【移动应用开发技术】分分钟带你读懂 ButterKnife 的源码

【移动应用开发技术】分分钟带你读懂ButterKnife的源码

为什么要写这一系列的博客呢？

为什么要写这一系列的博客呢？因为在Android开发的过程中，泛型，反射，注解这些知识进场会用到，几乎所有的框架至少都会用到上面的一两种知识，如Gson就用到泛型，反射，注解，Retrofit也用到泛型，反射，注解。学好这些知识对我们进阶非常重要，尤其是阅读开源框架源码或者自己开发开源框架。前言ButterKnife这个开源库火了有一段时间了，刚开始它的实现原理是使用反射实现的，性能较差。再后面的版本中逐渐使用注解+放射实现，性能提高了不少。ButterKnife是基于编译时的框架，它能够帮助我们减去每次写FindViewById的麻烦，截止到2017.5.1,在github上面的start已经超过15000.本篇博客要分析的ButterKnife的源码主要包括以下三个部分，版本号是8.5.1其中butterknife-annotations库主要用来存放自定义注解；butterknife-compiler主要是用来扫描哪些地方使用到我们的自定义注解，并进行相应的处理，生成模板代码等；butterknife主要是用来注入我们的代码的。我们先来先一下要怎样使用butterknife：ButterKnife的基本使用在moudle的build.gradle增加依赖dependencies{

compile'com.jakewharton:butterknife:8.5.1'

annotationProcessor'com.jakewharton:butterknife-compiler:8.5.1'

}publicclassSimpleActivityextendsActivity{

privatestaticfinalButterKnife.Action<View>ALPHA_FADE=newButterKnife.Action<View>(){

@Overridepublicvoidapply(@NonNullViewview,intindex){

AlphaAnimationalphaAnimation=newAlphaAnimation(0,1);

alphaAnimation.setFillBefore(true);

alphaAnimation.setDuration(500);

alphaAnimation.setStartOffset(index*100);

view.startAnimation(alphaAnimation);

}

};

@BindView(R2.id.title)TextViewtitle;

@BindView(R2.id.subtitle)TextViewsubtitle;

@BindView(R2.id.hello)Buttonhello;

@BindView(R2.id.list_of_things)ListViewlistOfThings;

@BindView(R2.id.footer)TextViewfooter;

@BindViews({R2.id.title,R2.id.subtitle,R2.id.hello})List<View>headerViews;

privateSimpleAdapteradapter;

@OnClick(R2.id.hello)voidsayHello(){

Toast.makeText(this,"Hello,views!",LENGTH_SHORT).show();

ButterKnife.apply(headerViews,ALPHA_FADE);

}

@OnLongClick(R2.id.hello)booleansayGetOffMe(){

Toast.makeText(this,"Letgoofme!",LENGTH_SHORT).show();

returntrue;

}

@OnItemClick(R2.id.list_of_things)voidonItemClick(intposition){

Toast.makeText(this,"Youclicked:"+adapter.getItem(position),LENGTH_SHORT).show();

}

@OverrideprotectedvoidonCreate(BundlesavedInstanceState){

super.onCreate(savedInstanceState);

setContentView(R.layout.simple_activity);

ButterKnife.bind(this);

//Contrivedcodetousetheboundfields.

title.setText("ButterKnife");

subtitle.setText("FieldandmethodbindingforAndroidviews.");

footer.setText("byJakeWharton");

hello.setText("SayHello");

adapter=newSimpleAdapter(this);

listOfThings.setAdapter(adapter);

}

}调用gradlebuild命令，我们在相应的目录下将可以看到生成类似这样的代码。publicclassSimpleActivity_ViewBinding<TextendsSimpleActivity>implementsUnbinder{

protectedTtarget;

privateViewview2130968578;

privateViewview2130968579;

@UiThread

publicSimpleActivity_ViewBinding(finalTtarget,Viewsource){

this.target=target;

Viewview;

target.title=Utils.findRequiredViewAsType(source,R.id.title,"field'title'",TextView.class);

target.subtitle=Utils.findRequiredViewAsType(source,R.id.subtitle,"field'subtitle'",TextView.class);

view=Utils.findRequiredView(source,R.id.hello,"field'hello',method'sayHello',andmethod'sayGetOffMe'");

target.hello=Utils.castView(view,R.id.hello,"field'hello'",Button.class);

view2130968578=view;

view.setOnClickListener(newDebouncingOnClickListener(){

@Override

publicvoiddoClick(Viewp0){

target.sayHello();

