compose jetpack canvas简单入门

原文出自这里
https://medium.com/mobile-app-development-publication/code-simple-android-jetpack-compose-drawing-app-886d1146ad20
文章标题为：使用简单代码在 Android Jetpack Compose 中开发绘图应用
文章有不少转载，由于版本的更新，它不能正常工作了，所以在这里更正一下
我所使用的环境为
compose_version = '1.0.0-beta01'
classpath "com.android.tools.build:gradle:7.0.0-alpha09"
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.4.30"
distributionUrl=https://services.gradle.org/distributions/gradle-6.8.2-bin.zip
代码如下：

class MainActivity : ComponentActivity() {
    private val action: MutableState<Pair<Float, Float>?> = mutableStateOf(null)
    private val path = Path()
    private val collectList = mutableListOf<Pair<Float, Float>>()
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            Canvas(
                modifier = Modifier
                    .fillMaxSize()
                    .pointerInteropFilter {
                        when (it.action) {
                            MotionEvent.ACTION_DOWN -> {
                                action.value = Pair(it.x, it.y)
                                path.moveTo(it.x, it.y)
                            }
                            MotionEvent.ACTION_MOVE -> {
                                action.value = Pair(it.x, it.y)
                                path.lineTo(it.x, it.y)
                            }
                            else -> false
                        }
                        true
                    }) {
                action.value?.let {
                    collectList.add(it)
                    drawPath(
                        path = path,
                        color = Color.Green,
                        alpha = 1f,
                        style = Stroke(10f)
                    )
                }
            }
        }
    }

}