ButterKnife原理分析

weixin_34402408發表於2018-01-18

ButterKnife原始碼地址

ButterKnife的分析文章很多了，這裡只是簡單分析原理，不想看程式碼，可以直接拉到後面總結。
ButterKnife最簡單的使用方法如下：

public class SimpleActivity extends Activity {
    @BindView(R.id.title) 
    TextView title;

    @Override 
     protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.simple_activity);
           ButterKnife.bind(this);
           title.setText("Butter Knife");
     }
}

從入口ButterKnife.bind(this)開始分析:

public static Unbinder bind(@NonNull Activity target) {
    View sourceView = target.getWindow().getDecorView();//獲得Activity的DecorView
    return createBinding(target, sourceView);
}

首先獲得Activity最頂層的DecorView，有了DecorView，後面就可以通過獲得佈局的所有View。往下走到了createBinding，引數是Activity和DecorView:

private static Unbinder createBinding(@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);
    }
}

createBinding首先要通過findBindingConstructorForClass去獲得該Activity對應的binding類，binding類在build/generated/source/apt/debug或release的包下面，比如，上面的SimpleActivity，通過findBindingConstructorForClass方法找到的類是以上目錄下的SimpleActivity_ViewBinding。這裡先不用管findBindingConstructorForClass是怎麼找到SimpleActivity_ViewBinding和SimpleActivity_ViewBinding是怎麼生成的，看看找到後，做了什麼事:return constructor.newInstance(target, source);找到後呼叫了構造方法。上面一大段無非是呼叫了SimpleActivity_ViewBinding的構造方法，來看看SimpleActivity_ViewBinding的程式碼:

public class SimpleActivity_ViewBinding implements Unbinder {
     private SimpleActivity target;
     private View view2130968578;

     @UiThread
     public SimpleActivity_ViewBinding(final SimpleActivity target, View source) 
     {
          this.target = target;
          target.title = Utils.findRequiredViewAsType(source, R.id.title, "field 'title'", TextView.class); 
     }
     public void unbind() {
         SimpleActivity target = this.target;
         this.target = null;
         target.title = null;
      }
}

上面的target就是最初的SimpleActivity，看到target.title就知道為什麼在SimpleActivity裡TextView title為什麼不能宣告為private變數。target.title是通過
Utils.findRequiredViewAsType獲得的，看引數，findViewById需要的東西都有了，可見findRequiredViewAsType肯定是findViewById的封裝方法，進去看看:

public static <T> T findRequiredViewAsType(View source, @IdRes int id, String who,
      Class<T> cls) {
    View view = findRequiredView(source, id, who);//獲得R.id.title的View
    return castView(view, id, who, cls);//將上面的View轉化成TextView型別
  }
public static View findRequiredView(View source, @IdRes int id, String who) {
    View view = source.findViewById(id);//這麼多行，就這句最重要
    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.");
  }
public static <T> T castView(View view, @IdRes int id, String who, Class<T> cls) {
    try {
      return cls.cast(view);
    } catch (ClassCastException e) {
      String name = getResourceEntryName(view, id);
      throw new IllegalStateException("View '"
          + name
          + "' with ID "
          + id
          + " for "
          + who
          + " was of the wrong type. See cause for more info.", e);
    }
  }

上面程式碼一大把，其實就做了兩件事，獲得R.id.title的View，將View轉化成真正的型別TextView，完成了id到View的繫結。

以上就是ButterKnife的原理,再總結下工作流程:
1、用@BindView註解View的ID和View變數。
2、apt會根據註解生成該Activity對應的Activity_ViewBinding類。
3、在Activity中呼叫 ButterKnife.bind(this)。
4、ButterKnife通過bind引數可以獲得當前的Activity類名和最頂層的DecorView。
5、通過類名獲得該類對應的Activity_ViewBinding類，並呼叫Activity_ViewBinding的構造方法初始化。
6、在Activity_ViewBinding的構造方法中通過封裝的findViewById獲得該id對應的View，並通過cast方法轉化成真正的View型別，比如TextView。

額外的話題
上面有兩個問題我們先沒有管：
1、SimpleActivity_ViewBinding是怎麼生成的？
2、findBindingConstructorForClass是怎麼找到SimpleActivity_ViewBinding？
第一個問題其實是java註解的應用，比如@BindView，apt通過這些註解生成一個新的java檔案，當然生成邏輯得自己寫，這裡比較複雜，可以參考原始碼。
第二個問題看原始碼:

static final Map<Class<?>, Constructor<? extends Unbinder>> BINDINGS = new LinkedHashMap();

private static Constructor<? extends Unbinder> findBindingConstructorForClass(Class<?> cls) {
        Constructor<? extends Unbinder> bindingCtor = (Constructor)BINDINGS.get(cls);
        if(bindingCtor != null) {
            if(debug) {
                Log.d("ButterKnife", "HIT: Cached in binding map.");
            }

            return bindingCtor;
        } else {
            String clsName = cls.getName();
            if(!clsName.startsWith("android.") && !clsName.startsWith("java.")) {
                try {
                    Class<?> bindingClass = cls.getClassLoader().loadClass(clsName + "_ViewBinding");
                    bindingCtor = bindingClass.getConstructor(new Class[]{cls, View.class});
                    if(debug) {
                        Log.d("ButterKnife", "HIT: Loaded binding class and constructor.");
                    }
                } catch (ClassNotFoundException var4) {
                    if(debug) {
                        Log.d("ButterKnife", "Not found. Trying superclass " + cls.getSuperclass().getName());
                    }

                    bindingCtor = findBindingConstructorForClass(cls.getSuperclass());
                } catch (NoSuchMethodException var5) {
                    throw new RuntimeException("Unable to find binding constructor for " + clsName, var5);
                }

                BINDINGS.put(cls, bindingCtor);
                return bindingCtor;
            } else {
                if(debug) {
                    Log.d("ButterKnife", "MISS: Reached framework class. Abandoning search.");
                }

                return null;
            }
        }
    }

首先在BINDINGS這個快取map裡去找對應的Activity_ViewBinding，找到了直接返回。沒找到話往下走。這時候因為有了Activity類名，要找到Activity_ViewBinding只需要找Activity類名+"_ViewBinding"字尾的類就行了，當然系統類不用找了，所以有個"android."和"java."字首的判斷，看下怎麼找的:

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

找到了Activity_ViewBinding類bindingClass，再獲得構造方法:

bindingCtor = bindingClass.getConstructor(new Class[]{cls, View.class});

最後把找到的Activity_ViewBinding快取起來:

BINDINGS.put(cls, bindingCtor);

返回Activity_ViewBinding的構造方法結束。

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