}

});

view.setOnLongClickListener(newView.OnLongClickListener(){

@Override

publicbooleanonLongClick(Viewp0){

returntarget.sayGetOffMe();

}

});

view=Utils.findRequiredView(source,R.id.list_of_things,"field'listOfThings'andmethod'onItemClick'");

target.listOfThings=Utils.castView(view,R.id.list_of_things,"field'listOfThings'",ListView.class);

view2130968579=view;

((AdapterView<?>)view).setOnItemClickListener(newAdapterView.OnItemClickListener(){

@Override

publicvoidonItemClick(AdapterView<?>p0,Viewp1,intp2,longp3){

target.onItemClick(p2);

}

});

target.footer=Utils.findRequiredViewAsType(source,R.id.footer,"field'footer'",TextView.class);

target.headerViews=Utils.listOf(

Utils.findRequiredView(source,R.id.title,"field'headerViews'"),

Utils.findRequiredView(source,R.id.subtitle,"field'headerViews'"),

Utils.findRequiredView(source,R.id.hello,"field'headerViews'"));

}

@Override

@CallSuper

publicvoidunbind(){

Ttarget=this.target;

if(target==null)thrownewIllegalStateException("Bindingsalreadycleared.");

target.title=null;

target.subtitle=null;

target.hello=null;

target.listOfThings=null;

target.footer=null;

target.headerViews=null;

view2130968578.setOnClickListener(null);

view2130968578.setOnLongClickListener(null);

view2130968578=null;

((AdapterView<?>)view2130968579).setOnItemClickListener(null);

view2130968579=null;

this.target=null;

}

}ButterKnife的执行流程总的来说，大概可以分为以下几步：第一步：在编译的时候扫描注解，并做相应的处理，生成java代码。这一步，可以拆分为几个小步骤：butterknife-annotations讲解我们知道ButterKnife自定义很多的注解，有BindArray，BindBitmap，BindColor，BindView等，这里我们以BindView为例子讲解就OK了，其他的也是基本类似的，这里就不再讲解了。//编译时注解

@Retention(CLASS)

//成员变量，(includesenumconstants)

@Target(FIELD)

public@interfaceBindView{

/**ViewIDtowhichthefieldwillbebound.*/

@IdResintvalue();

}Processor解析器说明我们先来看一些基本方法：在init方法里面得到一些辅助工具类，这样有一个好处，确保工具类是单例的，因为init方法只会在初始化的时候调用。publicsynchronizedvoidinit(ProcessingEnvironmentenv){

super.init(env);

//辅助工具类

elementUtils=env.getElementUtils();

typeUtils=env.getTypeUtils();

filer=env.getFiler();

}接着重写getSupportedAnnotationTypes方法，返回我们支持的注解类型。@Override

publicSet<String>getSupportedAnnotationTypes(){

Set<String>types=newLinkedHashSet<>();

for(Class<?extendsAnnotation>annotation:getSupportedAnnotations()){

types.add(annotation.getCanonicalName());

}

//返回支持注解的类型

returntypes;

}

privateSet<Class<?extendsAnnotation>>getSupportedAnnotations(){

Set<Class<?extendsAnnotation>>annotations=newLinkedHashSet<>();

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(BindInt.class);

annotations.add(BindString.class);

annotations.add(BindView.class);

annotations.add(BindViews.class);

annotations.addAll(LISTENERS);

returnannotations;

}接下来来看我们的重点，process方法。所做的工作大概就是拿到我们所有的注解信息，存进map集合，遍历map集合，做相应的处理，生成java代码。@Override

publicbooleanprocess(Set<?extendsTypeElement>elements,RoundEnvironmentenv){

//拿到所有的注解信息，TypeElement作为key，BindingSet作为value

Map<TypeElement,BindingSet>bindingMap=findAndParseTargets(env);

//遍历map里面的所有信息，并生成java代码

for(Map.Entry<TypeElement,BindingSet>entry:bindingMap.entrySet()){

TypeElementtypeElement=entry.getKey();

BindingSetbinding=entry.getValue();

JavaFilejavaFile=binding.brewJava(sdk);

try{

javaFile.writeTo(filer);

}catch(IOExceptione){

error(typeElement,"Unabletowritebindingfortype%s:%s",typeElement,e

.getMessage());

}

}

returnfalse;

