How can one know if an activity is started without

问题:

I have a use case where I mostly start an activity with a transition, but that's not the case when opening it from the navigation drawer.

To keep the transition smooth I have a Transition.TransitionListener in which I trigger some UI updating when the transition is done.

So I have something like this:

public class SomeActivity extends Activity {

    public void onCreate(Bundle savedInstanceState){ 
        // ...
        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.LOLLIPOP) {
            Transition sharedElementEnterTransition = getWindow().getSharedElementEnterTransition();
            sharedElementEnterTransition.addListener(new Transition.TransitionListener() {
                // ...
                @Override
                public void onTransitionEnd(Transition transition) {
                    doSomeUiUpdating();
                }
            });
        } else { // Pre-Lollipop
            doSomeUiUpdating();
        }
    }
}

This works well when starting the Activity with the animation, but how can I know if the Activity was started without a transition so that I can call doSomeUiUpdating()?

I'm sure there must be a simple method in Activity, Window, Transition or somewhere that I have overlooked. I don't want to relay on the calling Activity to set some bundle that telling if the animation is showing or not.

回答1:

You can try onTransitionStart of TransitionListener to set some boolean isAnimationStarted.

public class SomeActivity extends Activity {

    private boolean isAnimationStarted = false;

    public void onCreate(Bundle savedInstanceState) { 
        // ...
        if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.LOLLIPOP) {
            Transition sharedElementEnterTransition = getWindow().getSharedElementEnterTransition();
            sharedElementEnterTransition.addListener(new Transition.TransitionListener() {
                // ...
                @Override
                public void onTransitionEnd(Transition transition) {
                    doSomeUiUpdating();
                }

                @Override
                public void onTransitionStarted(Transition transition) {
                    isAnimationStarted = true;
                }
            });
        }
    }

    public void onStart() {
        if (!isAnimationStarted) {
            doSomeUiUpdating();
        }
    }

}

回答2:

Since you are starting an Activity, you'll be making use of an Intent to start it. You can add extras to Intents and check for them in the onCreate() of the called Activity.

Let's assume that we have 2 Activities – ActivityA, and ActivityB.

Now, let's assume that ActivityA is the calling Activity, and that ActivityB is the called Activity.

In ActivityA, let's say you've written some code to start ActivityB with a SharedElementTransition.

@Override
public void onClick(View v) {
    Intent startActivityBIntent = new Intent(getContext(), ActivityB.class);
    startActivityBIntent.putExtra("IS_SHARED_ELEMENT_TRANSITION_ENABLED", true);
    ActivityOptionsCompat activityOptionsCompat = ActivityOptionsCompat.makeSceneTransitionAnimation(getActivity(), view, ViewCompat.getTransitionName(view));
    startActivity(startActivityBIntent, activityOptionsCompat);
}

Now, if you notice above, I've passed an Intent extra with the putExtra() method. I've passed a Boolean value of true because I intend to start the Activity with a SharedElementTransition.

Now in ActivityB's onCreate() method, you can just check for the boolean value passed to the Intent. If you passed false, then you can add a conditional statement and perform your UI updating there. I've given you a small snippet below to help you get started:

private static final String isSharedElementTransitionEnabled = "IS_SHARED_ELEMENT_TRANSITION_ENABLED";    

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_b);

    // If you are postponing your SharedElementTransition, don't forget to call postponeEnterTransition() and override onPreDraw()

    if (!getIntent().getExtras().getBoolean(isSharedElementTransitionEnabled)) {
        //Do your UI updation here
    }
}

The good thing about doing it this way is that you can then have full control over how your content transitions and your shared element transitions will play out.