AlertDialog源码浅析

AlertDialog的构造方法一共有4种，分别是：

protected AlertDialog(Context context)

创建一个默认配置的AlertDialog对象

protected AlertDialog(Context context, boolean cancelable, OnCancelListener cancelListener)

创建一个AlertDialog对象并设置是否可取消和取消监听

protected AlertDialog(Context context, @StyleRes int themeResId)

创建一个AlertDialog对象并设置主题

AlertDialog(Context context, @StyleRes int themeResId, boolean createContextThemeWrapper)

创建一个AlertDialog对象并设置主题和是否创建主题包装

我们通常却很少这样写，我们通常使用Builer创建：

AlertDialog alertDialog=new AlertDialog.Builder(this).create();

我们使用 setNegativeButton，setButton，setView来为AlertDialog设置样式。

如果我们在AlertDialog上设置的话，方法是这样的：

public void setTitle(CharSequence title) {super.setTitle(title);mAlert.setTitle(title);}public void setCustomTitle(View customTitleView) {mAlert.setCustomTitle(customTitleView);}public void setMessage(CharSequence message) {mAlert.setMessage(message);}

private AlertController mAlert;

都是在给AlertController对象赋值

如果使用Builer来设置，那么他们的方法时这样的：

public Builder setIcon(Drawable icon) {P.mIcon = icon;return this;}public Builder setPositiveButton(@StringRes int textId, final OnClickListener listener) {P.mPositiveButtonText = P.mContext.getText(textId);P.mPositiveButtonListener = listener;return this;}public Builder setItems(@ArrayRes int itemsId, final OnClickListener listener) {P.mItems = P.mContext.getResources().getTextArray(itemsId);P.mOnClickListener = listener;return this;}

我们可以看到，无论设置什么，都是在给P的字段赋值，P是什么呢？

private final AlertController.AlertParams P;

P是一个AlertController.AlertParams对象，装载着对话框的各种设置，它有以下字段

 public static class AlertParams {public final Context mContext;public final LayoutInflater mInflater;public int mIconId = 0;public Drawable mIcon;public int mIconAttrId = 0;public CharSequence mTitle;public View mCustomTitleView;public CharSequence mMessage;public CharSequence mPositiveButtonText;public DialogInterface.OnClickListener mPositiveButtonListener;public CharSequence mNegativeButtonText;public DialogInterface.OnClickListener mNegativeButtonListener;public CharSequence mNeutralButtonText;public DialogInterface.OnClickListener mNeutralButtonListener;public boolean mCancelable;public DialogInterface.OnCancelListener mOnCancelListener;public DialogInterface.OnDismissListener mOnDismissListener;public DialogInterface.OnKeyListener mOnKeyListener;public CharSequence[] mItems;public ListAdapter mAdapter;public DialogInterface.OnClickListener mOnClickListener;public int mViewLayoutResId;public View mView;public int mViewSpacingLeft;public int mViewSpacingTop;public int mViewSpacingRight;public int mViewSpacingBottom;public boolean mViewSpacingSpecified = false;public boolean[] mCheckedItems;public boolean mIsMultiChoice;public boolean mIsSingleChoice;public int mCheckedItem = -1;public DialogInterface.OnMultiChoiceClickListener mOnCheckboxClickListener;public Cursor mCursor;public String mLabelColumn;public String mIsCheckedColumn;public boolean mForceInverseBackground;public AdapterView.OnItemSelectedListener mOnItemSelectedListener;public OnPrepareListViewListener mOnPrepareListViewListener;public boolean mRecycleOnMeasure = true;

