AlertDialog 引发的内存泄露

追根溯源

在咱们使用AlertDialog时,标准的写法以下：

AlertDialog.Builder builder = new AlertDialog.Builder(this)
        .setPositiveButton("confirm", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                login();
            }
        });
builder.create().show();
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但这样子的写法是存在内存泄露的,具体分析以下:

• 一、以上代码在非静态内部类OnClickListener方法中引用着Activity(activity.this.login()) OnClickListener→ Activiity

public void setButton(int whichButton, CharSequence text,
        DialogInterface.OnClickListener listener, Message msg) {
    if (msg == null && listener != null) {
        msg = mHandler.obtainMessage(whichButton, listener);
    }
    switch (whichButton) {
        case DialogInterface.BUTTON_POSITIVE:
            mButtonPositiveText = text;
            mButtonPositiveMessage = msg;
            break;
        case DialogInterface.BUTTON_NEGATIVE:
            mButtonNegativeText = text;
            mButtonNegativeMessage = msg;
            break;
        case DialogInterface.BUTTON_NEUTRAL:
            mButtonNeutralText = text;
            mButtonNeutralMessage = msg;
            break;
        default:
            throw new IllegalArgumentException("Button does not exist");
    }
}
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• 二、从以上源码中能够看到,在AlertDialog构建过程当中传入的参数 int whichButton, OnClickListener listener都包装成了msg来处理,这样子就形成了msg对listener的引用 msg→ OnClickListener

private final View.OnClickListener mButtonHandler = new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        final Message m;
        if (v == mButtonPositive && mButtonPositiveMessage != null) {
            m = Message.obtain(mButtonPositiveMessage);
        } else if (v == mButtonNegative && mButtonNegativeMessage != null) {
            m = Message.obtain(mButtonNegativeMessage);
        } else if (v == mButtonNeutral && mButtonNeutralMessage != null) {
            m = Message.obtain(mButtonNeutralMessage);
        } else {
            m = null;
        }
        if (m != null) {
            m.sendToTarget();
        }
        // Post a message so we dismiss after the above handlers are executed
        mHandler.obtainMessage(ButtonHandler.MSG_DISMISS_DIALOG, mDialogInterface)
                .sendToTarget();
    }
};
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• 三、以上源码中,在处理事件响应时,Dialog从消息队列中再次obtain一个Message实例,复制给m进行发送,Message m也会在Dialog销毁时跟着销毁,并无发现产生内存泄露的时机.

• 四、那什么状况下会产生内存泄露呢?

(1)Message是任何线程共用的，HandlerThread中，Looper会不停的从阻塞队列MessageQueue中取Message进行处理.当没有可消费Message对象时，就会开始阻塞，而此时最后一个被取出的Message就会被本地变量引用，一直不会释放引用，除非有新的message

(2)Dialog从消息队列中可能会恰巧取到一个“仍然被某个阻塞中的HandlerThread本地变量引用的Message实例”,代码msg = mHandler.obtainMessage(whichButton, listener),把listener赋给Message的obj，并一直保存在Dialog实例中 如此产生引用: Thread → Mesage → Listener → Dialog → Activity. 当Activity关闭时,Thread仍然引用着Activity, 这样内存泄漏就发生了.

解决方法

• 定义一个 DetachClickListener 类实现 DialogInterface.OnClickListener 接口

public class DetachClickListener implements DialogInterface.OnClickListener {
    public static DetachClickListener wrap(DialogInterface.OnClickListener delegate) {
        return new DetachClickListener(delegate);
    }
    private DialogInterface.OnClickListener mDelegate;
    private DetachClickListener(DialogInterface.OnClickListener delegate) {
        this.mDelegate = delegate;
    }
    @Override
    public void onClick(DialogInterface dialog, int which) {
        if (mDelegate != null) {
            mDelegate.onClick(dialog, which);
        }
    }
    public void clearOnDetach(Dialog dialog) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR2) {
            dialog.getWindow()
                    .getDecorView()
                    .getViewTreeObserver()
                    .addOnWindowAttachListener(new ViewTreeObserver.OnWindowAttachListener() {
                        @Override
                        public void onWindowAttached() {
                        }
                        @Override
                        public void onWindowDetached() {
                            mDelegate = null;
                        }
                    });
        }
    }
}
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• 使用方式

DetachClickListener clickListener = DetachClickListener.wrap(
                new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                    }
                });
AlertDialog alertDialog = new AlertDialog.Builder(this)
        .setPositiveButton("confirm", clickListener).create();
alertDialog.show();
// 在适当的时机调用该方法，防止内存泄漏
clickListener.clearOnDetach(alertDialog);
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以上写法在Dialog退出后,清除了对DialogInterface.OnClickListener的引用,在中间层截断, 故在Activity关闭时避免了内存泄露.