events:
所以對這個component life cycle有興趣的人，就可以implement LifeCycleObserver:
public class MyObserver implements LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
public void connectListener() {
...
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
public void disconnectListener() {
...
}
}
myLifecycleOwner.getLifecycle().addObserver(new MyObserver());Life Cycle Owner
擁有LifeCycle的components必須implement LifeCycleOwner interface (AppCompatActivity的某個super class是implement這個interface的），所以最快的讓自己的activity或是fragment變成life cycle owner就是去繼承有implement這個interface的base class：class MyActivity extends AppCompatActivity {
private MyLocationListener myLocationListener;
public void onCreate(...) {
myLocationListener = new MyLocationListener(this, getLifecycle(), location -> {
// update UI
});
Util.checkUserStatus(result -> {
if (result) {
myLocationListener.enable();
}
});
}
}class MyLocationListener implements LifecycleObserver { private boolean enabled = false; public MyLocationListener(Context context, Lifecycle lifecycle, Callback callback) { ... } @OnLifecycleEvent(Lifecycle.Event.ON_START) void start() { if (enabled) { // connect } } public void enable() { enabled = true; if (lifecycle.getCurrentState().isAtLeast(STARTED)) { // connect if not connected } } @OnLifecycleEvent(Lifecycle.Event.ON_STOP) void stop() { // disconnect if connected } }
可以看到MyLocationListener並沒有override什麼interface method，而是依賴於annotations。我們說MyLocationListener是一個life cycle aware object。
如果不繼承已知的life cycle owner class的話，也可以自己去implement LifeCycleOwner interface，但是要透過以下的templates:
public class MyActivity extends Activity implements LifecycleOwner {
private LifecycleRegistry mLifecycleRegistry;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mLifecycleRegistry = new LifecycleRegistry(this);
mLifecycleRegistry.markState(Lifecycle.State.CREATED);
}
@Override
public void onStart() {
super.onStart();
mLifecycleRegistry.markState(Lifecycle.State.STARTED);
}
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}
