[Rally] README

Change-Id: I652f659f08609bd77cd0c7c4d002a847bcd2958b

Author: JoseAlcerreca

Diff for: Rally/README.md

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+# Rally sample
+
+This sample is a [Jetpack Compose][compose] implementation of [Rally][rally], a Material Design study. This sample showcases:
+
+* [Material theming][materialtheming]
+* Custom layouts and reusable elements
+* Charts and tables
+* Animations
+
+<img src="screenshots/rally.gif"/>
+
+### Status: 🚧 In progress
+
+This sample is still in under development, and some features are not yet implemented.
+
+## Features
+
+### Purpose
+This sample is a simple introduction to Material Design in Compose and it focuses on creating custom layouts and reusable elements. It uses a very simple architecture with a single activity and some hard-coded sample data. The navigation mechanism is implemented as a placeholder for an eventual official implementation using [Android Architecture Components Navigation](https://developer.android.com/guide/navigation).
+
+### Custom layouts and reusable elements
+Rally contains screens that have very similar elements, which allows for reusing a lot of code and implementing composables that are "styled" programmatically as needed.
+
+For example, [AccountsScreen](app/src/main/java/com/example/compose/rally/ui/accounts/AccountsScreen.kt) and [BillsScreen](app/src/main/java/com/example/compose/rally/ui/bills/BillsScreen.kt) wrap the same [`StatementBody`](app/src/main/java/com/example/compose/rally/ui/components/DetailsScreen.kt) composable which takes a list of _items_, their colors, amounts and even a slot to compose the item itself. Instead of passing lists with all this meta information, it's much more convenient, reusable and performant to pass functions and delegate how to fetch this information to each caller, making `StatementBody` completely generic:
+
+```kotlin
+@Composable
+fun <T> StatementBody(
+    items: List<T>,
+    colors: (T) -> Color,
+    amounts: (T) -> Float,
+    rows: @Composable (T) -> Unit
+    ...
+```
+
+```kotlin
+@Composable
+fun AccountsBody(accounts: List<Account>) {
+    StatementBody(
+        items = accounts,
+        colors = { account -> account.color },
+        amounts = { account -> account.balance },
+        rows = { account -> AccountRow(...) }
+        ...
+```
+
+### Theming
+Rally follows [Material Design][materialtheming], customizing [colors](app/src/main/java/com/example/compose/rally/ui/theme/Color.kt) and [typography](app/src/main/java/com/example/compose/rally/ui/theme/Theme.kt) used in the app via the [RallyTheme](app/src/main/java/com/example/compose/rally/ui/theme/Theme.kt). Rally's design only contains a dark theme, therefore the theme does not contain any light colors.
+
+### Charts and animations
+This sample features a donut chart that combines drawing using [`Canvas`](https://developer.android.com/reference/kotlin/androidx/compose/ui/graphics/Canvas) with animations combining two animated parameters: `AngleOffset` and `Shift`. This creates the animation with minimum boilerplate:
+
+```kotlin
+private enum class AnimatedCircleProgress { START, END }
+
+private val CircularTransition = transitionDefinition<AnimatedCircleProgress> {
+    state(AnimatedCircleProgress.START) {
+        this[AngleOffset] = 0f
+        this[Shift] = 0f
+    }
+    state(AnimatedCircleProgress.END) {
+        this[AngleOffset] = 360f
+        this[Shift] = 30f
+    }
+    transition(fromState = AnimatedCircleProgress.START, toState = AnimatedCircleProgress.END) {
+        AngleOffset using tween(
+            delayMillis = 500,
+            durationMillis = 900,
+            easing = CubicBezierEasing(0f, 0.75f, 0.35f, 0.85f)
+        )
+        Shift using tween(
+            delayMillis = 500,
+            durationMillis = 900,
+            easing = LinearOutSlowInEasing
+        )
+    }
+}
+```
+
+<img src="screenshots/donut.gif"/>
+
+## License
+```
+Copyright 2020 The Android Open Source Project
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+```
+
+[compose]: https://developer.android.com/jetpack/compose
+[rally]: https://material.io/design/material-studies/rally.html
+[materialtheming]: https://material.io/design/material-theming/overview.html#material-theming