Canvas 对象必须与之前由 lockCanvas 返回的实例相同

Question

我有一个自定义的 SurfaceView，它带有绘制背景和位图的方法 doDraw()。问题是当我运行我得到一个错误

Caused by: java.lang.IllegalArgumentException: canvas object must be the same instance that was previously returned by lockCanvas

我不明白为什么会这样。我没有在我的代码中的任何其他地方声明任何其他画布。我只有另外两个 classes，MainActivity 和 SurfaceViewExample。 MainActivity 只是有一个打开 SurfaceViewExample 的意图，而 SurfaceViewExample 只是有一个被一些按钮调用的方法。

我们的观点class:

package com.thatoneprogrammerkid.gameminimum;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.view.SurfaceHolder;
import android.view.SurfaceView;

public class OurView extends SurfaceView implements SurfaceHolder.Callback {

    private SurfaceHolder holder;
    private Bitmap testimg;
    public int xCoord = 500;
    public int yCoord = 500;

    public OurView(Context context) {
        super(context);
        init();
    }

    public OurView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public OurView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init();
    }

    private void init() {
        holder = getHolder();
        holder.addCallback(this);
        testimg = BitmapFactory.decodeResource(getResources(),R.drawable.testimg);
    }

    void moveImage(int xChange, int yChange) {
        xCoord += xChange;
        yCoord += yChange;
        doDraw();
    }

    void doDraw() {
        Canvas myCanvas = holder.lockCanvas();
        if (myCanvas != null) {
            myCanvas.drawARGB(255, 55, 255, 255);
            myCanvas.drawBitmap(testimg, xCoord, yCoord, null);
        }
        holder.unlockCanvasAndPost(myCanvas);
    }

    @Override
    public void surfaceCreated(final SurfaceHolder holder) {
        doDraw();
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {}


    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {}

}

Answer 1

将 unlockCanvasAndPost() 移动到 if (myCanvas != null) { 语句中。我的猜测是 lockCanvas() 返回 null，而您正在尝试解锁 null 引用。

查看 the source code，"is it the same" 的测试先于 "is it locked at all" 的测试——并且 mCanvas 被初始化为非空值。

Canvas 对象必须与之前由 lockCanvas 返回的实例相同

Canvas object must be the same instance that was previously returned by lockCanvas

java

android

surfaceview

android-canvas