在使用Builder设置完后必须调用creat方法来创建AlertDialog对象

/*** Creates an {@link AlertDialog} with the arguments supplied to this* builder.* <p>* Calling this method does not display the dialog. If no additional* processing is needed, {@link #show()} may be called instead to both* create and display the dialog.*/public AlertDialog create() {// Context has already been wrapped with the appropriate theme.final AlertDialog dialog = new AlertDialog(P.mContext, 0, false);P.apply(dialog.mAlert);dialog.setCancelable(P.mCancelable);if (P.mCancelable) {dialog.setCanceledOnTouchOutside(true);}dialog.setOnCancelListener(P.mOnCancelListener);dialog.setOnDismissListener(P.mOnDismissListener);if (P.mOnKeyListener != null) {dialog.setOnKeyListener(P.mOnKeyListener);}return dialog;}

public void apply(AlertController dialog) {if (mCustomTitleView != null) {dialog.setCustomTitle(mCustomTitleView);} else {if (mTitle != null) {dialog.setTitle(mTitle);}if (mIcon != null) {dialog.setIcon(mIcon);}if (mIconId != 0) {dialog.setIcon(mIconId);}if (mIconAttrId != 0) {dialog.setIcon(dialog.getIconAttributeResId(mIconAttrId));}}if (mMessage != null) {dialog.setMessage(mMessage);}if (mPositiveButtonText != null) {dialog.setButton(DialogInterface.BUTTON_POSITIVE, mPositiveButtonText,mPositiveButtonListener, null);}if (mNegativeButtonText != null) {dialog.setButton(DialogInterface.BUTTON_NEGATIVE, mNegativeButtonText,mNegativeButtonListener, null);}if (mNeutralButtonText != null) {dialog.setButton(DialogInterface.BUTTON_NEUTRAL, mNeutralButtonText,mNeutralButtonListener, null);}if (mForceInverseBackground) {dialog.setInverseBackgroundForced(true);}// For a list, the client can either supply an array of items or an// adapter or a cursorif ((mItems != null) || (mCursor != null) || (mAdapter != null)) {createListView(dialog);}if (mView != null) {if (mViewSpacingSpecified) {dialog.setView(mView, mViewSpacingLeft, mViewSpacingTop, mViewSpacingRight,mViewSpacingBottom);} else {dialog.setView(mView);}} else if (mViewLayoutResId != 0) {dialog.setView(mViewLayoutResId);}/*dialog.setCancelable(mCancelable);dialog.setOnCancelListener(mOnCancelListener);if (mOnKeyListener != null) {dialog.setOnKeyListener(mOnKeyListener);}*/}

可以看到两种方法都是一样的，最后都是在给AlertController对象赋值

调用AlertDialog的show方法其实也会调用creat方法

/*** Creates an {@link AlertDialog} with the arguments supplied to this* builder and immediately displays the dialog.* <p>* Calling this method is functionally identical to:* <pre>*     AlertDialog dialog = builder.create();*     dialog.show();* </pre>*/public AlertDialog show() {final AlertDialog dialog = create();dialog.show();return dialog;}

我们再来看看setButton方法，它有4种不同参数的方法，不过有两种被弃用了

/*** Set a message to be sent when a button is pressed.** @param whichButton Which button to set the message for, can be one of*            {@link DialogInterface#BUTTON_POSITIVE},*            {@link DialogInterface#BUTTON_NEGATIVE}, or*            {@link DialogInterface#BUTTON_NEUTRAL}* @param text The text to display in positive button.* @param msg The {@link Message} to be sent when clicked.*/public void setButton(int whichButton, CharSequence text, Message msg) {mAlert.setButton(whichButton, text, null, msg);}/*** Set a listener to be invoked when the positive button of the dialog is pressed.** @param whichButton Which button to set the listener on, can be one of*            {@link DialogInterface#BUTTON_POSITIVE},*            {@link DialogInterface#BUTTON_NEGATIVE}, or*            {@link DialogInterface#BUTTON_NEUTRAL}* @param text The text to display in positive button.* @param listener The {@link DialogInterface.OnClickListener} to use.*/public void setButton(int whichButton, CharSequence text, OnClickListener listener) {mAlert.setButton(whichButton, text, listener, null);}