}这里我们进入findAndParseTargets方法，看里面到底是怎样将注解信息存进map集合的？findAndParseTargets方法里面针对每一个自定义注解（BindArray，BindBitmap，BindColor，BindView)等都做了处理，这里我们重点关注@BindView的处理即可。其他注解的处理思想也是一样的。我们先来看一下findAndParseTargets方法的前半部分，遍历env.getElementsAnnotatedWith(BindView.class)集合，并调用parseBindView方法去转化。privateMap<TypeElement,BindingSet>findAndParseTargets(RoundEnvironmentenv){

Map<TypeElement,BindingSet.Builder>builderMap=newLinkedHashMap<>();

Set<TypeElement>erasedTargetNames=newLinkedHashSet<>();

scanForRClasses(env);

//Processeach@BindViewelement.

for(Elementelement:env.getElementsAnnotatedWith(BindView.class)){

//wedon'tSuperficialValidation.validateElement(element)

//sothatanunresolvedViewtypecanbegeneratedbylaterprocessingrounds

try{

parseBindView(element,builderMap,erasedTargetNames);

}catch(Exceptione){

logParsingError(element,BindView.class,e);

}

}

//后半部分，待会再讲

}可以看到牵绊部分的主要逻辑在parseBindView方法里面，主要做了以下几步操作：判断被注解@BindView修饰的成员变量是不是合法的，private或者static修饰的，则出错。parseBindView方法分析完毕之后，我们在回过头来看一下findAndParseTargets方法的后半部分，主要做的工作是对bindingMap进行重排序。到这里为止，我们已经分析完ButterKnifeProcessor是怎样处理注解的相关知识，并存进map集合中的，下面我们回到process方法，看一下是怎样生成java模板代码的。生成代码的核心代码只有这几行跟踪进去，发现是调用square公司开源的库javapoet开生成代码的。关于javaPoet的使用可以参考官网地址privateTypeSpeccreateType(intsdk){TypeSpec.Builderresult=TypeSpec.classBuilder(bindingClassName.simpleName()).addModifiers(PUBLIC);if(isFinal){result.addModifiers(FINAL);}if(parentBinding!=null){

result.superclass(parentBinding.bindingClassName);

}else{

result.addSuperinterface(UNBINDER);

}

if(hasTargetField()){

result.addField(targetTypeName,"target",PRIVATE);

}

//如果是View或者是View的子类的话，添加构造方法

if(isView){

result.addMethod(createBindingConstructorForView());

}elseif(isActivity){//如果是Activity或者是Activity的子类的话，添加构造方法

result.addMethod(createBindingConstructorForActivity());

}elseif(isDialog){//如果是Dialog或者是Dialog的子类的话，添加构造方法

result.addMethod(createBindingConstructorForDialog());

}

//如果构造方法不需要View参数，添加需要View参数的构造方法

if(!constructorNeedsView()){

//Addadelegatingconstructorwithatargettype+viewsignatureforreflectiveuse.

result.addMethod(createBindingViewDelegateConstructor());

}

result.addMethod(createBindingConstructor(sdk));

if(hasViewBindings()||parentBinding==null){

//生成unBind方法

result.addMethod(createBindingUnbindMethod(result));

}

returnresult.build();}接着我们一起来看一下createBindingConstructor(sdk)方法，大概做的事情就是

-判断是否有设置监听，如果有监听，将View设置为final

-遍历viewBindings，调用addViewBinding生成findViewById形式的代码。privateMethodSpeccreateBindingConstructor(intsdk){MethodSpec.Builderconstructor=MethodSpec.constructorBuilder().addAnnotation(UI_THREAD).addModifiers(PUBLIC);//如果有方法绑定，比如@onClick，那么增加一个targetTypeName类型的方法参数target，并且是final类型的if(hasMethodBindings()){constructor.addParameter(targetTypeName,"target",FINAL);}else{//如果没有，不是final类型的constructor.addParameter(targetTypeName,"target");}//如果有注解的View，那么添加VIEW类型source参数if(constructorNeedsView()){constructor.addParameter(VIEW,"source");}else{//添加Context类型的context参数constructor.addParameter(CONTEXT,"context");}if(hasUnqualifiedResourceBindings()){

//AaptcanchangeIDsoutfromunderneathus,justsuppresssinceallwillworkat

//runtime.

constructor.addAnnotation(AnnotationSpec.builder(SuppressWarnings.class)

.addMember("value","$S","ResourceType")

.build());

}

//如果@OnTouch绑定View，添加@SuppressLint("ClickableViewAccessibility")

if(hasOnTouchMethodBindings()){

constructor.addAnnotation(AnnotationSpec.builder(SUPPRESS_LINT)

.addMember("value","$S","ClickableViewAccessibility")

.build());

