成员变量是在函数返回空？ [英] member variable is null after function return?

问题描述

我有一个扩展视图类。这个类有成员变量mCanvas

I have a class that extends View. This class has the member variable mCanvas

private Canvas mCanvas;

视图时大小创建此变量，所以适当的大小画布设置：

This variable is created when the view is resized, so the appropriate sizes for the canvas are set:

@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    int curW = mBitmap != null ? mBitmap.getWidth() : 0;
    int curH = mBitmap != null ? mBitmap.getHeight() : 0;
    if (curW >= w && curH >= h) {
        return;
    }

    if (curW < w) curW = w;
    if (curH < h) curH = h;

    Bitmap canvasBitmap = Bitmap.createBitmap(curW, curH, Bitmap.Config.ARGB_8888);
    mCanvas = new Canvas(canvasBitmap);
    if (mBitmap != null) {
        mCanvas.drawBitmap(mBitmap, 0, 0, null);
    }
    mBitmap = canvasBitmap;
}

但在我OnDraw函数我得到一个空指针异常，当我尝试让我的画布宽度/高度。我不确定时，实际上是获取调用onSizeChanged，我是假设在创建视图之前所以的onDraw。当它总是会被调用

But in my onDraw function i'm getting a null pointer exception when i try get the width/height of my canvas. I was unsure when onSizeChanged is actually getting called, i was assuming it would always get called when the view was created and therefore before onDraw.

但是，如果我的onDraw始于此：

But if my onDraw starts with this:

@Override
protected void onDraw(Canvas canvas) {
    if (mBitmap != null) {
        if(mCanvas == null)
        {
            Log.d("testing","mCanvas is null"
        }

LogCat中始终显示消息mCanvas为空当我到达的on​​Draw。

logCat always shows the message "mCanvas is null" when i reach onDraw.

所以我改变了code，这样如果mCanvas为空，当我读到的onDraw我只是重新创建它：

So i changed the code so that if mCanvas is null when i read onDraw i just create it again:

private void resizeCanvas()
{
    int curW = mBitmap != null ? mBitmap.getWidth() : 0;
    int curH = mBitmap != null ? mBitmap.getHeight() : 0;

    if (curW >= this.getWidth() && curH >= this.getHeight()) {
        return;
    }

    if (curW < this.getWidth()) curW = this.getWidth();
    if (curH < this.getHeight()) curH = this.getHeight();

    Bitmap canvasBitmap = Bitmap.createBitmap(curW, curH, Bitmap.Config.ARGB_8888);
    mCanvas = new Canvas(canvasBitmap);

    if (mBitmap != null) {
        mCanvas.drawBitmap(mBitmap, 0, 0, null);
    }

    mBitmap = canvasBitmap;
}

@Override
protected void onDraw(Canvas canvas) {
    if (mBitmap != null) {
        if(mCanvas == null)
        {
            resizeCanvas();
            if(mCanvas == null)
            {
                Log.d("test","canvas is still null");
            }

LogCat中仍然打印帆布依然空

logCat still prints "canvas is still null"

有人能解释这里发生了什么？我非常新的Andr​​oid和大多数这个code是从我一直在玩的touchpaint例子。

Can someone explain what is happening here? I'm very new with android and most of this code is from the touchpaint example that i've been playing with.

如果我检查resizeCanvas函数内部如果mCanvas为null，它总是说，这是不为空。但是，如果我只是检查调用该函数后，它始终为空。

If i check inside the resizeCanvas function if mCanvas is null it always says it is not null. But if i check just after calling that function it is always null.

推荐答案

我认为这个问题是在你的 resizeCanvas <$ C $，你可以从它初始化之前返回C> mCanvas 。

I think the problem is in your resizeCanvas as you can return from it before initializing mCanvas.

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