Question

这里是如何使用Kotlin在Try-Catch块中安全投射。这是一个语法问题。对于Kotlin，没有与此相关的答案，因此我将分享给大家，以节省一些时间。

通常，为了安全起见，人们使用以下格式：

fun functionName() {
    if (propertyName is ClassName1) {
        val variableName2 = propertyName as ClassName1()
        propertyName.memberFunction()
    }
}

以下代码将在出现编译错误时显示（我可能显示的太多了，但是我希望你们理解上下文）：

import android.graphics.Canvas
import android.view.SurfaceHolder
import java.lang.Exception

class MainThread(surfaceHolder: SurfaceHolder, gameView: GameView): Thread() {
    private lateinit var surfaceHolder: SurfaceHolder
    private lateinit var gameView: GameView
    private var running: Boolean = false
    private var canvas: Canvas? = null

    override fun run() {
        super.run()

        while(running) {
            try { 
                canvas = this.surfaceHolder.lockCanvas() 
                synchronized(surfaceHolder) {
                    this.gameView.update()
                    this.gameView.draw(canvas) // Error arises at this line
                }
            } catch (e: Exception) {
            } finally {
                if(canvas != null){
                    try {
                        surfaceHolder.unlockCanvasAndPost(canvas)
                    } catch (e: Exception){
                        e.printStackTrace()
                    }
                }
            }
        }
    }

Build Output Error读取：

Smart cast to 'Canvas' is impossible, 
because 'canvas' is a mutable property that could have been changed by this time

Answer 1

实际上，您必须将as关键字放在括号内。

以下代码将显示如何在try-catch块中实现智能强制转换：

    override fun run() {
        super.run()

        while(running) {
            try { 
                canvas = this.surfaceHolder.lockCanvas() 
                synchronized(surfaceHolder) {
                    this.gameView.update()
if (canvas is Canvas)
                    this.gameView.draw(canvas as Canvas) // Note change
                }
            } catch (e: Exception) {
            } finally {
                if(canvas != null){
                    try {
                        surfaceHolder.unlockCanvasAndPost(canvas)
                    } catch (e: Exception){
                        e.printStackTrace()
                    }
                }
            }
        }
    }

如何在试捕区中安全投放

1 个答案: