android canvas的问题

[老黑牛]

下面是我的自定义view的相关代码：

@Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        mCanvas = canvas;

        mCanvas.drawOval(mCirRect, mCP);
    }

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                mCP.setColor(mPressColor);
                mCanvas.drawOval(mCirRect, mCP);//①
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                mCP.setColor(mNormalColor);
                mCanvas.drawOval(mCirRect, mCP);//②
                invalidate();
                break;
            default:
                break;
        }
        return true;
    }

    @Override
    protected void onDetachedFromWindow() {
        super.onDetachedFromWindow();
        mCanvas = null;
    }

一共两个activity，一个叫 A，一个叫 B。这个自定义view是放在B的布局文件里的，A的布局文件里只有一个button，我在A中，点击button启动B，然后点击我的自定义view，再按返回键回到A，结果在A的界面中出现了我的自定义view 。但是将上述代码中的标注①②的代码行注释掉一切正常，请问这是什么原因？

[键盘舞者113]

你再贴点别的代码

[老黑牛]

public class RecordButton extends FrameLayout implements View.OnTouchListener {
    private static final String TAG = "RecordButton";

    private Canvas mCanvas;
    private RectF mCirRect;
    private Paint mCP;
    private float mRingWidth;
    private int mNormalColor;
    private int mPressColor;

    public RecordButton(@NonNull Context context) {
        this(context, null);
    }

    public RecordButton(@NonNull Context context, @Nullable AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public RecordButton(@NonNull Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        initView(context, attrs);
    }

    private void initView(Context context, AttributeSet attrs) {
        setWillNotDraw(false);
        mRingWidth = (int) (getResources().getDisplayMetrics().density * 10 + 0.5f);

        mNormalColor = getResources().getColor(android.R.color.holo_red_dark);
        mPressColor = getResources().getColor(android.R.color.holo_red_light);

        mCP = new Paint();
        mCP.setColor(mNormalColor);
        mCP.setAntiAlias(true);

        mCirRect = new RectF();

        setOnTouchListener(this);

    }

    @Override
    protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
        super.onLayout(changed, left, top, right, bottom);
        float width = getMeasuredWidth();
        float height = getMeasuredHeight();

        mCirRect.set(mRingWidth, mRingWidth, width - mRingWidth, height - mRingWidth);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        mCanvas = canvas;

        mCanvas.drawOval(mCirRect, mCP);
    }

    @Override
    public boolean onTouch(View v, MotionEvent event) {
        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                mCP.setColor(mPressColor);
                mCanvas.drawOval(mCirRect, mCP);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                mCP.setColor(mNormalColor);
                mCanvas.drawOval(mCirRect, mCP);
                invalidate();
                break;
            default:
                break;
        }
        return true;
    }

    @Override
    protected void onDetachedFromWindow() {
        super.onDetachedFromWindow();
        mCanvas = null;
    }
}

[老黑牛]

public class Main3Activity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main3);
        Button btn = findViewById(R.id.submit_btn);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(Main3Activity.this, Main2Activity.class);
                startActivity(intent);
            }
        });
    }
}

public class Main2Activity extends AppCompatActivity {
    private static final String TAG = "Main2Activity";

    private RecordButton btn;

    @SuppressLint("ClickableViewAccessibility")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);
        btn = findViewById(R.id.camera_record_btn);
    }
}

[键盘舞者113]

我使用你的代码的时候没有发生你那样的情况，我只发现你那个自定义View代码有一个错误，就是一点击当前Activity就崩溃，因为你自己吧Canvas保存下来，放在onTouch里绘画，我就这么说Canvas只能在onDraw函数里使用，其他位置不能使用，这相当于一个分配在内存里的地址，只有在onDraw对其开锁，其他函数关锁，不能访问

[老黑牛]

嗯，多谢。还有个问题： onDraw的入参Canvas指向的是谁？是谁调用的他？

[键盘舞者113]

VIiew由measure测量大小，layout确定位置，draw绘画， draw会调用onDraw(Canvas canvas)方法,Canvas画布，你可当做BItmap一样的图像在内存的数据结构

[老黑牛]

ok,3ks.待我去读读源码。