前言

ButterKnife又名黃油刀，是一款知名的Andorid框架，通過注解綁定，省去初始化控件等重復工作，簡化代碼，極大提高工作效率。

dependencies {
    ......
    implementation 'com.jakewharton:butterknife:8.8.1'
    annotationProcessor 'com.jakewharton:butterknife-compiler:8.8.1'
}

 @BindView(R.id.tv)
    TextView tv;

 @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ButterKnife.bind(this);
    }

@OnClick(R.id.tv)
    public void click(View view) {
        Toast.makeText(this, tv.getText().toString(), Toast.LENGTH_SHORT).show();
    }

使用非常簡單，但這里需要注意，使用的時候不能使用加private和static，并且生成的文件必須同包。

public static Unbinder bind(@NonNull Activity target) {
    View sourceView = target.getWindow().getDecorView();
    return bind(target, sourceView);
  }
 public static Unbinder bind(@NonNull Object target, @NonNull View source) {
    Class<?> targetClass = target.getClass();
    if (debug) Log.d(TAG, "Looking up binding for " + targetClass.getName());
    Constructor<? extends Unbinder> constructor = findBindingConstructorForClass(targetClass);

    if (constructor == null) {
      return Unbinder.EMPTY;
    }

    //noinspection TryWithIdenticalCatches Resolves to API 19+ only type.
    try {
      return constructor.newInstance(target, source);
    } catch (IllegalAccessException e) {
      throw new RuntimeException("Unable to invoke " + constructor, e);
    } catch (InstantiationException e) {
      throw new RuntimeException("Unable to invoke " + constructor, e);
    } catch (InvocationTargetException e) {
      Throwable cause = e.getCause();
      if (cause instanceof RuntimeException) {
        throw (RuntimeException) cause;
      }
      if (cause instanceof Error) {
        throw (Error) cause;
      }
      throw new RuntimeException("Unable to create binding instance.", cause);
    }
  }

我們從 ButterKnife.bind(this)入手，點進去調(diào)用了findBindingConstructorForClass()發(fā)現(xiàn)一個構(gòu)造方法，構(gòu)造方法不為空，就把這個方法初始化。

private static Constructor<? extends Unbinder> findBindingConstructorForClass(Class<?> cls) {
    Constructor<? extends Unbinder> bindingCtor = BINDINGS.get(cls);
    if (bindingCtor != null) {
      if (debug) Log.d(TAG, "HIT: Cached in binding map.");
      return bindingCtor;
    }
    String clsName = cls.getName();
    if (clsName.startsWith("android.") || clsName.startsWith("java.")
        || clsName.startsWith("androidx.")) {
      if (debug) Log.d(TAG, "MISS: Reached framework class. Abandoning search.");
      return null;
    }
    try {
      Class<?> bindingClass = cls.getClassLoader().loadClass(clsName + "_ViewBinding");
      //noinspection unchecked
      bindingCtor = (Constructor<? extends Unbinder>) bindingClass.getConstructor(cls, View.class);
      if (debug) Log.d(TAG, "HIT: Loaded binding class and constructor.");
    } catch (ClassNotFoundException e) {
      if (debug) Log.d(TAG, "Not found. Trying superclass " + cls.getSuperclass().getName());
      bindingCtor = findBindingConstructorForClass(cls.getSuperclass());
    } catch (NoSuchMethodException e) {
      throw new RuntimeException("Unable to find binding constructor for " + clsName, e);
    }
    BINDINGS.put(cls, bindingCtor);
    return bindingCtor;
  }

在findBindingConstructorForClass()方法中，我們可以看到，如果這個類是以android、java、androidx開始，那么都會直接返回null,否則就會調(diào)用cls.getClassLoader().loadClass(clsName + "_ViewBinding")生成這個類的對象。我們上述代碼對應的應該是MainActivity_ViewBinding.class。

 public MainActivity_ViewBinding(final MainActivity target, View source) {
    this.target = target;

    View view;
    view = Utils.findRequiredView(source, R.id.tv, "field 'tv' and method 'click'");
    target.tv = Utils.castView(view, R.id.tv, "field 'tv'", TextView.class);
    view7f07008d = view;
    view.setOnClickListener(new DebouncingOnClickListener() {
      @Override
      public void doClick(View p0) {
        target.click(p0);
      }
    });
  }

 public static View findRequiredView(View source, @IdRes int id, String who) {
    View view = source.findViewById(id);
    if (view != null) {
      return view;
    }
    String name = getResourceEntryName(source, id);
    throw new IllegalStateException("Required view '"
        + name
        + "' with ID "
        + id
        + " for "
        + who
        + " was not found. If this view is optional add '@Nullable' (fields) or '@Optional'"
        + " (methods) annotation.");
  }

然后調(diào)用里面的構(gòu)造方法，在findRequiredView中調(diào)用了findViewById()方法。

public abstract class DebouncingOnClickListener implements View.OnClickListener {
  static boolean enabled = true;

  private static final Runnable ENABLE_AGAIN = new Runnable() {
    @Override public void run() {
      enabled = true;
    }
  };

  @Override public final void onClick(View v) {
    if (enabled) {
      enabled = false;
      v.post(ENABLE_AGAIN);
      doClick(v);
    }
  }

  public abstract void doClick(View v);
}

下面同步調(diào)用了setOnClickListener也初始化了點擊事件，不同的是DebouncingOnClickListener是抽象類，實現(xiàn)了View.OnClickListener接口，在onClick事件中調(diào)用了抽象方法doClick()，然后doClick()在外面重寫了，這里實際上是模板方法模式，感興趣的同學可以去這篇文章看看。

至此，ButterKnife原理解析我們已經(jīng)分析完畢，至于MainActivity_ViewBinding.class代碼的生成則是使用的APT技術(shù)。