Understanding Compose layout
Compose is an Android UI toolkit based on declarative programming that treats composable UI elements as functions and uses the Kotlin DSL to build and compose them. Compose also provides a corresponding layout system and a set of basic functions for nesting and composing UI elements.
Compose's core layout system consists of two parts: layout and the constraints and measurement system.
- The layout system is an abstract way to combine UI elements. It places all elements on the screen based on the screen coordinate system. For combining UI elements, the layout system provides many different layout options. When using the layout system, you view the result as a series of tags, where each tag represents a UI element. Therefore, working with the layout system is similar to using traditional layout Views and ViewGroups, but with a more intuitive and easier-to-use syntax.
- The constraints and measurement system is based on the markup to evaluate the constraints and measurements of the UI layout, it is responsible for passing the layout data to the layout engine for the actual calculation. Constraints are a set of layout parameters that control the position and size of UI elements within the layout. In Compose, constraints are specified through parameters provided by layout system functions.
- The measurement system is a calculation of the size of UI components. It is very important for implementing certain layouts, especially when implementing large UI layouts. In Compose, you can choose to use the preset measurement system functions or customize the measurement system for specific UI components.
When you write Compose UI code in Kotlin, the code you write is compiled into bytecode and the underlying rendering engine is used to do the actual rendering. The engine converts bytecode into commands that the GPU understands, enabling UI elements to be displayed on the screen.
Compose principle
The core principle of Compose is declarative programming, which uses the style of functional programming to compose and build UI elements. This approach allows developers to build user interfaces in a more straightforward and understandable manner without having to worry about managing state or handling events.
Compose, by using higher-order functions, extension functions, and DSL in the Kotlin language, allows developers to write UI elements as independent small components that can be combined and reused at will. For example, a developer can write a custom UI control that can generate different text or images based on a given input value. This control can be combined with other UI controls to build higher level UI pages.
In Compose, however, there is no need to directly manipulate the low-level View and ViewGroup objects. That's because Compose makes the UI declarative and stateless, which means developers don't have to track state in code and manage a hierarchy of views, making it easier to maintain and refactor code.
Finally, Compose uses a data-binding-based approach to help implement data-driven UI, which allows UI controls to automatically update according to changes in data. This approach makes the relationship between the page's data and UI elements more explicit, reduces the risk of errors in the code, and speeds up development.
How Compose configures the layout
In Jetpack Compose, you can use the Modifier object to configure the UI controls and layouts returned by Composable functions. Various layouts and UI effects can be achieved by chain calling different methods of the Modifier object.
The following are some commonly used Modifier methods:
- padding(): Sets the padding around the view.
- background(): Set the background color or background drawable of the view.
- fillMaxWidth() and fillMaxHeight(): Fill the view to the width or height of its parent layout.
- width() and height(): Set the width or height of the view.
- wrapContentWidth() and wrapContentHeight(): Set the width or height of the view to the width or height of the content.
- clickable(): Sets the view as clickable and specifies a handler for the click event.
- align(): Aligns the view in the parent layout.
Various UI effects can be easily achieved using these methods. For example, you can use the padding() method to set the padding of a view to leave some white space around it:
Text(
text = "Hello, Compose!",
Modifier.padding(start = 16.dp, end = 16.dp, top = 8.dp, bottom = 8.dp)
)
In this example, we use the Modifier.padding() method to add some padding to a text view, leaving 8dp of space around it.
In addition to the above methods, you can also use more advanced Modifier methods, such as constrainAs() and aspectRatio(), etc., to more flexibly layout the control and set its size.
In addition to using the Modifier method, you can also use Style to set common properties for multiple views, and Theme to set the style and color of the entire application.
In summary, in Jetpack Compose, by using the Modifier object, you can add various layouts and effects to UI views and configure them in a simple and readable way.
Compose standard layout components
In Jetpack Compose, there are a variety of standard layout components that can be used to build UIs. Here are some commonly used standard layout components:
- Row and Column: These components allow you to arrange child components horizontally and vertically and specify the spacing between them.
- Box: The Box component is a container similar to FrameLayout, which can place subcomponents in different positions and sizes. The Box component can also be used as a base component for other layouts, such as Scaffold and Surface.
- BoxWithConstraints: Similar to Box, in the BoxWithConstraints component, you can specify the position and size of the child components. Unlike Box, BoxWithConstraints also allows you to check its actual size to determine how to lay out its child components.
- ConstraintLayout: Similar to ConstraintLayout, the ConstraintLayout component allows you to use constraints to lay out child components. Compared with traditional XML layout, ConstraintLayout is more flexible and intuitive, and can better support adaptability to screen size and orientation changes.
- ScrollView: The ScrollView component allows you to place subcomponents inside a scroll view so that the user can scroll and see the content in the view.
- LazyColumn and LazyRow: These components are scrollable columns and rows that only generate part of the subcomponents that are visible in the view, which can optimize performance.
- Spacer: The Spacer component can add space to keep a certain distance between subcomponents. Can be used in Row, Column and Box.
You can easily build complex UI layouts by using these standard layout components, combined with Modifier objects and other Composable functions.
Compose layout simple use code example
Compose provides a layout method based on code declaration, that is, using Composable function to declare UI layout. Here is an example of a simple Composable function to declare a vertical linear layout with two text boxes:
@Composable
fun MyLayout() {
Column(Modifier.padding(16.dp)) {
Text("First Textbox")
TextField(value = "", onValueChange = {})
Text("Second Textbox")
TextField(value = "", onValueChange = {})
}
}
In this example, we use Composable functions such as Column and TextField, which are provided through the Compose library. This Column function creates a vertical layout and specifies the padding modifier to give some space to the subviews. The TextField function creates a text input box, and specifies the corresponding value and event handler through the value and onValueChange attributes.
In this way, we can use Composable functions to create arbitrarily complex layouts and UI elements, such as lists, grids, images, and so on. And inside the Composable function, we can also use all the functions and structures provided by the Kotlin language, such as loops, conditional statements, etc., to process data and control UI behavior.
Declaring UI layouts in this way allows designers and developers to collaborate better because they can share and reuse layouts and UI controls as code components without worrying too much about the underlying implementation details. At the same time, this method also allows developers to control UI elements more flexibly and quickly respond to user needs and changes.
The full text mainly explains the cutting-edge technology in Android development; Jetpack Compose layout, there is still a lot to learn about Compose; advanced learning can refer to the document "Android Core Technology Manual" , which records 30 large and small technical sections . Check the details category to get it!
Summarize
The above are some commonly used standard layout components in Jetpack Compose. Row and Column can be used to arrange subcomponents in horizontal and vertical directions; Box can be used to contain and position subcomponents; ConstraintLayout can be used to layout subcomponents using constraints; ScrollView can be used to place subcomponents in a scrollable view; LazyColumn and LazyRow can generate only part of child components that are visible in the view; Spacer can be used to add space to child components.
UI layout can be easily implemented using these standard layout components and Composable functions. Composable functions provide a way to build UI logic, while components provide common layout and interaction patterns. By combining these components and functions, complex UIs can be built while keeping the code readable and maintainable.