Android系统封装了AlertDialog，用来给我们使用。我们可以通过

AlertDialog.Builder builder = new AlertDialog.Builder(this).setTitle("Title").setMessage("message").create().show();
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这个其实就是典型的Builder设计模式，通过封装复杂的dialog对象，将组建和构建分离，当用户使用的时候可以直接调用组建，并最后创建出Dialog对象。

通过Dialog源码的分析，我们能够更好的了解Builder设计模式。

public class AlertDialog extends Dialog implements DialogInterface {private AlertController mAlert;AlertDialog(Context context, @StyleRes int themeResId, boolean createContextThemeWrapper) {super(context, createContextThemeWrapper ? resolveDialogTheme(context, themeResId) : 0,createContextThemeWrapper);mWindow.alwaysReadCloseOnTouchAttr();mAlert = new AlertController(getContext(), this, getWindow());}@Overridepublic void setTitle(CharSequence title) {super.setTitle(title);mAlert.setTitle(title);}public static class Builder {public Builder(Context context, int themeResId) {P = new AlertController.AlertParams(new ContextThemeWrapper(context, resolveDialogTheme(context, themeResId)));}public Builder setTitle(@StringRes int titleId) {P.mTitle = P.mContext.getText(titleId);return this;}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 AlertDialog show() {final AlertDialog dialog = create();dialog.show();return dialog;}}
}
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通过上面的代码我们可以看到，创建AlertDialog.Build的时候会创建一个AlertController.AlertParams对象，这个对象里面封装了所有的Dialog的属性，并且在调用Builder的类似于setTitle方法的时候会将参数赋值给AlertParams，当所有的组件的属性赋值好之后，就调用create()方法，这个方法里面就创建出AlertDialog对象，并调用P.apply(dialog.mAlert)方法，将AlertDialog构造函数创建出来的AlertController对象传递给P:

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));}}// 代码省略
}
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在apply方法中，可以发现只是把AlertParams中的参数设置到AlertController中，当我们调用create方法的时候，就是讲AlertDialog对象的组件组装起来，当我调用show的时候就会调用dialog的show方法：

public void show() {// 如果已经在显示状态，returnif (mShowing) {if (mDecor != null) {if (mWindow.hasFeature(Window.FEATURE_ACTION_BAR)) {mWindow.invalidatePanelMenu(Window.FEATURE_ACTION_BAR);}mDecor.setVisibility(View.VISIBLE);}return;}mCanceled = false;if (!mCreated) {dispatchOnCreate(null);}onStart();// 获取DecorViewmDecor = mWindow.getDecorView();// 获取布局参数WindowManager.LayoutParams l = mWindow.getAttributes();if ((l.softInputMode& WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION) == 0) {WindowManager.LayoutParams nl = new WindowManager.LayoutParams();nl.copyFrom(l);nl.softInputMode |=WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION;l = nl;}// 将decorView添加到WindowManager中mWindowManager.addView(mDecor, l);mShowing = true;// 发送一个现实Dialog的消息sendShowMessage();
}
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在show方法中：

1. 通过diapatchOnCreate方法，调用Dialog的onCreate方法，并最终调用installContent方法。

   public void installContent() {/* We use a custom title so never request a window title */mWindow.requestFeature(Window.FEATURE_NO_TITLE);// 设置窗口的视图int contentView = selectContentView();mWindow.setContentView(contentView);setupView();setupDecor();
   }
   private int selectContentView() {if (mButtonPanelSideLayout == 0) {return mAlertDialogLayout;}if (mButtonPanelLayoutHint == AlertDialog.LAYOUT_HINT_SIDE) {return mButtonPanelSideLayout;}// TODO: use layout hint side for long messages/listsreturn mAlertDialogLayout;
   }
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   而我们可以看到在调用selectContentView的时候，会去获取mButtonPanelSideLayout视图，并通过WindowManager的setContentView方法来将视图加载。

   public AlertController(Context context, DialogInterface di, Window window) {final TypedArray a = context.obtainStyledAttributes(null,R.styleable.AlertDialog, R.attr.alertDialogStyle, 0);// 获取视图mAlertDialogLayout = a.getResourceId(R.styleable.AlertDialog_layout, R.layout.alert_dialog);a.recycle();
   }
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   再通过setupView方法初始化AlertDialog布局中的各个部分

2. 调用AlertDialog的onStart方法

3. 最后将DecorView添加到WindowManager中