目的：分析ButterKnife如何进行view与onClick事件的绑定
原理分析
通过观察BindView注解发现，该注解是存在于编译器的：
@Retention(CLASS) @Target(FIELD)
public @interface BindView {
/** View ID to which the field will be bound. */
@IdRes int value();
}
那么猜想肯定需要通过注解处理器来处理该注解注释的字段或方法。
找到引用：
kapt "com.jakewharton:butterknife-compiler:8.8.1"
找到注解编译器源码：ButterKnifeProcessor extends AbstractProcessor{}
通过重写getSupportedAnnotationTypes方法来获取支持的注解类型：
@Override public Set getSupportedAnnotationTypes() {
Set types = new LinkedHashSet<>();
for (Class annotation : getSupportedAnnotations()) {
types.add(annotation.getCanonicalName());
}
return types;
}
private Set> getSupportedAnnotations() {
Set> annotations = new LinkedHashSet<>();
annotations.add(BindAnim.class);
annotations.add(BindArray.class);
annotations.add(BindBitmap.class);
annotations.add(BindBool.class);
annotations.add(BindColor.class);
annotations.add(BindDimen.class);
annotations.add(BindDrawable.class);
annotations.add(BindFloat.class);
annotations.add(BindFont.class);
annotations.add(BindInt.class);
annotations.add(BindString.class);
annotations.add(BindView.class);
annotations.add(BindViews.class);
annotations.addAll(LISTENERS);
return annotations;
}
在获取到所有的上述类型后，会进入process方法进行处理：
@Override public boolean process(Set elements, RoundEnvironment env) {
Map bindingMap = findAndParseTargets(env);
for (Map.Entry 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;
}
该方法异常简短，主要是做了很多的封装，其中BindingSet类就是一个封装了将要生成的中间类的代码的类。在该类中，会通过解析的注解的属性来将对应的语句添加到BindingSet.Builder中，也可通过其中的addMethod向中间类中添加方法。
因此findAndParseTargets方法的主要工作就是解析各个注解信息并生成对应的语句放入BindingSet中。在所有的解析完毕后，会轮询该Map集合，然后通过JavaFile来生成中间类。
下边详细看一下findAndParseTargets方法：
private Map findAndParseTargets(RoundEnvironment env) {
Map builderMap = new LinkedHashMap<>();
Set erasedTargetNames = new LinkedHashSet<>();
// Process each @BindView element.
for (Element element : env.getElementsAnnotatedWith(BindView.class)) {
// we don't SuperficialValidation.validateElement(element)
// so that an unresolved View type can be generated by later processing rounds
try {
parseBindView(element, builderMap, erasedTargetNames);
} catch (Exception e) {
logParsingError(element, BindView.class, e);
}
}
// Process each annotation that corresponds to a listener.
for (Class listener : LISTENERS) {
findAndParseListener(env, listener, builderMap, erasedTargetNames);
}
// Associate superclass binders with their subclass binders. This is a queue-based tree walk
// which starts at the roots (superclasses) and walks to the leafs (subclasses).
Deque> entries =
new ArrayDeque<>(builderMap.entrySet());
Map bindingMap = new LinkedHashMap<>();
while (!entries.isEmpty()) {
Map.Entry entry = entries.removeFirst();
TypeElement type = entry.getKey();
BindingSet.Builder builder = entry.getValue();
TypeElement parentType = findParentType(type, erasedTargetNames);
if (parentType == null) {
bindingMap.put(type, builder.build());
} else {
BindingSet parentBinding = bindingMap.get(parentType);
if (parentBinding != null) {
builder.setParent(parentBinding);
bindingMap.put(type, builder.build());
} else {
// Has a superclass binding but we haven't built it yet. Re-enqueue for later.
entries.addLast(entry);
}
}
}
return bindingMap;
}
在该方法中，会处理收集每种注解类型的信息，上边只粘出了BindView和OnClick的片段。以BindView为例：
private void parseBindView(Element element, Map builderMap,
Set erasedTargetNames) {
TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();
// Start by verifying common generated code restrictions.
boolean hasError = isInaccessibleViaGeneratedCode(BindView.class, "fields", element)
|| isBindingInWrongPackage(BindView.class, element);
// Verify that the target type extends from View.
TypeMirror elementType = element.asType();
if (elementType.getKind() == TypeKind.TYPEVAR) {
TypeVariable typeVariable = (TypeVariable) elementType;
elementType = typeVariable.getUpperBound();
}
Name qualifiedName = enclosingElement.getQualifiedName();
Name simpleName = element.getSimpleName();
// Assemble information on the field.
int id = element.getAnnotation(BindView.class).value();
BindingSet.Builder builder = builderMap.get(enclosingElement);
Id resourceId = elementToId(element, BindView.class, id);
if (builder != null) {
String existingBindingName = builder.findExistingBindingName(resourceId);
} else {
builder = getOrCreateBindingBuilder(builderMap, enclosingElement);
}
String name = simpleName.toString();
TypeName type = TypeName.get(elementType);
boolean required = isFieldRequired(element);
builder.addField(resourceId, new FieldViewBinding(name, type, required));
// Add the type-erased version to the valid binding targets set.
erasedTargetNames.add(enclosingElement);
}
会收集修饰字段的修饰符，字段名称，字段类型等信息，然后通过builder.addField将该字段添加到BindingSet.Builder中。
在收集到所有的类型后，最终会通过：
JavaFile javaFile = binding.brewJava(sdk, debuggable);
javaFile.writeTo(filer);
生成中间类。binding.brewJava方法：
JavaFile brewJava(int sdk, boolean debuggable) {
TypeSpec bindingConfiguration = createType(sdk, debuggable);
return JavaFile.builder(bindingClassName.packageName(), bindingConfiguration)
.addFileComment("Generated code from Butter Knife. Do not modify!")
