Customized Jetpack Compose structure with Kotlin DSL | by Evgeny Landarsky | Dec, 2022 | Zero Tech

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Whereas Jetpack Compose gives a variety of elements to create an attractive consumer interface, it’s possible you’ll discover someday that your subsequent structure must be applied with a customized element. Jetpack Compose, for instance, would not have a handy out-of-the-box element for working with a grid-based consumer interface. What we perceive by comfort:

  • versatile chance to outline row and column sizes
  • placement of content material at any particular location with out the necessity to place neighbors

We wish to have a element that permits us to create UI as within the following picture:

In Jetpack Compose we have already got related elements: LazyVerticalGrid Y LazyHorizontalGrid. Here’s a frequent utilization of LazyHorizontalGrid:

Look intently however not the identical as what we want. Lazy grids work with row and column rely solely and place gadgets sequentially. Whereas we wish to management not solely the counts but in addition the sizes. Additionally, we want to have the ability to place a component at any place.

Constructing on the lazy grids API, we’ll outline our element API and name it GridPad:

Since we wish to management the location of the content material, our element will be capable of add components with Kotlin DSL through merchandise sections.

Let’s take a better look GridPadCells. This class accommodates details about the sizes of rows and columns.

Let’s again up a bit and keep in mind what we want. We want to have the ability to specify a measurement for every row and column in a grid.

Subsequently, GridPadCells cowl all our necessities. You may discover, to retailer rowSizes and columnSizes utilizing ImmutableList. Because of this implementation of an inventory, the composition compiler marks this class as steady and we need not mark GridPadCells with @Immutable both @Steady annotation. As a result of it will be important? That is what the official documentation says:

Not all courses have to be steady, however a steady class unlocks lots of flexibility for the composition compiler to make optimizations when a steady kind is utilized in some locations, which is why it is such an vital idea in composition.

Here is a fantastic article that additional explains the fundamentals of Jetpack Compose, similar to stability, jumpabilityY reset skill. I like to recommend studying it to higher perceive the fundamentals of Jetpack Compose and what they’ll have an effect on.

There are two extra courses we’ve not coated but: GridPadCellSize Y TotalSize. The GridPadCellSize it is only a container that holds details about the scale of a particular row or column:

The TotalSize is a helper class that accommodates details about the sum of all sorts of sizes for a row or column and is utilized in a measurement stage:

And the ultimate class here’s a constructor class. This class isn’t required, but it surely helps scale back boilerplate code. Our GridPad requires two lists of sizes, which within the fundamental use case would be the identical. For that cause, we have to create a mutable class with default initialization and the flexibility to override chosen sizes:

The second fundamental aim we would like to realize is to put the content material in any grid cell with any span width and peak.

Some vital edge circumstances we’ll deal with:

  • a cell can include multiple aspect
  • content material can overlap one another
  • content material that goes exterior the grid can be ignored

Right here is the signature of GridPad:

Let’s take an in depth take a look at GridPadScope. GridPadScope it is the scope of DSL that limits what customers can put into our composable layouts. Solely content material with the article is accepted:

It appears simple sufficient on high, however it is a bit sophisticated within the particulars. Right here it’s GridPadScope definition:

Listed here are two vital issues: @GridPadScopeMarker Y GridPadItemScope. The @GridPadScopeMarke is a @DslMarker:

The GridPadItemScope is a context receiver:

The above mixture helps to make strict use of our API: a developer can’t place different composables with out wrapping them in a component. The developer additionally can’t put a component inside one other aspect. If you wish to be taught extra about context receivers, I like to recommend that you simply learn this text.

As you may see, all we have simply explored is an interface. Let’s take a better take a look at the implementations. Earlier than you proceed, keep in mind what the composable lifecycle appears like. For customized design, we have to implement three sub-stages of the design stage:

Subsequently, we have to measure the kids, decide the scale of the structure and place the content material. Here’s a very excessive stage of GridPad implementation:

The one interface definition isn’t sufficient to implement the logic of including content material. For that cause, we have to have some implementation, and that implementation is GridPadScopeImpl. That is the place all use calls GridPad are redirected to the interface implementation:

The GridPadScopeImpl It is like a container that collects all top-level DSL broadcasts, creates an inventory to show, and gives the flexibility so as to add composables:

Within the above code, probably the most vital half is a DSL name transformation merchandise inside GridPad to metaclass GridPadContent which shops data for future measurements and areas.

let’s return to GridPad and end the element. After accumulating all of the details about the location content material, all we want is to precisely calculate the sizes of every cell, measure the composables, and place them within the correct place. Right here we won’t delve into the calculation of the situation logic, we are going to deal with the API associated to Jetpack Compose. Step one is to measure the kids primarily based on their location and span measurement:

A lot of the code above works by calculating the bounds of the composables to name the measure of the positioned aspect.

The final contact after measuring all the kids is to outline the scale of the element and place the weather:

Lots of work has been accomplished right here. The above examples could seem a bit sophisticated, however Jetpack Compose mixed with Kotlin DSL gives prospects to implement fancy APIs to your customized layouts. When confronted with an issue, be at liberty to consult with the supply code of already accomplished elements and get impressed there, it is the easiest way to be taught one thing new.

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Custom Jetpack Compose layout with Kotlin DSL | by Evgeny Landarsky | Dec, 2022