可能我们用的多是第二种方法，第一种很少见，但其实第一种可以说是第二种方法的内部实现，看代码

/*** Sets a click listener or a message to be sent when the button is clicked.* You only need to pass one of {@code listener} or {@code msg}.** @param whichButton Which button, can be one of*            {@link DialogInterface#BUTTON_POSITIVE},*            {@link DialogInterface#BUTTON_NEGATIVE}, or*            {@link DialogInterface#BUTTON_NEUTRAL}* @param text The text to display in positive button.* @param listener The {@link DialogInterface.OnClickListener} to use.* @param msg The {@link Message} to be sent when clicked.*/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");}}

如果设置了监听，监听就会被封装到message中，最终都会被赋值给对应按钮的message中，如果视图被点击就会取对应的信息，使用handler发送：

 private final View.OnClickListener mButtonHandler = new View.OnClickListener() {@Overridepublic 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 executedmHandler.obtainMessage(ButtonHandler.MSG_DISMISS_DIALOG, mDialogInterface).sendToTarget();}};

如果是设置的监听，会使用AlertController中的mHandler发送，它在AlertController的构造方法中被初始化，也就是说调用了creat方法或者show方法，mHandler都会被初始化

protected AlertController(Context context, DialogInterface di, Window window) {mContext = context;mDialogInterface = di;mWindow = window;mHandler = new ButtonHandler(di);final TypedArray a = context.obtainStyledAttributes(null,R.styleable.AlertDialog, R.attr.alertDialogStyle, 0);mAlertDialogLayout = a.getResourceId(R.styleable.AlertDialog_layout, R.layout.alert_dialog);mButtonPanelSideLayout = a.getResourceId(R.styleable.AlertDialog_buttonPanelSideLayout, 0);mListLayout = a.getResourceId(R.styleable.AlertDialog_listLayout, R.layout.select_dialog);mMultiChoiceItemLayout = a.getResourceId(R.styleable.AlertDialog_multiChoiceItemLayout,R.layout.select_dialog_multichoice);mSingleChoiceItemLayout = a.getResourceId(R.styleable.AlertDialog_singleChoiceItemLayout,R.layout.select_dialog_singlechoice);mListItemLayout = a.getResourceId(R.styleable.AlertDialog_listItemLayout,R.layout.select_dialog_item);mShowTitle = a.getBoolean(R.styleable.AlertDialog_showTitle, true);a.recycle();/* We use a custom title so never request a window title */window.requestFeature(Window.FEATURE_NO_TITLE);}

如果是设置的Message,那么就会使用Message对象的target Handler

/*package*/ Handler target;

它可以使用public void setTarget(Handler target)来赋值

看个例子：

AlertDialog alertDialog=new AlertDialog.Builder(this).create();Message message=new Message();message.setTarget(new Handler(){@Overridepublic void handleMessage(Message msg) {super.handleMessage(msg);showToast("哈哈");}});alertDialog.setButton(DialogInterface.BUTTON_POSITIVE,"显示",message);alertDialog.show();

所以明白了吧，AlertDialog的按钮监听其实是使用handler实现的，那么就会有内存泄漏问题

在AlertDialog构建过程中传入的参数 int whichButton, OnClickListener listener都包装成了msg来处理,这样子就造成了msg对listener的引用 msg→ OnClickListener

有两种情况可能会发生内存泄漏：

(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.

那么发现了问题如何解决呢？看一下示例代码

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;}@Overridepublic void onClick(DialogInterface dialog, int which) {if (mDelegate != null) {mDelegate.onClick(dialog, which);}}public void clearOnDetach(Dialog dialog) {dialog.getWindow().getDecorView().getViewTreeObserver().addOnWindowAttachListener(new ViewTreeObserver.OnWindowAttachListener() {@Overridepublic void onWindowAttached() {}@Overridepublic void onWindowDetached() {mDelegate = null;}});}}DetachClickListener clickListener = DetachClickListener.wrap(new DialogInterface.OnClickListener() {@Overridepublic void onClick(DialogInterface dialog, int which) {}
});AlertDialog dialog = new AlertDialog.Builder(context).setPositiveButton("confirm", clickListener).create();
dialog.show();
clickListener.clearOnDetach(dialog);

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.

参考网址：https://blog.csdn.net/awangyunke/article/details/78933854

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