调停者模式

一、概述

一般问题：系统内部经常会出现多个类互相耦合，形成网状结构。任意一个类发生改变，所有调用者也会受到影响，造成阅读和维护困难。

核心方案：用一个中介对象来封装多个耦合对象的交互，使各对象不需要显示地互相调用，从而使其耦合松散。

设计意图：调停者是对系统内部运转的协调，是从架构设计之初就应该考虑的问题。那么，架构设计应该面向接口，调停者模式的示意性类图如下：

引入调停者模式的意图是将一对多抓化成一对一，降低整个系统的交互复杂度。

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二、应用场景

Android 锁屏应用采用了调停者模式，锁屏是一个特殊的与系统关系密切的App，PhoneWindowManager通过KeyguardService对锁屏有着频繁的影响（如ScreenOn/Off、BootComplete、SwtichUser等），KeyguardUpdateMonitor收集的外部信息变化（如指纹识别、sim卡状态等）也对锁屏View有着频繁影响。同时，它们也依赖锁屏提供的一些状态信息，以及View变化的回调。总之，如果没有中介者，那系统和KeyguardUpdateMonitor与锁屏View的耦合将非常严重。因此，在锁屏架构中出现了KeyguardViewMediator类作为调停者，来统一处理这些复杂交互。

KeyguardViewMediator设计图如下：

如下是google对KeyguardViewMediator类的作用解释，列举了一些KeyguardViewMediator在协调系统和keyguard之间交互的例子：

/**
     * Mediates requests related to the keyguard.  This includes queries about the
     * state of the keyguard, power management events that effect whether the keyguard
     * should be shown or reset, callbacks to the phone window manager to notify
     * it of when the keyguard is showing, and events from the keyguard view itself
     * stating that the keyguard was succesfully unlocked.
     *
     * Note that the keyguard view is shown when the screen is off (as appropriate)
     * so that once the screen comes on, it will be ready immediately.
     *
     * Example queries about the keyguard:
     * - is {movement, key} one that should wake the keygaurd?
     * - is the keyguard showing?
     * - are input events restricted due to the state of the keyguard?
     *
     * Callbacks to the phone window manager:
     * - the keyguard is showing
     *
     * Example external events that translate to keyguard view changes:
     * - screen turned off -> reset the keyguard, and show it so it will be ready
     *   next time the screen turns on
     * - keyboard is slid open -> if the keyguard is not secure, hide it
     *
     * Events from the keyguard view:
     * - user succesfully unlocked keyguard -> hide keyguard view, and no longer
     *   restrict input events.
     */

由于一般Mediator类的代码比较复杂，这里不再贴代码，只列一下KeyguardViewMediator对这些交互的一些封装列表：

// used for handler messages
        private static final int SHOW = 1;
        private static final int HIDE = 2;
        private static final int RESET = 3;
        private static final int VERIFY_UNLOCK = 4;
        private static final int NOTIFY_FINISHED_GOING_TO_SLEEP = 5;
        private static final int NOTIFY_SCREEN_TURNING_ON = 6;
        private static final int KEYGUARD_DONE = 7;
        private static final int KEYGUARD_DONE_DRAWING = 8;
        private static final int SET_OCCLUDED = 9;
        private static final int KEYGUARD_TIMEOUT = 10;
        private static final int DISMISS = 11;
        private static final int START_KEYGUARD_EXIT_ANIM = 12;
        private static final int KEYGUARD_DONE_PENDING_TIMEOUT = 13;
        private static final int NOTIFY_STARTED_WAKING_UP = 14;
        private static final int NOTIFY_SCREEN_TURNED_ON = 15;
        private static final int NOTIFY_SCREEN_TURNED_OFF = 16;
        private static final int NOTIFY_STARTED_GOING_TO_SLEEP = 17;
        private static final int SYSTEM_READY = 18;
        private static final int REQUEST_UNLOCK = 100;

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三、总结

优点：集中控制交互，将一对多转化成一对一，降低了耦合度

缺点：中介类会庞大，变得复杂难以维护

总结：调停者模式是一种行为型设计模式，是对交互的封装。它的目的在于降低系统内部各组件之间的耦合，集中控制交互，所以它是内向闭合的，要与外观模式的外向公开特性区分。同时，中介类的内部设计要有条理，规范注释，避免随着系统需求不断升级而变得过于臃肿，难以维护。

 类有功能和定位双重属性，读代码能理解功能，熟悉业务；观整体才能理解定位，进而看到设计模式，知其所以然。