Migration Guide: 0.4 to 0.5 (#107)

Migration Guide: 0.4 to 0.5

content/learn/book/migration-guides/0.4-0.5/_index.md

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+title = "0.4 to 0.5"
+weight = 1
+sort_by = "weight"
+template = "book-section.html"
+page_template = "book-section.html"
+[extra]
+long_title = "Migration Guide: 0.4 to 0.5"
++++
+
+<!-- TODO: link to release blog post here -->
+
+## `commands: &mut Commands` SystemParam is now `mut commands: Commands`
+
+```rust
+// 0.4
+fn foo(commands: &mut Commands) {
+}
+
+// 0.5
+fn foo(mut commands: Commands) {
+}
+```
+
+Systems using the old `commands: &mut Commands` syntax in 0.5 will fail to compile when calling `foo.system()`.
+
+This change was made because {{rust_type(type="struct" crate="bevy_ecs" version="0.5.0" name="Commands" no_mod=true)}}
+now holds an internal {{rust_type(type="struct" crate="bevy_ecs" version="0.5.0" name="World" no_mod=true)}}
+reference to enable safe entity allocations.
+
+Note: The internal {{rust_type(type="struct" crate="bevy_ecs" version="0.5.0" name="World" no_mod=true)}} reference requires two lifetime parameters to pass Commands into a non-system function: ```commands: &'a mut Commands<'b>```
+
+## {{rust_type(type="struct" crate="bevy_ecs" version="0.5.0" name="Commands" no_mod=true)}} `insert()` API is now used for a single component
+
+```rust
+// 0.4
+// component
+commands.insert_one(entity, MyComponent)
+commands.insert(entity, (MyComponent,))
+// bundle
+commands.insert(entity, Bundle)
+
+
+// 0.5
+// component
+commands.insert(entity, MyComponent)
+// bundle
+commands.insert_bundle(entity, MyBundle)
+```
+
+Instead of using `commands.insert_one()` for a single component, use `commands.insert()`.
+
+This means that `commands.insert()` will no longer accept a bundle as an argument. For bundles, use `commands.insert_bundle()`.
+
+This change helps to clarify the difference between components and bundles, and brings {{rust_type(type="struct" crate="bevy_ecs" version="0.5.0" name="Commands" no_mod=true)}} into alignment with other Bevy APIs. It also eliminates the confusion associated with calling `commands.insert()` on a tuple for the single-component case.
+
+## {{rust_type(type="struct" crate="bevy_core" version="0.5.0" name="Timer" no_mod=true)}} uses `Duration`
+
+```rust
+// 0.4
+if timer.tick(time.delta_seconds()).finished() { /* do stuff */ }
+timer.elapsed() // returns a bool
+
+// 0.5
+if timer.tick(time.delta()).finished() { /* do stuff */ }
+timer.elapsed() // returns a `Duration`
+```
+
+Most of the methods of {{rust_type(type="struct" crate="bevy_core" version="0.5.0" name="Timer" no_mod=true)}}
+now use `Duration` instead of `f32`.
+
+This change allows timers to have consistent, high precision. For convenience, there is also an
+`elapsed_secs` method that returns `f32`.  Otherwise, when you need an `f32`, use the
+`as_secs_f32()` method on `Duration` to make the conversion.
+
+## Simplified Events
+
+```rust
+// 0.4
+fn event_reader_system(
+    mut my_event_reader: Local<EventReader<MyEvent>>,
+    my_events: Res<Events<MyEvent>>,
+) {
+    for my_event in my_event_reader.iter(&my_events) {
+        // do things with your event
+    }
+}
+
+// 0.5
+fn event_reader_system(mut my_event_reader: EventReader<MyEvent>) {
+    for my_event in my_event_reader.iter() {
+        // do things with your event
+    }
+}
+```
+You no longer need two system parameters to read your events. One `EventReader` is sufficient.
+
+Following the above example of using an `EventReader` to read events, you can now use `EventWriter` to create new ones.
+```rust
+// 0.4
+fn event_writer_system(
+    mut my_events: ResMut<Events<MyEvent>>,
+) {
+    my_events.send(MyEvent);
+}
+
+// 0.5
+fn event_writer_system(
+    mut my_events: EventWriter<MyEvent>
+) {
+    my_events.send(MyEvent);
+}
+```
+
+## `AppBuilder::add_resource` is now called `AppBuilder::insert_resource`
+
+This is a small change to have function names on `AppBuilder` consistent with the `Commands` API.
+
+## TextBundle
+
+This bundle has been reworked to allow multiple differently-styled sections of text within a single bundle. `Text::with_section` was added to simplify the common case where you're only interested in one text section.
+
+```rust
+// 0.4
+TextBundle {
+    text: Text {
+        value: "hello!".to_string(),
+        font: asset_server.load("fonts/FiraSans-Bold.ttf"),
+        style: TextStyle {
+            font_size: 60.0,
+            color: Color::WHITE,
+            ..Default::default()
+        },
+    },
+    ..Default::default()
+}
+
+// 0.5
+TextBundle {
+    text: Text::with_section(
+        "hello!",
+        TextStyle {
+            font: asset_server.load("fonts/FiraSans-Bold.ttf"),
+            font_size: 60.0,
+            color: Color::WHITE,
+        },
+        TextAlignment::default()
+    ),
+    ..Default::default()
+}
+```

content/learn/book/migration-guides/_index.md

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+title = "Migration Guides"
+weight = 7
+sort_by = "weight"
+template = "book-section.html"
+page_template = "book-section.html"
++++
+
+Bevy is still in the "experimentation phase", which means each release has its fair share of breaking changes. We provide migration guides for major releases to help users migrate their apps to the latest and greatest Bevy release.