浅析 JetPack Compose 是如何安装到View视图上

public fun ComponentActivity.setContent(
    parent: CompositionContext? = null,
    content: @Composable () -> Unit
) {
    //获取decorView->R.id.content->内第一个view,在compose中，默认为composeView
    val existingComposeView = window.decorView
        .findViewById<ViewGroup>(android.R.id.content)
        .getChildAt(0) as? ComposeView
    👉 -> 如果view不为null,则证明已经安装过，则重新设置内容
    if (existingComposeView != null) with(existingComposeView) {
        //设置父Compose内容
        setParentCompositionContext(parent)
        //设置当前页面内容
        setContent(content)
    👉 -> 如果上面获取的view是空，也就是现在还没有安装，则生成一个新的去安装到 R.id.content 上
    } else ComposeView(this).apply {
        //设置父Compose内容
        setParentCompositionContext(parent)
        //设置当前页面内容
        setContent(content)
        //设置TreeLifecycleOwner,实际上是修复Appcompat1.3及以下的bug，忽视即可
        setOwners()
        //设置内容视图,activity的方法
        setContentView(this, DefaultActivityContentLayoutParams)
    }
}

fun setParentCompositionContext(parent: CompositionContext?) {
        parentContext = parent
}

fun setContent(content: @Composable () -> Unit) {
    shouldCreateCompositionOnAttachedToWindow = true
    this.content.value = content
    //如果已经挂接到窗口，即view完全显示时，isAttachedToWindow=true
    //第一次调用时，这里为false,所以当onAttachToWindows调用时，下面的方法才会被调用
    if (isAttachedToWindow) {
        createComposition()
    }
}

 override fun onAttachedToWindow() {
        super.onAttachedToWindow()
        previousAttachedWindowToken = windowToken
        //这里当onAttached时为true,因为上一步已经赋值为true
        if (shouldCreateCompositionOnAttachedToWindow) {
            ensureCompositionCreated()
        }
    }

private fun ensureCompositionCreated() {
    //整个流程中，我们没看见composition的初始化，所以这里为null
    if (composition == null) {
        try {
            👉 1.  ///结合步骤1来看，就是一个用于判断是否add过的变量
            creatingComposition = true
            👉 2. // 先看 resolveParentCompositionContext() -> 得到一个顶级CompositionContext
            👉 3. // 拿着parentContext，传入setContent方法
            composition = setContent(resolveParentCompositionContext()) {
              Content()
            }
        } finally {
            creatingComposition = false
        }
    }
}

private fun resolveParentCompositionContext() = parentContext
    //👉1. 
    ?: findViewTreeCompositionContext()?.also { cachedViewTreeCompositionContext = it }
    ?: cachedViewTreeCompositionContext
    // 👉2. 
    ?: windowRecomposer.also { cachedViewTreeCompositionContext = it }

fun View.findViewTreeCompositionContext(): CompositionContext? { 
  //先取自己的compositionContenxt，如果取到直接返回，否则不断向上找父级的context.
var found: CompositionContext? = compositionContext
if (found != null) return found
var parent: ViewParent? = parent
while (found == null && parent is View) {
  found = parent.compositionContext
  parent = parent.getParent()
}
return found
}

@OptIn(InternalComposeUiApi::class)
internal val View.windowRecomposer: Recomposer
    get() {
        ...
        //这个rootView就是R.id.content对应的FrameLayout的第一个子view,即ComposeView
        val rootView = contentChild
        // 因为上述是初始化状态，所以 rootParentRef 第一次也为null,所以我们继续往下看
        return when (val rootParentRef = rootView.compositionContext) {
            //调用窗口Recomposer创建一个新的，并且把根content传入进去
            null -> WindowRecomposerPolicy.createAndInstallWindowRecomposer(rootView)
            ...
        }
    }

private val View.contentChild: View
get() {
  var self: View = this
  var parent: ViewParent? = self.parent
  while (parent is View) {
      if (parent.id == android.R.id.content) return self
      self = parent
      parent = self.parent
  }
  return self
}

internal fun createAndInstallWindowRecomposer(rootView: View): Recomposer {
        //使用默认的工厂创建一个Recomposer
        val newRecomposer = factory.get().createRecomposer(rootView)
        //将其赋值给rootView的compositionContext，而compositionContext也是一个扩展函数，通用使用tag保存
        rootView.compositionContext = newRecomposer
      ...
}
-- View.compositionContext
var View.compositionContext: CompositionContext?
get() = getTag(R.id.androidx_compose_ui_view_composition_context) as? CompositionContext
set(value) {
  setTag(R.id.androidx_compose_ui_view_composition_context, value)
}

internal fun ViewGroup.setContent(
parent: CompositionContext,
content: @Composable () -> Unit
): Composition {
GlobalSnapshotManager.ensureStarted()
val composeView =
  if (childCount > 0) {
      //第一次调用这里肯定为null，注意下面方法调用
      getChildAt(0) as? AndroidComposeView
  } else {
      removeAllViews(); null
    //初始化一个AndroidComposeView，并将我们最外面的content函数传入，
      //将初始化的AndroidComposeView添加到ComposeView上
  } ?: AndroidComposeView(context).also { addView(it.view, DefaultLayoutParams) }
  //这里留一个伏笔，即我们的下个阶段将
return doSetContent(composeView, parent, content)
}