ShaderBrushSpan.android.kt

/*
 * Copyright 2022 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.platform.style

import android.graphics.Shader
import android.text.TextPaint
import android.text.style.CharacterStyle
import android.text.style.UpdateAppearance
import androidx.compose.ui.geometry.Size
import androidx.compose.ui.geometry.isUnspecified
import androidx.compose.ui.graphics.ShaderBrush
import androidx.compose.ui.text.platform.setAlpha

/**
 * A span that applies [ShaderBrush] to TextPaint after receiving a specified size
 */
internal class ShaderBrushSpan(
    val shaderBrush: ShaderBrush,
    val alpha: Float
) : CharacterStyle(), UpdateAppearance {
    var size: Size = Size.Unspecified
    private var cachedShader: Pair<Size, Shader>? = null

    override fun updateDrawState(textPaint: TextPaint) {
        textPaint.setAlpha(alpha)

        if (size.isUnspecified) return

        val finalCachedShader = cachedShader

        val shader = if (finalCachedShader == null || finalCachedShader.first != size) {
            // if cached shader is not initialized or the size has changed, recreate the shader
            shaderBrush.createShader(size)
        } else {
            // reuse the earlier created shader
            finalCachedShader.second
        }

        textPaint.shader = shader
        cachedShader = size to shader
    }
}