# Layout System ## What is a Layout? At its core, a **Layout** is a negotiation of space. It is the process where the system decides where to place a widget and how big that widget should be. In Digia, you don't use absolute pixels (e.g., "Place this button at x=50, y=100"). Instead, you describe the **relationship** between widgets (e.g., "Place this button in the center of the screen"). ## Why Understanding Layout Matters Digia uses a **Server-Driven UI** model. You define the layout JSON once, and it renders using the native rendering engines of the target platform (Flutter, Android/Jetpack Compose, or iOS/SwiftUI). While Digia handles the translation for you, understanding how these engines "think" helps you build interfaces that: 1. **Adapt Responsively**: Work on both an iPhone SE and an iPad Pro. 2. **Avoid Errors**: Prevent "Pixel Overflow" or "Clipped Content" issues. 3. **Perform**: Render efficiently without unnecessary calculations. ## Core Layout Principles Modern UI frameworks all follow a similar pattern known as **Declarative Layout**. ### 1. Flutter The motto of Flutter's layout engine is: > **"Constraints go down. Sizes go up. Parent sets position."** 1. **Constraints Down**: A parent (like a Column) tells its child: *"You can be as wide as the screen, but only 50px high."* 2. **Sizes Up**: The child (like a Button) calculates its content and replies: *"Okay, I will be 100px wide and 50px high."* 3. **Parent Sets Position**: The parent then decides coordinates: *"I will place you at x=0, y=0."* ### 2. Android (Jetpack Compose) Compose uses a similar logic called **Single Pass Layout**. > **"Constraints flow down, measurements flow up."** Parents pass `Constraints` to children, and children return `Placeables` (with a width and height). Parents are strictly forbidden from measuring a child twice, ensuring high performance. ### 3. iOS (SwiftUI) SwiftUI follows a three-step process: > **"Parent proposes a size. Child chooses its own size. Parent places the child."** 1. **Proposal**: The parent offers the child a size (e.g., the whole screen). 2. **Choice**: The child decides how much of that it needs (e.g., "I only need 200x50"). 3. **Placement**: The parent respects that choice and positions the child (e.g., centered). ## The WYSIWYG Reality Digia Studio acts as a **unified layout engine** in your browser. It mimics these native behaviors so that what you see is *mostly* what you get. However, each platform has subtle text rendering engines, spacing defaults, and safe area calculations. > \[!WARNING] **Always Test on Device** While the Digia Studio preview is 99% accurate, the final 1% (text kerning, shadow rendering, precise safe areas) depends on the physical device. Use **Digia Preview** on a real phone to verify your layouts before publishing. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.digia.tech/ui-building-blocks/layout-system.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.