android - 创 build 置 Activity 时出现 boolean 参数问题

Question

在为设置页面创建 Activity 后，我注意到 if (mCurrentValue !== value) 中的 mCurrentValue !== value 返回警告:

Identity equality for arguments of types Boolean? and Boolean can be unstable because of implicit boxing

我尝试通过在 override fun onXchange(value:Boolean) { 中的 Boolean 旁边添加一个 ? 来解决此警告，但随后返回以下错误:

'onXchange' overrides nothing

关于如何解决这个问题有什么想法吗？

Activity 类

import android.content.Intent
import android.graphics.Color
import android.graphics.drawable.ColorDrawable
import android.os.Bundle
import android.preference.PreferenceManager
import android.support.v7.app.AppCompatActivity
import android.view.MenuItem

class MySettingsActivity : AppCompatActivity(), MySettingsFragment.PreferenceXchangeListener {
    private var mCurrentValue: Boolean? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        val mSharedPreferences = PreferenceManager.getDefaultSharedPreferences(this)
        mCurrentValue = mSharedPreferences.getBoolean("preference_a", false)
        if (mCurrentValue as Boolean)
        {
            setTheme(R.style.MyDarkAppCompatTheme)
        }
        else
        {
            setTheme(R.style.MyLightAppCompatTheme)
        }
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_mysettings)

        val settingsFragment = MySettingsFragment()
        supportFragmentManager
                .beginTransaction()
                .replace(R.id.settings_container, settingsFragment)
                .commit()

        val myActionBar = actionBar

        if (myActionBar != null)
        {
            myActionBar.setTitle(R.string.settings)
            myActionBar.setBackgroundDrawable(ColorDrawable(Color.BLACK))
            myActionBar.setDisplayHomeAsUpEnabled(true)
            myActionBar.setDisplayShowHomeEnabled(true)
            myActionBar.setHomeAsUpIndicator(resources.getDrawable(R.drawable.ic_arrow_back_white, null))
        }
    }

    override fun onXchange(value:Boolean?) {
        if (mCurrentValue !== value) {
            mCurrentValue = value
            recreate()
        }
    }


    override fun onOptionsItemSelected(item: MenuItem): Boolean {
        when (item.itemId) {
            android.R.id.home -> {
                val intent = parentActivityIntent
                intent?.addFlags(Intent.FLAG_ACTIVITY_NO_ANIMATION)
                onBackPressed()
                return true
            }

            else ->
                return super.onOptionsItemSelected(item)
        }
    }
}

fragment 类

class MySettingsFragment : PreferenceFragmentCompat(), Preference.OnPreferenceChangeListener {
    override fun onCreatePreferences(savedInstanceState: Bundle?, rootKey: String?) {
        addPreferencesFromResource(R.xml.app_preferences)

        val mCheckBoxPreference = findPreference("preference_a") as CheckBoxPreference
        mCheckBoxPreference.onPreferenceChangeListener = this
    }

    private var mPreferenceXchangeListener: PreferenceXchangeListener? = null

    interface PreferenceXchangeListener {
        fun onXchange(value:Boolean)
    }

    override fun onAttach(context: Context) {
        super.onAttach(context)

        try
        {
            mPreferenceXchangeListener = context as MySettingsFragment.PreferenceXchangeListener
        }
        catch (e:ClassCastException) {
            Log.e(TAG, "onAttach::::: PreferenceXchangeListener must be set in parent Activity")
        }
    }

    override fun onPreferenceChange(preference: Preference, newValue:Any):Boolean {
        val preferenceKey = preference.key

        if (preferenceKey == "preference_a")
        {
            (preference as CheckBoxPreference).isChecked = newValue as Boolean

            mPreferenceXchangeListener!!.onXchange(newValue)
            return true
        }

        return false
    }

    companion object {
        private val TAG = MySettingsFragment::class.java.simpleName
    }
}

Answer 1

所以首先我可以解释它描述的问题。让我们提出一个非常人为的场景:

BooleanProvider.java

class BooleanProvider {
    @NonNull Boolean wrapMyBoolean(boolean state) {
        return new Boolean(state);
    }
}

BooleanProviderTest.kt

class BooleanProviderTest {
    @Test fun `it returns a true value when true is provided`() {
        assertSame(BooleanProvider().wrapMyBoolean(true), true)
    }
}

这个测试实际上会失败，因为 Boolean 的实例不一样。在 Java 代码中，我们初始化了一个新的 Boolean 实例(而不是静态定义的 Boolean.TRUE 和 Boolean.FALSE 实例)当一个原语被自动装箱到一个 java.lang.Boolean 时，你会得到)。因此，为避免潜在的意外结果，建议您不要通过引用来比较这些类型。

最简单的解决方法是将相等性测试更改为 != 而不是 !==。这将执行身份相等性测试而不是引用相等性测试，并将适本地处理任一侧的空值。无论如何，这就是您在 99% 的时间里想要的。

其次，如果您不需要以特定方式处理空值，您也可以只将mCurrentValue 声明为非空类型，并为其指定一个默认值。只需将其声明为:

private var mCurrentValue: Boolean = false // false is the default here