ButterKnife第三方库源码分析

ButterKnife原理其实也很简单

我的Github博客详解

ButterKnife是大名鼎鼎JakeWharton热门开源项目的其中一个，让开发者不再重复的进行findViewById的操作。

配合android studio的插件，一键自动生成xml文件所有view的实例。

ButterKnife为什么会那么神奇，自动帮助开发者省去了繁琐的操作，他的实现的原理到底是怎么样的呢？

下面我们从代码使用上，一步一步的分析ButterKnife的实现原理

分析ButterKnife版本：com.jakewharton:butterknife:7.0.1

使用ButterKnife快速初始化xml布局对象.

public class MainActivity extends AppCompatActivity {

@Bind(R.id.tv01) TextView tv01;
@Bind(R.id.tv02) TextView tv02;
@Bind(R.id.tv03) TextView tv03;
@Bind(R.id.tv04) TextView tv04;
@Bind(R.id.activity_main) LinearLayout activityMain;

@OnClick(R.id.tv01)
public void test(View v) {

}

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

}

一段很常规使用ButterKnife快速初始化xml布局对象的代码。为什么当onCreate方法的ButterKnife.bind(this);

调用完毕，xml所有的布局对象都初始化好了呢？

我们进入到@Bind注解里看看究竟，看看是否找得到线索

@Retention(CLASS) @Target(FIELD)
public @interface Bind {
/** View ID to which the field will be bound. */
int[] value();
}

可以看到@Retention(CLASS)，这句表示：保留时间 编译时，也就是工程编译时运行的注解.

常规获取View对象的方式是这样的：

tv01 = (TextView) findViewById(R.id.tv01);

//现在变成这样
@Bind(R.id.tv01) TextView tv01;

所以说，ButterKnife这个库应该拿到了R.id.tv01 这个id值，也拿到了tv01成员变量，在通过findViewById给tv01赋值

但是，ButterKnife在哪里进行这样的操作呢？

从@Bind注解来看，应该是编译时拿到了id值

我们在build目录下找到了ButterKnife生成的新文件：

build\generated\source\apt\debug\com\butterknifedemo\MainActivity$$ViewBinder

// Generated code from Butter Knife. Do not modify!
package com.butterknifedemo;

import android.view.View;
import butterknife.ButterKnife.Finder;
import butterknife.ButterKnife.ViewBinder;

public class MainActivity$$ViewBinder<T extends com.butterknifedemo.MainActivity> implements ViewBinder<T> {
@Override public void bind(final Finder finder, final T target, Object source) {
View view;
view = finder.findRequiredView(source, 2131427413, "field 'tv01' and method 'test'");
target.tv01 = finder.castView(view, 2131427413, "field 'tv01'");
view.setOnClickListener(
new butterknife.internal.DebouncingOnClickListener() {
@Override public void doClick(
android.view.View p0
) {
target.test(p0);
}
});
view = finder.findRequiredView(source, 2131427414, "field 'tv02'");
target.tv02 = finder.castView(view, 2131427414, "field 'tv02'");
view = finder.findRequiredView(source, 2131427415, "field 'tv03'");
target.tv03 = finder.castView(view, 2131427415, "field 'tv03'");
view = finder.findRequiredView(source, 2131427416, "field 'tv04'");
target.tv04 = finder.castView(view, 2131427416, "field 'tv04'");
view = finder.findRequiredView(source, 2131427412, "field 'activityMain'");
target.activityMain = finder.castView(view, 2131427412, "field 'activityMain'");
}

@Override public void unbind(T target) {
target.tv01 = null;
target.tv02 = null;
target.tv03 = null;
target.tv04 = null;
target.activityMain = null;
}
}

我们发现ButterKnife在build目录下生成了一个类，这个类竟然帮助我们完成了findVieweById的操作

那这个类是怎么制作出来的呢？

现在，我们直接去看ButterKnife源码：

//先进入里面看看
ButterKnife.bind(this);

//显然Activity对象作为target往下传递了
//Finder.ACTIVITY 是什么呢
public static void bind(Activity target) {
bind(target, target, Finder.ACTIVITY);
}

//Finder.ACTIVITY 原来是 ButterKnife 内部枚举
//return ((Activity) source).findViewById(id); 注意看句代码
public final class ButterKnife {
private ButterKnife() {
throw new AssertionError("No instances.");
}

/** DO NOT USE: Exposed for generated code. */
@SuppressWarnings("UnusedDeclaration") // Used by generated code.
public enum Finder {
VIEW {
@Override protected View findView(Object source, int id) {
return ((View) source).findViewById(id);
}

@Override public Context getContext(Object source) {
return ((View) source).getContext();
}
},
ACTIVITY {
@Override protected View findView(Object source, int id) {
return ((Activity) source).findViewById(id);
}

@Override public Context getContext(Object source) {
return (Activity) source;
}
},
DIALOG {
@Override protected View findView(Object source, int id) {
return ((Dialog) source).findViewById(id);
}

