Share ViewModels between fragments OR share LiveData between ViewModels

问题

Basically I want a way to properly share ViewModels between fragments OR share LiveData between ViewModels.

My scenario:

I have 2 fragments (FragmentA & FragmentB) - each has its own ViewModels:

FragmentA has ViewModelA, FragmentB has ViewModelB.

ViewModelA has LiveDataA1, ViewModelB has LiveDataB1 and LiveDataB2

ViewModelB is only allowed to have LiveDataB2 and ViewModelA cannot have it.

Problem is I want FragmentA to observe LiveDataB2 from ViewModelB.

Approach#1:

Aside from ViewModelA, ViewModelB is also be used in FragmentA (so it's like 2 ViewModels in FragmentA). So FragmentA will observe LiveDataB2 from ViewModelB.

This is my current implementation now. But I feel like it's not proper to have another ViewModel that is intended for other fragments. I thinking that each Fragment should only have 1 ViewModel.

Approach#2:

Create a new SharedViewModel. So we will have 3 ViewModels now: ViewModelA has LiveDataA1, ViewModelB has LiveDataB1, SharedViewModel has LiveDataB2. (Here I move LiveDataB2 from ViewModelB to SharedViewModel)

Aside from ViewModelA, SharedViewModel is also be used in FragmentA. So FragmentA will observe LiveDataB2 from SharedViewModel.

So I guess its the same as #1 but I guess but here I'm thinking that SharedViewModel is just a util ViewModel to just like get the shared data needed. So here we are like putting all the LiveDatas that can be common/shared between FragmentA and FragmentB (or even with other fragments)

Approach#3:

Share LiveData between ViewModels. I think this is wild and I don't know how to implement this. But I'm thinking that there will a new LiveDataA2 in ViewModelA that refers to the same instance as LiveDataB2 in ViewModelB.

So FragmentA will only have ViewModelA and can observe LiveDataA2. If there is a change in LiveDataB2 in ViewModelB, FragmentA will have it.

Badly need some advise here on which the proper way!

回答1:

Shared ViewModel is the right approach

You should use a single Shared ViewModel for both fragments FragmentA and FragmentB with multiple live data objects LiveDataA1, LiveDataB1 and LiveDataB2, with this approach your FragmentA can easily observe LiveDataB2.

Now, the problem with your approaches are:

Approach#1:

If FragmentA creates an instance of ViewModelB you will not get ViewModelB associate with FragmentB so ViewModelB lost its state.

Approach#2:

Somehow the problem is you are creating separate ViewModel when there is a recommended way i.e Shared ViewModel as per official docs

Approach#3:

Sharing LiveData objects is not the right approach as it is associated with lifecycle owner.

来源：https://stackoverflow.com/questions/60796010/share-viewmodels-between-fragments-or-share-livedata-between-viewmodels