ButterKnife的分析文章很多了，这里只是简单分析原理，不想看代码，可以直接拉到后面总结。
ButterKnife最简单的使用方法如下：
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");
}
}
从入口ButterKnife.bind(this)开始分析:
public static Unbinder bind(@NonNull Activity target) {
View sourceView = target.getWindow().getDecorView();//获得Activity的DecorView
return createBinding(target, sourceView);
}
首先获得Activity最顶层的DecorView，有了DecorView，后面就可以通过获得布局的所有View。往下走到了createBinding，参数是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首先要通过findBindingConstructorForClass去获得该Activity对应的binding类，binding类在build/generated/source/apt/debug或release的包下面，比如，上面的SimpleActivity，通过findBindingConstructorForClass方法找到的类是以上目录下的SimpleActivity_ViewBinding。这里先不用管findBindingConstructorForClass是怎么找到SimpleActivity_ViewBinding和SimpleActivity_ViewBinding是怎么生成的，看看找到后，做了什么事:return constructor.newInstance(target, source);找到后调用了构造方法。上面一大段无非是调用了SimpleActivity_ViewBinding的构造方法，来看看SimpleActivity_ViewBinding的代码:
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就知道为什么在SimpleActivity里TextView title为什么不能声明为private变量。target.title是通过
Utils.findRequiredViewAsType获得的，看参数，findViewById需要的东西都有了，可见findRequiredViewAsType肯定是findViewById的封装方法，进去看看:
public static <T> T findRequiredViewAsType(View source, @IdRes int id, String who,
Class<T> cls) {
View view = findRequiredView(source, id, who);//获得R.id.title的View
return castView(view, id, who, cls);//将上面的View转化成TextView类型
}
public static View findRequiredView(View source, @IdRes int id, String who) {
View view = source.findViewById(id);//这么多行，就这句最重要
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);
}
}
上面代码一大把，其实就做了两件事，获得R.id.title的View，将View转化成真正的类型TextView，完成了id到View的绑定。
以上就是ButterKnife的原理,再总结下工作流程:
1、用@BindView注解View的ID和View变量。
2、apt会根据注解生成该Activity对应的Activity_ViewBinding类。
3、在Activity中调用 ButterKnife.bind(this)。
4、ButterKnife通过bind参数可以获得当前的Activity类名和最顶层的DecorView。
5、通过类名获得该类对应的Activity_ViewBinding类，并调用Activity_ViewBinding的构造方法初始化。
6、在Activity_ViewBinding的构造方法中通过封装的findViewById获得该id对应的View，并通过cast方法转化成真正的View类型，比如TextView。
额外的话题
上面有两个问题我们先没有管：
1、SimpleActivity_ViewBinding是怎么生成的？
2、findBindingConstructorForClass是怎么找到SimpleActivity_ViewBinding？
第一个问题其实是java注解的应用，比如@BindView，apt通过这些注解生成一个新的java文件，当然生成逻辑得自己写，这里比较复杂，可以参考源码。
第二个问题看源码:
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这个缓存map里去找对应的Activity_ViewBinding，找到了直接返回。没找到话往下走。这时候因为有了Activity类名，要找到Activity_ViewBinding只需要找Activity类名+"_ViewBinding"后缀的类就行了，当然系统类不用找了，所以有个"android."和"java."前缀的判断，看下怎么找的:
Class<?> bindingClass = cls.getClassLoader().loadClass(clsName + "_ViewBinding");
找到了Activity_ViewBinding类bindingClass，再获得构造方法:
bindingCtor = bindingClass.getConstructor(new Class[]{cls, View.class});
最后把找到的Activity_ViewBinding缓存起来:
BINDINGS.put(cls, bindingCtor);
返回Activity_ViewBinding的构造方法结束。
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