@Override public Context getContext(Object source) {
return ((Dialog) source).getContext();
}
};

//findViewBinderForClass这个方法通过Activity对象去查找返回了一个ViewBinder类，
//然后viewBinder.bind(finder, target, source);
static void bind(Object target, Object source, Finder finder) {
Class<?> targetClass = target.getClass();
try {
if (debug) Log.d(TAG, "Looking up view binder for " + targetClass.getName());
ViewBinder<Object> viewBinder = findViewBinderForClass(targetClass);
if (viewBinder != null) {
viewBinder.bind(finder, target, source);
}
} catch (Exception e) {
throw new RuntimeException("Unable to bind views for " + targetClass.getName(), e);
}
}

这里有两个问题：

1. findViewBinderForClass通过Activity字节码如何找到的viewBinder？

2. viewBinder是什么？

首先，看viewBinder是什么：

/** DO NOT USE: Exposed for generated code. */
public interface ViewBinder<T> {
void bind(Finder finder, T target, Object source);
void unbind(T target);
}

原来是个接口，注释说自动生成代码用的，看看刚刚在build找到的类：

public class MainActivity$$ViewBinder
<T extends com.butterknifedemo.MainActivity> implements ViewBinder<T> {

@Override public void bind(final Finder finder, final T target, Object source) {

原来viewBinder.bind(finder, target, source);这行代码调用了MainActivity$$ViewBinder类里的bind方法，

帮助我们完成findViewById工作

现在我们知道了，平时我们调用ButterKnife.bind(this);

最终都会调用对应生成的$$ViewBinder类里的bind方法帮助我们完成繁琐的操作

问题2：怎么通过Activity字节码找到viewBinder对象的？

private static ViewBinder<Object> findViewBinderForClass(Class<?> cls)
throws IllegalAccessException, InstantiationException {
ViewBinder<Object> viewBinder = BINDERS.get(cls);
if (viewBinder != null) {
if (debug) Log.d(TAG, "HIT: Cached in view binder map.");
return viewBinder;
}
String clsName = cls.getName();
if (clsName.startsWith(ANDROID_PREFIX) || clsName.startsWith(JAVA_PREFIX)) {
if (debug) Log.d(TAG, "MISS: Reached framework class. Abandoning search.");
return NOP_VIEW_BINDER;
}
try {
Class<?> viewBindingClass =
Class.forName(clsName + ButterKnifeProcessor.SUFFIX);//SUFFIX = "$$ViewBinder";
//noinspection unchecked
viewBinder = (ViewBinder<Object>) viewBindingClass.newInstance();
if (debug) Log.d(TAG, "HIT: Loaded view binder class.");
} catch (ClassNotFoundException e) {
if (debug) Log.d(TAG, "Not found. Trying superclass " + cls.getSuperclass().getName());
viewBinder = findViewBinderForClass(cls.getSuperclass());
}
BINDERS.put(cls, viewBinder);
return viewBinder;
}

原来先从BINDERS.get(cls);里面取，空的话在通过Class.forName(clsName + ButterKnifeProcessor.SUFFIX);

创建出一个新对象出来，前提是这个$$ViewBinder已经生成好了

什么时候生成这个类，怎么生成的？通过注解@Bind我们应该猜到，是工程编译时就生成好了的

在ButterKnife源码中我们发现了这一个类：

public final class ButterKnifeProcessor extends AbstractProcessor {
public static final String SUFFIX = "$$ViewBinder";
public static final String ANDROID_PREFIX = "android.";
public static final String JAVA_PREFIX = "java.";
static final String VIEW_TYPE = "android.view.View";
private static final String COLOR_STATE_LIST_TYPE = "android.content.res.ColorStateList";
private static final String DRAWABLE_TYPE = "android.graphics.drawable.Drawable";
private static final String NULLABLE_ANNOTATION_NAME = "Nullable";
private static final String ITERABLE_TYPE = "java.lang.Iterable<?>";
private static final String LIST_TYPE = List.class.getCanonicalName();
private static final List<Class<? extends Annotation>> LISTENERS = Arrays.asList(//
OnCheckedChanged.class, //
OnClick.class, //
OnEditorAction.class, //
OnFocusChange.class, //
OnItemClick.class, //
OnItemLongClick.class, //
OnItemSelected.class, //
OnLongClick.class, //
OnPageChange.class, //
OnTextChanged.class, //
OnTouch.class //
);

