codelabs

collectAsState (Code A) is safe for any kind of Flow (cold/hot it doesn't matter). If you look at how collectAsState is implemented then you will see that it uses a LaunchedEffect deep down (collectAsState -> produceState -> LaunchedEffect)

I summed height of all elements in the design. It was 308. Then, I think 310 will be an ideal number. No problem, when I make it 310. Everything seems good.

Recomposition happens when either the parameter values change or a mutable state variable within the composable function is updated.

A stateless function, sometimes referred to as an "immutable" function, is one where no data is stored in the function. This means that if you call the function repeatedly, it would be the same as if you had called it for the first time. The function retains no memory of any variables each time it is called.

Hoisting the state of a composable means that the state variables are kept outside of the composable and the values of those state variables are passed into the composable as normal parameters. This allows you to reuse the composable. This is especially important if you are creating a library of composables that you want to reuse from one project to another. Users of the library can be assured that there are no state dependencies that the composable requires. If you want a truely stateless composable, you would avoid doing things like passing in viewmodels. A viewmodel is very much dependent on the app in which it resides and will have code in it that is very specific to the app. A stateless composable has no dependencies on the app and therefore can be reused elsewhere.

That doesn't mean that you can't use viewmodels in your composables. Initially Google was against this when Compose first came out but realized that it was very awkward for developers. If a developer had no reason to make a reusable composable outside of the screen that it appears on, hoisting the state for every composable becomes a pain resulting in unnecessary boilerplate code. So the general rule is that if your composable is not going to be reused elsewhere and you need access to viewmodel data, you can either pass in the viewmodel as a parameter or access it within the composable through other means.

Aside from viewmodels, you may still want to make a composable stateful even when it needs to be reused. A good example of this is if you are using a TextField. As you are typing in text, you want the text to appear. Without using a state variable to retain the typed characters, you won't see the TextField update. For this reason, using a local state variable to store the entered characters is acceptable. While this doesn't make the composable stateless, it is still something you need to implement. However, even in this example, you can still make it stateless by passing the typed characters back up to the hoisted function and storing it in a viewmodel state variable that in turn triggers the hoisted function to recompose the composable and pass in the text that the TextField needs. But this is rather convoluted and a big round trip just to get characters shown in the TextField and therefore isn't recommended. You might want to do it though if you have a very complex TextField composable and you need to process the characters in your viewmodel as they are being typed - such as might be the case for doing a suggestion lookup for a url.

So even if your composable is stateless but your viewmodel has a mutable state variable that the hoisted function is observing, if the mutable state of that variable changes, the hoisted function will recompose. But whether the composable that the hoisted composable is calling gets recomposed depends on whether the parameter values to that composable change. If they change, a recomposition will occur.

I've worked with Compose since the early alpha stages and became quickly disappointed with Google's lame attempt at providing a more modern approach to navigating a single-activity app. When you consider that Android's view-based system was entirely replaced with the declaration approach that Compose uses, you have to seriously wonder why they would stick with a navigation controller that doesn't allow you pass objects from one screen to another. There was also the issue that adding animation when transitioning from one screen to another was an afterthought. There is an add-on that supports animation transitions.

But perhaps the worst thing about Compose was its lack of handling device configuration changes. Under the older view-based system, you defined your layouts in xml files and placed these in resource folders that had qualifiers in the folder name that would aid Android in picking the correct layout based on things like screen density, orientation, screen size, etc. That went out the window with Compose. Eventually Google did add APIs to handle composables that need to be selected based on screen sizes and densities. But ultimately, you end up writing this decision logic within your composable and your code starts to look like spaghetti. Google's Android team completely forgot about the most basic "Separation of Concerns" when they chose to mix UI layouts with the logic that determines which layout gets selected. Designing your composables is one thing and how they get selected is another. Don't mix the two. Your composables become increasingly less reusable when you integrate those APIs. The original developers of Android (who weren't from Google) knew well enough to have the operating system manage the layout selection based upon changes to device configurations.

I chose to create my own framework that handles navigation and manages composables in almost an identical way that the view-based system works. It also remedies the issue of not being able to pass objects from screen to screen. If you are interested, you can check it out at:

https://github.com/JohannBlake/Jetmagic

So to answer you question about whether Compose Navigation is good for navigating, I would have to say no. I have worked with it and have found it severely lacking. If you want to be just-another-run-of-the-mill-Android-developer, then use Compose Navigation. But if you want to chart your own path and liberate yourself from Google's poor design practices, then use Jetmagic or even better, create your own navigation framework. It isn't that hard.

Remember that each of the on variables is a callback. onScreenChange implicitly passes the variable it as String, but it is not necessary to use the variable within your callback.

1. it is implicitly passed, but in the GitHub link you gave, the programmer decided to rename it to onScreenChange. That callback then was passed to OverviewBody, which overrides the default {}, as you said. What I don't understand about your question is where onAccountClick is coming from. That is not a parameter of OverviewBody, according to the GitHub code.

2. content has a parameter for another callback, which you noticed. The body variable for each class in the enum is then invoked, and the callback parameter is passed to the invocation. In the future, that callback will only be used by OverviewBody. Remember that body(callback) is the same as body.invoke(callback).

I hope this helps. Please let me know if anything needs some more explanation.

For further reference:

• Kotlin Lambda Docs

Additional Content Edit:

1. I see the parameters in your code, not the one on GitHub. My apologies for missing that.
2. body is a variable function. This means that the variable function can be invoked, either by writing body(...) or body.invoke(...). The String parameter it, which is passed implicitly, is ignored and the default values are used, unless specified like you suggested (onAccountClick=it).

Try this basic Map code, hope you understand and if you have any doubts please feel free to ask in the comments section.

Similarly to for example suspend functions, @Composable functions are processed by the compiler in a very special way. This is to allow automatic recompositions and also to implicitly pass the context between components.

Documentation for @Composable specifies:

A useful mental model for Composable functions is that an implicit "composable context" is passed into a Composable function, and is done so implicitly when it is called from within another Composable function. This "context" can be used to store information from previous executions of the function that happened at the same logical point of the tree.

We can see it in action by writing a simple composable function and analyzing the resulting bytecode. Taking this source code:

From the documentation of varargs:

When you call a vararg -function, you can pass arguments individually, for example asList(1, 2, 3). If you already have an array and want to pass its contents to the function, use the spread operator (prefix the array with *):

Room DAO functions should always suspend so it can't block the main UI thread, you can refer this google recommended example: https://developer.android.com/codelabs/android-room-with-a-view-kotlin#5