}

//如果parentBinding不为空，调用父类的构造方法

if(parentBinding!=null){

if(parentBinding.constructorNeedsView()){

constructor.addStatement("super(target,source)");

}elseif(constructorNeedsView()){

constructor.addStatement("super(target,source.getContext())");

}else{

constructor.addStatement("super(target,context)");

}

constructor.addCode("\n");

}

//添加成员变量

if(hasTargetField()){

constructor.addStatement("this.target=target");

constructor.addCode("\n");

}

if(hasViewBindings()){

if(hasViewLocal()){

//Localvariableinwhichallviewswillbetemporarilystored.

constructor.addStatement("$Tview",VIEW);

}

//遍历viewBindings，生成source.findViewById($L)代码

for(ViewBindingbinding:viewBindings){

addViewBinding(constructor,binding);

}

for(FieldCollectionViewBindingbinding:collectionBindings){

constructor.addStatement("$L",binding.render());

}

if(!resourceBindings.isEmpty()){

constructor.addCode("\n");

}

}

if(!resourceBindings.isEmpty()){

if(constructorNeedsView()){

constructor.addStatement("$Tcontext=source.getContext()",CONTEXT);

}

if(hasResourceBindingsNeedingResource(sdk)){

constructor.addStatement("$Tres=context.getResources()",RESOURCES);

}

for(ResourceBindingbinding:resourceBindings){

constructor.addStatement("$L",binding.render(sdk));

}

}

returnconstructor.build();}下面我们一起来看一下addViewBinding方法是怎样生成代码的。

privatevoidaddViewBinding(MethodSpec.Builderresult,ViewBindingbinding){if(binding.isSingleFieldBinding()){//Optimizethecommoncasewherethere'sasinglebindingdirectlytoafield.FieldViewBindingfieldBinding=binding.getFieldBinding();//注意这里直接使用了target.的形式，所以属性肯定是不能private的CodeBlock.Builderbuilder=CodeBlock.builder().add("target.$L=",fieldBinding.getName());booleanrequiresCast=requiresCast(fieldBinding.getType());

if(!requiresCast&&!fieldBinding.isRequired()){

builder.add("source.findViewById($L)",binding.getId().code);

}else{

builder.add("$T.find",UTILS);

builder.add(fieldBinding.isRequired()?"RequiredView":"OptionalView");

if(requiresCast){

builder.add("AsType");

}

builder.add("(source,$L",binding.getId().code);

if(fieldBinding.isRequired()||requiresCast){

builder.add(",$S",asHumanDescription(singletonList(fieldBinding)));

}

if(requiresCast){

builder.add(",$T.class",fieldBinding.getRawType());

}

builder.add(")");

}

result.addStatement("$L",builder.build());

return;

}

**ButterKnife是怎样实现代码注入的**

使用过ButterKnife得人基本都知道，我们是通过bind方法来实现注入的，即自动帮我们findViewById，解放我们的双手，提高工作效率。下面我们一起来看一下bind方法是怎样实现注入的。@NonNull@UiThreadbr/>@UiThreadViewsourceView=target.getWindow().getDecorView();returncreateBinding(target,sourceView);}可以看到bind方法很简单，逻辑基本都交给createBinding方法去完成。我们一起进入createBinding方法来看一下到底做了什么privatestaticUnbindercreateBinding(@NonNullObjecttarget,@NonNullViewsource){Class<?>targetClass=target.getClass();if(debug)Log.d(TAG,"Lookingupbindingfor"+targetClass.getName());//从Class中查找constructorConstructor<?extendsUnbinder>constructor=findBindingConstructorForClass(targetClass);if(constructor==null){

returnUnbinder.EMPTY;

}

//noinspectionTryWithIdenticalCatchesResolvestoAPI19+onlytype.

try{

//反射实例化构造方法

returnconstructor.newInstance(target,source);

}catch(IllegalAccessExceptione){

thrownewRuntimeException("Unabletoinvoke"+constructor,e);

}catch(InstantiationExceptione){

thrownewRuntimeException("Unabletoinvoke"+constructor,e);

}catch(InvocationTargetExceptione){

Throwablecause=e.getCause();

if(causeinstanceofRuntimeException){

throw(RuntimeException)cause;

}

if(causeinstanceofError){

throw(Error)cause;

}

thrownewRuntimeException("Unabletocreatebindinginstance.",cause);

}}其实createBinding来说，主要做了这几件事情

-传入class，通过findBindingConstructorForClass方法来实例化constructor

-利用反射来初始化constructor对