Retrofit中的Builder模式

1、Retrofit的構(gòu)造

public final class Retrofit {
  private final Map<Method, ServiceMethod> serviceMethodCache = new LinkedHashMap<>();

  private final okhttp3.Call.Factory callFactory;
  private final HttpUrl baseUrl;
  private final List<Converter.Factory> converterFactories;
  private final List<CallAdapter.Factory> adapterFactories;
  private final Executor callbackExecutor;
  private final boolean validateEagerly;

  Retrofit(okhttp3.Call.Factory callFactory, HttpUrl baseUrl,
      List<Converter.Factory> converterFactories, List<CallAdapter.Factory> adapterFactories,
      Executor callbackExecutor, boolean validateEagerly) {
    this.callFactory = callFactory;
    this.baseUrl = baseUrl;
    this.converterFactories = unmodifiableList(converterFactories); // Defensive copy at call site.
    this.adapterFactories = unmodifiableList(adapterFactories); // Defensive copy at call site.
    this.callbackExecutor = callbackExecutor;
    this.validateEagerly = validateEagerly;
  }
.......
public static final class Builder {
    private Platform platform;
    private okhttp3.Call.Factory callFactory;
    private HttpUrl baseUrl;
    private List<Converter.Factory> converterFactories = new ArrayList<>();
    private List<CallAdapter.Factory> adapterFactories = new ArrayList<>();
    private Executor callbackExecutor;
    private boolean validateEagerly;

    Builder(Platform platform) {
      this.platform = platform;
      // Add the built-in converter factory first. This prevents overriding its behavior but also
      // ensures correct behavior when using converters that consume all types.
      converterFactories.add(new BuiltInConverters());
    }

    public Builder() {
      this(Platform.get());
    }
    ......
public Retrofit build() {
      ......
      return new Retrofit(callFactory, baseUrl, converterFactories, adapterFactories,
          callbackExecutor, validateEagerly);
    }

以上是Retrofit的構(gòu)造過程(其實(shí)在Builder構(gòu)造中也用到了簡(jiǎn)單工廠模式)。
2、Builder模式的特點(diǎn)
當(dāng)一個(gè)類有大量屬性需要初始化的時(shí)候，為了避免對(duì)外的API接口繁多和屬性建立的先后順序混亂，這個(gè)時(shí)候可以使用Builder模式來設(shè)置類的屬性，這樣可以把屬性和屬性的構(gòu)造過程完全分離，耦合性降低并且可擴(kuò)展性增加。
其實(shí)Retrofit中用了大量的Builder模式，再比如
ServiceMethod的構(gòu)造。
其實(shí)特點(diǎn)很好總結(jié)：

1. 外部類有個(gè)包含Builder參數(shù)的構(gòu)造函數(shù)
2. 在構(gòu)造函數(shù)中把Builder中各個(gè)屬性的值賦給外部類中的對(duì)應(yīng)的屬性值
3. 在Builder類中有一個(gè)builder方法，此方法返回值為外部類
4. 為了客戶端鏈?zhǔn)秸{(diào)用設(shè)置Builder屬性值，把Builder中的方法返回值都設(shè)置為Builder(builder方法除外)

3、Builder模式的使用
在項(xiàng)目中用Builder模式封裝了WebView，因?yàn)閃ebView有大量的屬性需要構(gòu)造，把Builder模式用在這里是在合適不過了，Builder模式的運(yùn)用-WebView的構(gòu)造。