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ButterKnife中注解

Posted on Edited on In

ButterKnife 是一款知名老牌 Android 开发框架，通过注解绑定视图，避免了 findViewById() 的操作，广受好评！由于它是在编译时对注解进行解析完成相关代码的生成，所以在项目编译时会略耗时，但不会影响运行时的性能。

简单使用

在Activity中通过注解@BindView绑定视图，@onClick绑定事件。

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public class MainActivity extends AppCompatActivity {
    @BindView(R.id.tv_title)
    TextView title;

    @OnClick(R.id.bt)
    public void onClick() {
        title.setText("hello world");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ButterKnife.bind(this); // 绑定
    }
}

原理

ButterKnife和Activity是从ButterKnife.bind(this)建立绑定关系。

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@NonNull @UiThread
public static Unbinder bind(@NonNull Activity target) {
   View sourceView = target.getWindow().getDecorView();
   return bind(target, sourceView);
}

@NonNull @UiThread
  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);
    }
    // ...
  }

根据 Class 得到一个继承了Unbinder的Constructor，最后通过反射constructor.newInstance(target, source)得到Unbinder子类的一个实例。

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@Nullable @CheckResult @UiThread
  private static Constructor<? extends Unbinder> findBindingConstructorForClass(Class<?> cls) {
    Constructor<? extends Unbinder> bindingCtor = BINDINGS.get(cls);
    if (bindingCtor != null || BINDINGS.containsKey(cls)) {
      if (debug) Log.d(TAG, "HIT: Cached in binding map.");
      return bindingCtor;
    }
    String clsName = cls.getName();
    // ···
    try {
      Class<?> bindingClass = cls.getClassLoader().loadClass(clsName + "_ViewBinding");
      //noinspection unchecked
      bindingCtor = (Constructor<? extends Unbinder>) bindingClass.getConstructor(cls, View.class);
    } catch (ClassNotFoundException e) {
      bindingCtor = findBindingConstructorForClass(cls.getSuperclass());
    } catch (NoSuchMethodException e) {
      throw new RuntimeException("Unable to find binding constructor for " + clsName, e);
    }
    BINDINGS.put(cls, bindingCtor);
    return bindingCtor;
  }

先检查BINDINGS是否存在 Class 对应的 Constructor，如果存在则直接返回，否则去构造对应的 Constructor。其中BINDINGS是一个LinkedHashMap： Map<Class<?>, Constructor<? extends Unbinder>> BINDINGS = new LinkedHashMap<>()，缓存了对应的 Class 和 Constructor 以提高效率！

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Class<?> bindingClass = cls.getClassLoader().loadClass(clsName + "_ViewBinding");

最终bind()方法返回的是MainActivity_ViewBinding类的实例。

MainActivity_ViewBinding类是在编译期由annotationProcessor生成的。文件内容如下：

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public class MainActivity_ViewBinding implements Unbinder {
  private MainActivity target;

  private View view213213213;

  @UiThread
  public MainActivity_ViewBinding(MainActivity target) {
    this(target, target.getWindow().getDecorView());
  }

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

    View view;
    target.title = Utils.findRequiredViewAsType(source, R.id.tv_title, "field 'title'", TextView.class);
    view = Utils.findRequiredView(source, R.id.bt, "method 'onClick'");
    view213213213 = view;
    view.setOnClickListener(new DebouncingOnClickListener() {
      @Override
      public void doClick(View p0) {
        target.onClick();
      }
    });
  }

  @Override
  @CallSuper
  public void unbind() {
    MainActivity target = this.target;
    if (target == null) throw new IllegalStateException("Bindings already cleared.");
    this.target = null;

    target.title = null;

    view213213213.setOnClickListener(null);
    view213213213 = null;
  }
}

核心还是利用findViewById()转成指定类型的 View，并完成事件绑定。

注解处理器

注解处理器参考注解annotation 和 注解 in kotlin。在ButterKnife中使用如下：

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@Retention(RUNTIME) @Target(FIELD)
public @interface BindView {
  /** View ID to which the field will be bound. */
  @IdRes int value();
}

#ButterKnifeProcessor 注解器

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@AutoService(Processor.class)
@IncrementalAnnotationProcessor(IncrementalAnnotationProcessorType.DYNAMIC)
@SuppressWarnings("NullAway") // TODO fix all these...
public final class ButterKnifeProcessor extends AbstractProcessor {
  // TODO remove when http://b.android.com/187527 is released.
  private static final String OPTION_SDK_INT = "butterknife.minSdk";
  private static final String OPTION_DEBUGGABLE = "butterknife.debuggable";
  static final Id NO_ID = new Id(NO_RES_ID);
  static final String VIEW_TYPE = "android.view.View";
  static final String ACTIVITY_TYPE = "android.app.Activity";
  static final String DIALOG_TYPE = "android.app.Dialog";
  
  // ···
  
  @Override public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {
    Map<TypeElement, BindingSet> bindingMap = findAndParseTargets(env);

    for (Map.Entry<TypeElement, BindingSet> entry : bindingMap.entrySet()) {
      TypeElement typeElement = entry.getKey();
      BindingSet binding = entry.getValue();

      JavaFile javaFile = binding.brewJava(sdk, debuggable);
      try {
        javaFile.writeTo(filer);
      } catch (IOException e) {
        error(typeElement, "Unable to write binding for type %s: %s", typeElement, e.getMessage());
      }
    }

    return false;
  }
  
  // ···
  
}
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