add in predictive back snippets for NavHost and predictive back handler

compose/snippets/src/main/java/com/example/compose/snippets/predictiveback/PredictiveBackSnippets.kt

@@ -0,0 +1,143 @@
+package com.example.compose.snippets.predictiveback
+
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Modifier
+import androidx.navigation.compose.NavHost
+import androidx.navigation.compose.rememberNavController
+import androidx.navigation.NavHostController
+import androidx.compose.animation.scaleOut
+import androidx.compose.ui.graphics.TransformOrigin
+import androidx.compose.animation.EnterTransition
+import android.os.SystemClock
+import androidx.activity.compose.PredictiveBackHandler
+import androidx.compose.ui.unit.dp
+import androidx.compose.animation.core.Animatable
+import androidx.navigation.compose.composable
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.material3.Surface
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberCoroutineScope
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.geometry.Offset
+import androidx.compose.ui.input.pointer.util.VelocityTracker
+import androidx.compose.ui.platform.LocalDensity
+import kotlin.coroutines.cancellation.CancellationException
+
+
+@Composable
+private fun PredictiveBackOverrideExit(
+    modifier: Modifier,
+) {
+    val navController = rememberNavController()
+
+    // [START android_compose_predictiveback_navhost]
+    NavHost(
+        navController = navController,
+        startDestination = "home",
+        popExitTransition = {
+            scaleOut(
+                targetScale = 0.9f,
+                transformOrigin = TransformOrigin(pivotFractionX  = 0.5f, pivotFractionY = 0.5f)
+            )
+        },
+        popEnterTransition = {
+            EnterTransition.None
+        },
+        modifier = modifier,
+    )
+    // [END android_compose_predictiveback_navhost]
+    {
+        composable("home") {
+            HomeScreen(
+                modifier = modifier,
+                navController = navController,
+            )
+        }
+        composable("settings") {
+            SettingsScreen(
+                modifier = modifier,
+                navController = navController,
+            )
+        }
+    }
+}
+
+@Composable
+private fun HomeScreen(
+    modifier: Modifier = Modifier, navController: NavHostController
+) {
+
+}
+
+@Composable
+private fun SettingsScreen(
+    modifier: Modifier = Modifier, navController: NavHostController
+) {
+
+}
+
+@Composable
+private fun PredictiveBackHandlerManualProgress() {
+
+    Surface(
+        modifier = Modifier.fillMaxSize()
+    ) {
+        var drawerState by remember {
+            mutableStateOf(DrawerState.Closed)
+        }
+
+        val translationX = remember {
+            Animatable(0f)
+        }
+
+        val drawerWidth = with(LocalDensity.current) {
+            DrawerWidth.toPx()
+        }
+        translationX.updateBounds(0f, drawerWidth)
+
+        val coroutineScope = rememberCoroutineScope()
+
+        suspend fun closeDrawer(velocity: Float = 0f) {
+            translationX.animateTo(targetValue = 0f, initialVelocity = velocity)
+            drawerState = DrawerState.Closed
+        }
+        suspend fun openDrawer(velocity: Float = 0f) {
+            translationX.animateTo(targetValue = drawerWidth, initialVelocity = velocity)
+            drawerState = DrawerState.Open
+        }
+
+        val velocityTracker = remember {
+            VelocityTracker()
+        }
+
+        // [START android_compose_predictivebackhandler]
+        PredictiveBackHandler(drawerState == DrawerState.Open) { progress ->
+            try {
+                progress.collect { backEvent ->
+                    val targetSize = (drawerWidth - (drawerWidth * backEvent.progress))
+                    translationX.snapTo(targetSize)
+                    velocityTracker.addPosition(
+                        SystemClock.uptimeMillis(),
+                        Offset(backEvent.touchX, backEvent.touchY)
+                    )
+                }
+                closeDrawer(velocityTracker.calculateVelocity().x)
+            } catch (e: CancellationException) {
+                openDrawer(velocityTracker.calculateVelocity().x)
+            }
+            velocityTracker.resetTracking()
+        }
+        // [END android_compose_predictivebackhandler]
+
+    }
+}
+
+private enum class DrawerState {
+    Open,
+    Closed
+}
+
+private val DrawerWidth = 300.dp
+