Kotlin中的嵌套let块

发布于02月16日

Is it a good practice to use multiple nested let in Kotlin or should I introduce a local variable instead? Is there any overhead?

webView?.let { webview ->
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
        webview.setLayerType(View.LAYER_TYPE_HARDWARE, null)
    } else {
        webview.setLayerType(View.LAYER_TYPE_SOFTWARE, null)
    }

    webview.webViewClient = WebViewClient()

    webview.settings.let { settings ->
        settings.javaScriptEnabled = true
        settings.setSupportZoom(false)
        settings.builtInZoomControls = false
        settings.displayZoomControls = false
        settings.loadsImagesAutomatically = true
    }
}

推荐答案

Since let is inlined (see: what is inlining, also the official docs), its cost is the exact same as declaring the local variable yourself. If you're using it with a safe call, it also adds a null check, which you'd have to do manually as well.

因此，使用它基本上没有开销，只要代码对您来说足够清晰，就可以随意使用它.

For example, code like this:

webview.settings.let { settings ->
    settings.javaScriptEnabled = true
    settings.setSupportZoom(false)
}

... would roughly translate to bytecode equivalent to this Java code:

Settings settings = webview.getSettings();
settings.setJavaScriptEnabled(true);
settings.setSupportZoom(false);

Except settings would probably be called something generic like var10000, but that's besides the point. Point is that no function instances are created and no let function is called or anything like that, therefore no overhead.

For any future concerns, you can actually check this yourself by using the Kotlin plugin's bytecode viewer and decompiler.

Kotlin中的嵌套let块

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