# Session 226 - Data Flow Through SwiftUI

###### tags: `WWDC-2019` `SwiftUI`

https://developer.apple.com/wwdc19/226

- Tools for data flow (正式名稱叫 Property Wrapper)
- Property
- @Environment
- @Binding
- @State
- BindableObject / @ObjectBinding
- Data access as a Dependency
- 配合 `Combine.framework`
- 想像成 RAC + Redux

---

- @State
- Each `@State` is a single source of truth
- Read and write **with ownership**
- Views are a function of state, not of a sequence of events
- **Be Careful**
- Limit use if possible
- Use derived `Binding` or value
- Prefer `BindableObject` for presistence
- Example: Button highlighting
- Most of the time: Good for prototyping, not for practice

- @Binding
- Read and write **without ownership**
- Derivable from `@State` / `@ObjectBinding` / `@Binding`
- 用 `$` prefix 存取 data source
- First class reference to data
- Great for reusability

- BindableObject Protocol
- External data
- Reference type
- Great for the model you already have
- 透過 `Combine` 送出變化

- @ObjectBinding
- 接受符合 `BindableObject` protocol 的 `class` object
- `BindableObject` 送出 `didChange()` 時就會收到變化

- @EnvironmentObject
- 用在 subview 需要根據 superview 的 model 而變化
- Data applicable to an entire hierarchy
- Convenience for indirection
- Accent color, right-to-left, and more
- 一種 `整個 view 的全域變數` 的概念
- model`.environmentObject()`

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@State 跟 BindableObject 的差異

| @State | BindableObject |
| ----------------- | ----------------- |
| View-local | External |
| Value type | Reference type |
| Framework managed | Developer managed |