尝试在空对象引用上调用虚拟方法“void android.graphics.Canvas.drawText()”答案

【问题标题】：Attempt to invoke virtual method 'void android.graphics.Canvas.drawText()' on a null object reference尝试在空对象引用上调用虚拟方法“void android.graphics.Canvas.drawText()”
【发布时间】：2018-06-24 21:30:07
【问题描述】：

我想在屏幕上绘制文字，但收到错误消息：

java.lang.NullPointerException: 尝试在空对象引用上调用虚拟方法 'void android.graphics.Canvas.drawText(java.lang.String, float, float, android.graphics.Paint)'

我的代码：

package com.example.root.runningmanv2

import android.content.Context
import android.graphics.Paint
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.view.SurfaceView
import android.view.WindowManager

class GamingActivity : AppCompatActivity() {
    private lateinit var gameView: SurfaceGame

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        //set fullscreen
        window.setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                WindowManager.LayoutParams.FLAG_FULLSCREEN)
        gameView = SurfaceGame(this)
        setContentView(gameView)
    }

    override fun onPause() {
        super.onPause()
    }

    override fun onResume() {
        super.onResume()
        gameView.start()
    }

    inner class SurfaceGame(context: Context): SurfaceView(context), Runnable{
        val paint = Paint(Paint.ANTI_ALIAS_FLAG)
        val ourHolder = holder
        val canvas = ourHolder.lockCanvas()

        fun start(){
            val thread = Thread(this)
            thread.start()
        }

        override fun run() {
            drawFigure()
        }

        fun drawFigure(){
            paint.setARGB(255, 255, 255, 255)
            canvas.drawText("Hello", 500f, 500f, paint)
            ourHolder.unlockCanvasAndPost(canvas)
        }

    }
}

如何解决？

【问题讨论】：

标签： android kotlin

【解决方案1】：

您自己从 SurfaceView.Callback 类中获取 holder 而不是在 SurfaceGame 类中获取它

class GamingActivity : SurfaceHolder.Callback, AppCompatActivity() {

    private var holder: SurfaceHolder? = null
    private var pos = 0

    public override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val view = SurfaceView(this)
        setContentView(view)
        view.holder.addCallback(this)
    }

    override fun surfaceCreated(holder: SurfaceHolder) {
        this.holder = holder
        pos = 0
        Thread({
            while (this.holder != null) {
                drawFigure(this.holder)
                Thread.sleep(100)
            }
        }).start()
    }

    override fun surfaceChanged(holder: SurfaceHolder, frmt: Int, w: Int, h: Int) {
        this.holder = holder
    }

    override fun surfaceDestroyed(holder: SurfaceHolder) {
        this.holder = null
    }

    private fun drawFigure(holder: SurfaceHolder?) {

        if (holder == null) return

        val canvas = holder.lockCanvas()
        val paint = Paint(Paint.ANTI_ALIAS_FLAG)
        paint.setARGB(255, 0, 0, 0)
        paint.textSize = 96f
        canvas.drawText("Hello", 100f + pos, 500f, paint)
        pos++
        holder.unlockCanvasAndPost(canvas)
    }

}

【讨论】：