android - obj.apply{ func1() } 的结果与 obj.func1() 不同

Question

在 Kotlin 中，apply{}定义为 inline fun <T> T.apply(block: T.() -> Unit): T我以为使用这个功能是为了最小化这段代码

obj.func1()
obj.func2()
obj.func3()
...

至

obj.apply {
    func1()
    func2()
    func3()
}

但现在我认为它有一些完全不同的实际用例。在我的 android studio 项目中，我得到了这个日志

2020-12-08 13:3:22.648 28006-28006/com.skb.skara D/VideoViewActivity: onStart: pos 2677
2020-12-08 13:3:22.649 28006-28006/com.skb.skara D/VideoViewActivity: initializePlayer: seeked to 0
2020-12-08 13:3:22.650 28006-28006/com.skb.skara D/VideoViewActivity: onStart: pos 2677

对于此代码

private var currentPosition: Int

override fun onStart() {
    super.onStart()
    Log.d(TAG, "onStart: pos $currentPosition")
    initializePlayer()
    Log.d(TAG, "onStart: pos $currentPosition")
    if (currentPosition == 0) {
        Log.d(TAG, "onStart: vid start")
        fullscreen_video.start()
    }
    show()
}

private fun initializePlayer() {
    fullscreen_video.apply {
        setVideoPath(videoUrl)
        seekTo(currentPosition)
        Log.d(TAG, "initializePlayer: seeked to $currentPosition")
    }
}

然后我改了 initialisePlayer作用于此

private fun initializePlayer() {
    fullscreen_video.setVideoPath(videoUrl)
    fullscreen_video.seekTo(currentPosition)
    Log.d(TAG, "initializePlayer: seeked to $currentPosition")
}

然后我得到了这个日志

2020-12-08 13:12:53.548 28006-28006/com.skb.skara D/VideoViewActivity: onStart: 2677
2020-12-08 13:12:53.550 28006-28006/com.skb.skara D/VideoViewActivity: initializePlayer: seeked to 2677
2020-12-08 13:12:53.550 28006-28006/com.skb.skara D/VideoViewActivity: onStart: 2677

有人请解释这种行为并解释范围函数的使用 apply{}

Answer 1

小心，VideoView有自己的currentPosition .访问 currentPosition在其apply block 将获得 VideoView 的 currentPosition .

private fun initializePlayer() {
    fullscreen_video.apply {
        setVideoPath(videoUrl)
        seekTo(currentPosition) // This will use VideoView.currentPosition
        Log.d(TAG, "initializePlayer: seeked to $currentPosition") // This, too
    }
}

解释一下 apply :

The context object is available as a receiver (this). And this always can be omitted.

如果您想访问 currentPosition您在上面声明，请使用 this@YourActivity.currentPosition .替换 YourActivity与您的 Activity 。