OnBackPressedDispatcher 的正确使用

Question

有时，我想自己处理用户按下的后退按钮。我的（示例）代码如下所示：

class TestActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.test)
    }

    override fun onBackPressed() {

        if (checkSomeCondition()) {
            // do nothing
        } else {
            super.onBackPressed()
        }
    }

    private fun checkSomeCondition() = false
}

当按下返回时我会收到通知，然后我决定是否要处理它，或者让系统通过调用 来处理它super.onBackPressed()。

由于 onBackPressed() 现已弃用，我使用以下方法替换它OnBackPressedDispatcher：

class TestActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.test)

        onBackPressedDispatcher.addCallback(this, object : OnBackPressedCallback(true) {
            override fun handleOnBackPressed() {
                if (checkSomeCondition()) {
                    // do nothing
                } else {
                    onBackPressedDispatcher.onBackPressed()
                }
            }
        })
    }

    private fun checkSomeCondition() = false
}

这段代码的问题是：通过调用onBackPressedDispatcher.onBackPressed()，我调用了自己的回调，创建了一个无限循环，而不是像我那样将其交给系统super.onBackPressed()。

当暂时禁用我的回调时，可以避免这种情况，例如：

    onBackPressedDispatcher.addCallback(this, object : OnBackPressedCallback(true) {
        override fun handleOnBackPressed() {
            if (checkSomeCondition()) {
                // do nothing
            } else {
                this.isEnabled = false
                onBackPressedDispatcher.onBackPressed()
                this.isEnabled = true
            }
        }
    })

但这看起来相当尴尬，而且似乎并不是有意为之。

OnBackPressedDispatcher让我自己处理后退按钮或将其交给系统的正确用法是什么？

PS：我已经看到了关于弃用 onBackPressed 的问题，但它没有回答我更具体的问题。

Answer 1

根据“系统后退基础知识”视频，预测后退手势的全部要点是提前了解要处理的后退内容。

这意味着您永远不应该拥有即时逻辑，例如checkSomeCondition作为调用的一部分handleOnBackPressed。

相反，正如自定义后退导航文档中所述，只要您用于签入的条件发生变化，您就应该提前更新isEnabled状态。这可确保仅当条件为真时才调用回调，而当条件为假时则禁用回调，从而允许发生默认行为。OnBackPressedCallback checkSomeCondition

class TestActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.test)

        // Instead of always setting enabled to true at the beginning,
        // you need to check the state ahead of time to know what the
        // initial enabled state should be
        val isConditionAlreadySet = checkSomeCondition()
        val callback = object : OnBackPressedCallback(
            isConditionAlreadySet
        ) {
            override fun handleOnBackPressed() {
                // Since you've handled isEnabled correctly, you know
                // your condition is set correctly here, so you can
                // unconditionally do what you need to do to handle back
            }
        }
        // This is the key part - now you know ahead of time
        // when the condition changes, which lets you control
        // when you want to handle back or not
        setOnSomeConditionChangedListener { isConditionMet ->
          callback.isEnabled = isConditionMet
        }
        onBackPressedDispatcher.addCallback(this, callback)
    }

    private fun checkSomeCondition() = false
    private fun setOnSomeConditionChangedListener(
        (isConditionMet: Boolean) -> Unit
    ) {
        // The implementation here will depend on what
        // conditions checkSomeCondition() actually depends on
    }

}