FocusProperties.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.focus

import androidx.compose.ui.Modifier
import androidx.compose.ui.modifier.ModifierLocalConsumer
import androidx.compose.ui.modifier.ModifierLocalProvider
import androidx.compose.ui.modifier.ModifierLocalReadScope
import androidx.compose.ui.modifier.modifierLocalOf
import androidx.compose.ui.node.ModifiedFocusNode
import androidx.compose.ui.platform.InspectorInfo
import androidx.compose.ui.platform.InspectorValueInfo
import androidx.compose.ui.platform.debugInspectorInfo

/**
 * A Modifier local that stores [FocusProperties] for a sub-hierarchy.
 *
 * @see [focusProperties]
 */
internal val ModifierLocalFocusProperties =
    modifierLocalOf<FocusProperties> { DefaultFocusProperties }

internal object DefaultFocusProperties : FocusProperties {
    @Suppress("UNUSED_PARAMETER")
    override var canFocus: Boolean
        get() = true
        set(value) {
            error("Attempting to change DefaultFocusProperties")
        }
}

/**
 * Properties that are applied to [focusTarget]s that can read the [ModifierLocalFocusProperties]
 * Modifier Local.
 *
 * @see [focusProperties]
 */
interface FocusProperties {
    /**
     * When set to false, indicates that the [focusTarget] that this is applied to can no longer
     * take focus. If the [focusTarget] is currently focused, setting this property to false will
     * end up clearing focus.
     */
    var canFocus: Boolean
}

/**
 * This modifier allows you to specify properties that are accessible to [focusTarget]s further
 * down the modifier chain or on child layout nodes.
 *
 * @sample androidx.compose.ui.samples.FocusPropertiesSample
 */
fun Modifier.focusProperties(scope: FocusProperties.() -> Unit): Modifier = this.then(
    FocusPropertiesModifier(
        focusPropertiesScope = scope,
        inspectorInfo = debugInspectorInfo {
            name = "focusProperties"
            properties["scope"] = scope
        }
    )
)

internal class FocusPropertiesModifier(
    val focusPropertiesScope: FocusProperties.() -> Unit,
    inspectorInfo: InspectorInfo.() -> Unit
) : ModifierLocalConsumer,
    ModifierLocalProvider<FocusProperties>,
    InspectorValueInfo(inspectorInfo) {

    private var parentFocusProperties: FocusProperties? = null

    override fun onModifierLocalsUpdated(scope: ModifierLocalReadScope) {
        parentFocusProperties = scope.run { ModifierLocalFocusProperties.current }
    }

    override val key = ModifierLocalFocusProperties

    override val value: FocusProperties
        get() {
            // Start with default focus properties.
            val properties = object : FocusProperties {
                override var canFocus: Boolean = DefaultFocusProperties.canFocus
            }

            // Populate with the specified focus properties.
            properties.apply(focusPropertiesScope)

            // Current focus properties can be overridden by a parent's value, unless the parent's
            // properties are DefaultFocusProperties (The previous focus modifier sets the modifier
            // local to the DefaultFocusProperties to prevent the propagation of its focus
            // properties to its children).
            val parentProperties = parentFocusProperties
            if (parentProperties != null && parentProperties != DefaultFocusProperties) {
                properties.canFocus = parentProperties.canFocus
            }

            return properties
        }

    override fun equals(other: Any?) =
        other is FocusPropertiesModifier && focusPropertiesScope == other.focusPropertiesScope

    override fun hashCode() = focusPropertiesScope.hashCode()
}

internal fun ModifiedFocusNode.setUpdatedProperties(properties: FocusProperties) {
    if (properties.canFocus) activateNode() else deactivateNode()
}