AndroidFont.kt
/*
* Copyright 2021 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
*
* http://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.
*/
package androidx.compose.ui.text.font
import android.content.Context
import android.graphics.Typeface
import androidx.compose.ui.text.ExperimentalTextApi
/**
* Font for use on Android.
*
* All [AndroidFont] produce an [android.graphics.Typeface] which may be used to draw text on
* Android. This is the main low-level API for introducing a new Font description to Compose on
* Android for both blocking and async load.
*
* You may subclass this to add new types of font descriptions that may be used in
* [FontListFontFamily]. For example, you can add a [FontLoadingStrategy.Blocking] font that
* returns a Typeface from a local resource not supported by an existing [Font]. Or, you can create
* an [FontLoadingStrategy.Async] font that loads a font file from your server.
*
* When introducing new font descriptors, it is recommended to follow the patterns of providing a
* public Font constructor and a private implementation class:
*
* 1. Declare an internal or private subclass of AndroidFont
* 2. Expose a public Font(...) constructor that returns your new type.
*
* Font constructors are
*
* 1. Regular functions named `Font` that return type `Font`
* 2. The first argument is the font name, or similar object that describes the font uniquely
* 3. If the font has a provider, loader, or similar argument, put it after the font name.
* 4. The last two arguments are FontWeight and FontStyle.
*
* Examples of Font constructors:
*
* ```
* fun Font("myIdentifier", MyFontLoader, FontWeight, FontStyle): Font
* fun Font(CustomFontDescription(...), MyFontLoader, FontWeight, FontStyle): Font
* fun Font(CustomFontDescription(...), FontWeight, FontStyle): Font
*```
*
* @param loadingStrategy loadingStrategy this font will provide in fallback chains
* @param typefaceLoader a loader that knows how to load this [AndroidFont], may be shared between
* several fonts
*/
abstract class AndroidFont
@ExperimentalTextApi
constructor(
final override val loadingStrategy: FontLoadingStrategy,
val typefaceLoader: TypefaceLoader,
variationSettings: FontVariation.Settings,
) : Font {
@OptIn(ExperimentalTextApi::class)
// TODO(b/241016309) deprecate this once FontVariation is non-experimental
constructor(
loadingStrategy: FontLoadingStrategy,
typefaceLoader: TypefaceLoader,
) : this(loadingStrategy, typefaceLoader, FontVariation.Settings())
@Suppress("OPT_IN_MARKER_ON_WRONG_TARGET")
@ExperimentalTextApi
@get:ExperimentalTextApi
val variationSettings: FontVariation.Settings = variationSettings
/**
* Loader for loading an [AndroidFont] and producing an [android.graphics.Typeface].
*
* This interface is not intended to be used by application developers for text display. To load
* a typeface for display use [FontFamily.Resolver].
*
* [TypefaceLoader] allows the introduction of new types of font descriptors for use in
* [FontListFontFamily]. A [TypefaceLoader] allows a new subclass of [AndroidFont] to be used by
* normal compose text rendering methods.
*
* Examples of new types of fonts that [TypefaceLoader] can add:
* - [FontLoadingStrategy.Blocking] [Font] that loads Typeface from a local resource not
* supported by an existing font
* - [FontLoadingStrategy.OptionalLocal] [Font] that "loads" a platform Typeface only available
* on some devices.
* - [FontLoadingStrategy.Async] [Font] that loads a font from a backend via a network request.
*
* During resolution from [FontFamily.Resolver], an [AndroidFont] subclass will be queried for
* an appropriate loader.
*
* The loader attached to an instance of an [AndroidFont] is only required to be able to load
* that instance, though it is advised to create one loader for all instances of the same
* subclass and share them between [AndroidFont] instances to avoid allocations or allow
* caching.
*
* Implementers of custom font resources should try to ensure that their implementations are
* usable in the context of Compose Previews, which don't provide access to the full Android
* runtime. If not possible, it is advised to document the behavior in Compose Preview.
*/
interface TypefaceLoader {
/**
* Immediately load the font in a blocking manner such that it will be available this frame.
*
* This method will be called on a UI-critical thread, however it has been determined that
* this font is required for the current frame. This method is allowed to perform small
* amounts of I/O to load a font file from disk.
*
* This method should never perform expensive I/O operations, such as loading from a remote
* source. If expensive operations are required to complete the font, this method may choose
* to throw. Note that this method will never be called for fonts with
* [FontLoadingStrategy.Async].
*
* It is possible for [loadBlocking] to be called for the same instance of [AndroidFont] in
* parallel. Implementations should support parallel concurrent loads, or de-dup.
*
* @param context current Android context for loading the font
* @param font the font to load which contains this loader as [AndroidFont.typefaceLoader]
* @return [android.graphics.Typeface] for loaded font, or null if the font fails to load
*/
fun loadBlocking(context: Context, font: AndroidFont): Typeface?
/**
* Asynchronously load the font, from either local or remote sources such that it will cause
* text reflow when loading completes.
*
* This method will be called on a UI-critical thread, and should not block the thread for
* font loading from sources slower than the local filesystem. More expensive loads should
* dispatch to an appropriate thread.
*
* This method is always called in a timeout context and must return it's final value within
* 15 seconds. If the Typeface is not resolved within 15 seconds, the async load is
* cancelled and considered a permanent failure. Implementations should use structured
* concurrency to cooperatively cancel work.
*
* Compose does not know what resources are required to satisfy a font load.
* Subclasses implementing [FontLoadingStrategy.Async] behavior should ensure requests are
* de-duped for the same resource.
*
* It is possible for [awaitLoad] to be called for the same instance of [AndroidFont] in
* parallel. Implementations should support parallel concurrent loads, or de-dup.
*
* @param context current Android context for loading the font
* @param font the font to load which contains this loader as [AndroidFont.typefaceLoader]
* @return [android.graphics.Typeface] for loaded font, or null if not available
*
*/
suspend fun awaitLoad(context: Context, font: AndroidFont): Typeface?
}
}
// keep generating AndroidFontKt to avoid API change
private fun generateAndroidFontKtForApiCompatibility() {}