是否在 UI 线程上调用了 Android.App.Application 活动生命周期回调？

Question

我正在调查我们使用的第三方库 (Xamarin.Insights) 更新后开始出现的性能下降。在我的调查过程中，我在一个大停顿期间停止了应用程序，显示Android.App.Android.OnActivityDestroyed 中隐藏了一个互斥锁。

调用堆栈显示该库利用ActivityLifecycleCallbacks 接口的注册实例来跟踪我们应用程序中的活动生命周期事件。我正在尝试确定是否在 UI 线程上调用了这些回调。

直觉上我认为它们是在 UI 线程上调用的，但我找不到确凿的证据来证实这一点。

• ActivityLifecycleCallbacks 是否在 UI 线程上调用？
• 我可以提供一个链接来证明这一点吗？

Answer 1

Android.App.Application.ActivityLifecycleCallbacks 的回调方法确实是在 UI 线程上调用的。

实现Android.App.Application.ActivityLifecycleCallbacks 接口并记录回调方法的线程ID 表明线程ID 与调用活动的OnCreate 方法的线程相同：

OnActivityCreated on thread 1
OnCreate on thread 1
OnActivityStarted on thread 1
OnActivityResumed on thread 1

下面的示例可以验证这一点：

[Activity (Label = "UIThreadsCallbacks", MainLauncher = true, Icon = "@drawable/icon")]
public class MainActivity : Activity
{
    protected override void OnCreate (Bundle bundle)
    {
        base.OnCreate (bundle);

        // Set our view from the "main" layout resource
        SetContentView (Resource.Layout.Main);

        Console.WriteLine ("OnCreate on thread " + System.Threading.Thread.CurrentThread.ManagedThreadId);
    }
}

[Application]
public class MyApplication : Application
{
    public MyApplication(IntPtr javaReference, JniHandleOwnership transfer) : base(javaReference, transfer) { 
    }

    public override void OnCreate ()
    {
        RegisterActivityLifecycleCallbacks(new LifecycleCallbacks());
        base.OnCreate ();
    }
}

public class LifecycleCallbacks : Java.Lang.Object, Android.App.Application.IActivityLifecycleCallbacks
{
    public void OnActivityCreated (Activity activity, Bundle savedInstanceState)
    {
        Console.WriteLine ("OnActivityCreated on thread " + System.Threading.Thread.CurrentThread.ManagedThreadId);
    }

    public void OnActivityDestroyed (Activity activity)
    {
        Console.WriteLine ("OnActivityDestroyed on thread " + System.Threading.Thread.CurrentThread.ManagedThreadId);
    }

    public void OnActivityPaused (Activity activity)
    {
        Console.WriteLine ("OnActivityPaused on thread " + System.Threading.Thread.CurrentThread.ManagedThreadId);
    }

    public void OnActivityResumed (Activity activity)
    {
        Console.WriteLine ("OnActivityResumed on thread " + System.Threading.Thread.CurrentThread.ManagedThreadId);
    }

    public void OnActivitySaveInstanceState (Activity activity, Bundle outState)
    {
        Console.WriteLine ("OnActivitySaveInstanceState on thread " + System.Threading.Thread.CurrentThread.ManagedThreadId);
    }

    public void OnActivityStarted (Activity activity)
    {
        Console.WriteLine ("OnActivityStarted on thread " + System.Threading.Thread.CurrentThread.ManagedThreadId);
    }

    public void OnActivityStopped (Activity activity)
    {
        Console.WriteLine ("OnActivityStopped on thread " + System.Threading.Thread.CurrentThread.ManagedThreadId);
    }
}

Answer 2

你可以很容易的查看是哪个线程调用了这个方法：

1. 通过记录Thread.currentThread().getName()，如果是主线程，您会看到类似Main Thread 的内容。
2. 通过检查当前线程Looper.getMainLooper() == Looper.getMyLooper()的Looper，如果是主线程则为true。
3. 通过在方法中设置断点并在 IDE/调试器的堆栈跟踪中检查线程名称。

Answer 3

是的，它确实是在 UI 线程上调用的。 ActivityLifecycleCallbacks 由 Activity.onXXX() 方法调用。例如，下面是 Activity.onCreate() 的源代码。 getApplication().dispatchActivityCreated(this, savedInstanceState); 是调用ActivityLifecycleCallbacks.onActivityCreate()。 所以它在 UI 线程上被调用。

@MainThread
@CallSuper
protected void onCreate(@Nullable Bundle savedInstanceState) {
    if (DEBUG_LIFECYCLE) Slog.v(TAG, "onCreate " + this + ": " + savedInstanceState);
    if (mLastNonConfigurationInstances != null) {
        mFragments.restoreLoaderNonConfig(mLastNonConfigurationInstances.loaders);
    }
    if (mActivityInfo.parentActivityName != null) {
        if (mActionBar == null) {
            mEnableDefaultActionBarUp = true;
        } else {
            mActionBar.setDefaultDisplayHomeAsUpEnabled(true);
        }
    }
    if (savedInstanceState != null) {
        Parcelable p = savedInstanceState.getParcelable(FRAGMENTS_TAG);
        mFragments.restoreAllState(p, mLastNonConfigurationInstances != null
                ? mLastNonConfigurationInstances.fragments : null);
    }
    mFragments.dispatchCreate();
    getApplication().dispatchActivityCreated(this, savedInstanceState);
    if (mVoiceInteractor != null) {
        mVoiceInteractor.attachActivity(this);
    }
    mCalled = true;
}