Finish old activity and start a new one or vice ve

问题:

I know, that I get the same result with both code snippets

finish();
startActivity(newActivity);

and

startActivity(newActivity);
finish();

I'd like to know your opinion, if there is a big difference between them. Is one better than the other? If so, why?

回答1:

When you do startActivity(), all that does is post your intent in a queue of events. The actual starting of the activity happens asynchronously in the near future. So I don't see a big difference between the two.

回答2:

The animation is clearly different (at least on 4.1 onwards). Calling finish() first starts to fade away the first activity earlier and you can briefly see a black background before the new activity fades in. Calling startActivity() first fades in the new activity on top of the old one and the black background is not visible.

回答3:

In addition to the Emmanuels answer:

Both methods startActivity and finish will be scheduled after the end of the invoking method, since both are processed by the UI thread.

回答4:

I would do the second choice, I'm not backing this on anything that I have looked up from official sources, but, it makes more sense to launch the new activity before you call finish, that way the new activity pops up via an intent, and the now background activity can call all its cleaning up methods.

If you were to do it the other way around, maybe the intent wont have time to fire before the cleaning up is done. I.e. will the activity call startActivity() after the finish() call?

I hope you understand what I'm trying to state, I would do the second option just to be safe.

回答5:

I had the similar issue:

Activity A: singleInstance
Activity B: singleInstance
Activity C: singleInstance

A starts B 
B starts C
C wants to start A:

here if I use:

finish();
startActivity(A);

something wired happens: Activity B comes to foreground instead of A! but if I change the code like this:

startActivity(A);
finish();

everything seems ok and Activity A comes visible.

I don't know what's the problem, but it seems that in the first case, C is finished before executing the startActivity command so that the back stack handles the situation and shows its top activity which is B! but in the second case, everything happens normally.

回答6:

I usually do startActivity() before finish() as I think that'd make sure the new screen comes out before the previous goes out.

I have a login page in my app. After user login successfully, the login activity is gone and main activity is up. It works fine in the Android 4.

Today I wanted to rewrite it in Material design. However, I got a big problem. New Android studio creates blank activity with material design which I think takes many resources. The same process, but I got the error

11-26 18:20:44.450 18397-18397/? I/Choreographer: Skipped 42 frames!  The application may be doing too much work on its main thread.
11-26 18:20:44.485 18397-18408/? I/art: Background partial concurrent mark sweep GC freed 2864(191KB) AllocSpace objects, 4(43MB) LOS objects, 13% free, 100MB/116MB, paused 8.056ms total 39.767ms

It said my apps takes many resources when mainActivity starts in my phone logcat. I have nothing in mainActivity just that it is the default Material Design layout.

I reversed the order and now it works without error on my phone.