.build();
}
就是直接调用JavaFile来创建类。而createType就是通过在解析时添加到Binding.Builder中的语句字段来生成类的代码，比如包名类名等。
最终生成的中间类代码如下：
// Generated code from Butter Knife. Do not modify!
package com.jf.jlfund.view.activity;
import butterknife.internal.DebouncingOnClickListener;
import butterknife.internal.Utils;
import com.jf.jlfund.R;
public class FundSaleOutActivity_ViewBinding implements Unbinder {
private FundSaleOutActivity target;
private View view2131231970;
@UiThread
public FundSaleOutActivity_ViewBinding(FundSaleOutActivity target) {
this(target, target.getWindow().getDecorView());
}
@UiThread
public FundSaleOutActivity_ViewBinding(final FundSaleOutActivity target, View source) {
this.target = target;
View view;
target.rootView = Utils.findRequiredViewAsType(source, R.id.rl_fundSaleOut_rootView, "field 'rootView'", RelativeLayout.class);
target.commonTitleBar = Utils.findRequiredViewAsType(source, R.id.commonTitleBar_fundSaleOut, "field 'commonTitleBar'", CommonTitleBar.class);
target.etAmount = Utils.findRequiredViewAsType(source, R.id.et_fundSaleOut, "field 'etAmount'", EditText.class);
view = Utils.findRequiredView(source, R.id.tv_fundSaleOut_saleAll, "field 'tvSaleAll' and method 'onClick'");
target.tvSaleAll = Utils.castView(view, R.id.tv_fundSaleOut_saleAll, "field 'tvSaleAll'", TextView.class);
view2131231970 = view;
view.setOnClickListener(new DebouncingOnClickListener() {
@Override
public void doClick(View p0) {
target.onClick(p0);
}
});
}
@Override
@CallSuper
public void unbind() {
FundSaleOutActivity target = this.target;
if (target == null) throw new IllegalStateException("Bindings already cleared.");
this.target = null;
target.rootView = null;
target.commonTitleBar = null;
target.etAmount = null;
target.tvSaleAll = null;
view2131231970.setOnClickListener(null);
view2131231970 = null;
}
}
其中，Utils.findRequiredViewAsType的作用就是source.findViewById并将查找的view转换成具体的类型。
以上的工作都发生在编译器，那么如何在运行期来进行view的绑定呢？
回想每次在Activity中使用都要事先进行：
Unbinder unbinder = ButterKnife.bind(this);
bind方法：
@NonNull @UiThread
public static Unbinder bind(@NonNull Activity target) {
View sourceView = target.getWindow().getDecorView();
return createBinding(target, sourceView);
}
看语句是先获取该Activity的DecorView，然后作为rootView进行控件的绑定：
private static Unbinder createBinding(@NonNull Object target, @NonNull View source) {
Class targetClass = target.getClass();
Constructor constructor = findBindingConstructorForClass(targetClass);
if (constructor == null) {
return Unbinder.EMPTY;
}
//noinspection TryWithIdenticalCatches Resolves to API 19+ only type.
try {
return constructor.newInstance(target, source);
} catch (Exception e) {
//...
}
}
@Nullable @CheckResult @UiThread
private static Constructor findBindingConstructorForClass(Class cls) {
Constructor bindingCtor = BINDINGS.get(cls);
if (bindingCtor != null) {
return bindingCtor;
}
String clsName = cls.getName();
if (clsName.startsWith("android.") || clsName.startsWith("java.")) {
return null;
}
try {
Class bindingClass = cls.getClassLoader().loadClass(clsName + "_ViewBinding");
//noinspection unchecked
bindingCtor = (Constructor) bindingClass.getConstructor(cls, View.class);
} catch (ClassNotFoundException e) {}
BINDINGS.put(cls, bindingCtor);
return bindingCtor;
}
方法说明：
- 根据传入的target来获取对应的在编译器生成的target_ViewBinding类。
- 获取该类的构造函数
- 通过传入的参数执行该类的构造函数
上边方法执行完毕后，在编译期生成的类就会被执行在Activity的onCreate方法中。在回顾一下生成的类的代码片段：
target.rootView = Utils.findRequiredViewAsType(source, R.id.rl_fundSaleOut_rootView, "field 'rootView'", RelativeLayout.class);
target.commonTitleBar = Utils.findRequiredViewAsType(source, R.id.commonTitleBar_fundSaleOut, "field 'commonTitleBar'", CommonTitleBar.class);
其中，findRequiredViewAsType就是调用source.findViewById然后转化成指定的类型。而这个source就是我们传入的DecorView。
至此，view的绑定也就完成了。事件的绑定也类似。还有要记得，在Activity的onDestory中要调用：
unbinder.unbind();
该操作会把注解生成的view都给置空。
总结
- 通过注解处理器生成对应的中间类**_ViewBinding，并将findViewById与事件点击等操作写入该类的构造函数中。
- 在运行期，通过在Activity的onCreate方法中bind(this)来获取该中间构造函数，并通过反射构造函数来实现view的绑定。