问题
Background
Let's take the next sceneraio:
- the user uses an app X which has multiple activities (like the gmail app).
- after navigating the app X a bit, he goes to your app.
- in your app, you need to start this X app using an intent, to go to a specific activity within it.
- now the user goes to this specific activity on the app X.
- the user presses the back button, hoping to go back to your app instead of staying on the app X.
another similar scenario:
- the user navigates on your app between activities.
- your app went to the background (using the home button, for example).
- your app shows a notification that once clicked, it will open a specific activity of your app.
- the user clicks on the notification and goes to the specific activity of your app.
- the user presses the back button, hoping to return to the app that was shown before clicking on the notification, instead of going to the previous activity on your app that was shown the last time it left it.
It seems that the last step on both scenarios isn't the default behavior.
This is why I've searched what is the best combination of flags for this purpose.
The problem
It seems that the only flag that can achieve the behavior i've described is Intent.FLAG_ACTIVITY_MULTIPLE_TASK (together with Intent.FLAG_ACTIVITY_NEW_TASK).
According to the android API, however, this flag isn't recommended for normal use:
Do not use this flag unless you are implementing your own top-level application launcher. ...
Because the default system does not include graphical task management, you should not use this flag unless you provide some way for a user to return back to the tasks you have launched.
This information seems as confusing as the rest of the descriptions about intents.
There are other intents flags that I've seen, like Intent.FLAG_ACTIVITY_CLEAR_TASK that achieve a similar result, but they have weird behaviors and/or they use high API
The question
Is it safe to use this flag? Are there any good alternatives to it?
What is the danger of using this flag and what is the meaning of the description on the API of using it?
回答1:
In your first scenario, if your app needs to start an activity of another app, it can just start this activity within the same task as your application. There is no reason to use any special Intent flags for this (you don't need FLAG_ACTIVITY_NEW_TASK or FLAG_ACTIVITY_MULTIPLE_TASK). In step 5 of your scenario, the user will return to your application (because BACK just takes him to the previous activity in the current task). This is the standard, default behaviour.
In your second scenario, my first response is "that isn't the standard behaviour". That means that users probably won't expect to be able to go back to the task that they were doing prior to clicking on the notification. However, if you really want to implement this, then I would suggest that you create a special Activity that is launched from the notification and this special Activity should have a different taskAffinity than the rest of the Activities in your application. In this case, when the special Activity is launched from the notification, it will not bring your application's task to the foreground. It will just create a new task containing just the special Activity. When the special Activity is showing, the user can press the BACK key and this will return him to the task that he was working on prior to clicking on your notification.
In general you should NOT use FLAG_ACTIVITY_MULTIPLE_TASK. The main reason is that if you have several tasks containing your application (or parts of it), it is pretty much impossible for the user to return to a specific one. There is no way to provide different launch icons or different application names (for the different tasks)(, so that the user will see multiple tasks in the "recent tasks" list, but will not be able to tell which one is which. You will have a hard time cleaning up what you are doing and you will just make more problems than you can deal with. There are about a million side effects of using this flag and for general applicaitons there is just no need to do it.
来源：https://stackoverflow.com/questions/19771051/is-it-safe-to-use-intent-flag-activity-multiple-task