@Override public Set<String> getSupportedAnnotationTypes() {
Set<String> types = new LinkedHashSet<String>();

types.add(Bind.class.getCanonicalName());

for (Class<? extends Annotation> listener : LISTENERS) {
types.add(listener.getCanonicalName());
}

types.add(BindBool.class.getCanonicalName());
types.add(BindColor.class.getCanonicalName());
types.add(BindDimen.class.getCanonicalName());
types.add(BindDrawable.class.getCanonicalName());
types.add(BindInt.class.getCanonicalName());
types.add(BindString.class.getCanonicalName());

return types;
}

extends AbstractProcessor，继承这一个类，表示它可以在工程编译是运行里面的process方法，

ButterKnife就是通过编译时，apt会自动查找集成AbstractProcessor的类，调用process方法

在process方法中找到存在ButterKnife的注解信息，获取在注解对应下的数据，例如id值

上面的代码我们还可以看到，Bind，OnClick等等注解已经存储好了，就等着遍历配对处理获取数据

看process方法：

@Override public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {
Map<TypeElement, BindingClass> targetClassMap = findAndParseTargets(env);

for (Map.Entry<TypeElement, BindingClass> entry : targetClassMap.entrySet()) {
TypeElement typeElement = entry.getKey();
BindingClass bindingClass = entry.getValue();

try {
JavaFileObject jfo = filer.createSourceFile(bindingClass.getFqcn(), typeElement);
Writer writer = jfo.openWriter();
writer.write(bindingClass.brewJava());
writer.flush();
writer.close();
} catch (IOException e) {
error(typeElement, "Unable to write view binder for type %s: %s", typeElement,
e.getMessage());
}
}

return true;
}

从JavaFileObject，Writer这个类就可以知道，ButterKnife把一些东西写到文件中去了，应该猜到

那些自动生成的java文件就从这里出来的

代码生成java文件，代码加载java文件去运行，有点意思

先看看findAndParseTargets方法做了什么：

private Map<TypeElement, BindingClass> findAndParseTargets(RoundEnvironment env) {
Map<TypeElement, BindingClass> targetClassMap = new LinkedHashMap<TypeElement, BindingClass>();
Set<String> erasedTargetNames = new LinkedHashSet<String>();

// Process each @Bind element.
for (Element element : env.getElementsAnnotatedWith(Bind.class)) {
try {
parseBind(element, targetClassMap, erasedTargetNames);
} catch (Exception e) {
logParsingError(element, Bind.class, e);
}
}

// Process each annotation that corresponds to a listener.
for (Class<? extends Annotation> listener : LISTENERS) {
findAndParseListener(env, listener, targetClassMap, erasedTargetNames);
}

// Process each @BindBool element.
for (Element element : env.getElementsAnnotatedWith(BindBool.class)) {
try {
parseResourceBool(element, targetClassMap, erasedTargetNames);
} catch (Exception e) {
logParsingError(element, BindBool.class, e);
}
}

....

可以看到，正在查找Bind之类的注解，猜都猜到通过定位注解获取注解下面的值了吧

现在，我们回过头看看把什么东西写到文件中去了，看着行代码： writer.write(bindingClass.brewJava());

String brewJava() {
StringBuilder builder = new StringBuilder();
builder.append("// Generated code from Butter Knife. Do not modify!\n");
builder.append("package ").append(classPackage).append(";\n\n");

if (!resourceBindings.isEmpty()) {
builder.append("import android.content.res.Resources;\n");
}
if (!viewIdMap.isEmpty() || !collectionBindings.isEmpty()) {
builder.append("import android.view.View;\n");
}
builder.append("import butterknife.ButterKnife.Finder;\n");
if (parentViewBinder == null) {
builder.append("import butterknife.ButterKnife.ViewBinder;\n");
}
builder.append('\n');

builder.append("public class ").append(className);
builder.append("<T extends ").append(targetClass).append(">");

if (parentViewBinder != null) {
builder.append(" extends ").append(parentViewBinder).append("<T>");
} else {
builder.append(" implements ViewBinder<T>");
}
builder.append(" {\n");

emitBindMethod(builder);
builder.append('\n');
emitUnbindMethod(builder);

builder.append("}\n");
return builder.toString();
}

可以一目了然的看到使用了StringBuilder手动拼接字符串的方式，生成了java文件，挺不容易的。

问题又来了，这些java文件静态不变化的部分可以写死，那些动态灵活的部分呢？例如View的id，对象名称

继续深入看源码：

//bind方法代码拼接
private void emitBindMethod(StringBuilder builder) {
builder.append("  @Override ")
.append("public void bind(final Finder finder, final T target, Object source) {\n");

// Emit a call to the superclass binder, if any.
if (parentViewBinder != null) {
builder.append("    super.bind(finder, target, source);\n\n");
}

if (!viewIdMap.isEmpty() || !collectionBindings.isEmpty()) {
// Local variable in which all views will be temporarily stored.
builder.append("    View view;\n");

// Loop over each view bindings and emit it.
for (ViewBindings bindings : viewIdMap.values()) {

//进入里面看看
emitViewBindings(builder, bindings);
}

private void emitViewBindings(StringBuilder builder, ViewBindings bindings) {
builder.append("    view = ");

List<ViewBinding> requiredViewBindings = bindings.getRequiredBindings();
if (requiredViewBindings.isEmpty()) {
builder.append("finder.findOptionalView(source, ")
.append(bindings.getId())//这个就是View的id了
.append(", null);\n");
} else {
if (bindings.getId() == View.NO_ID) {
builder.append("target;\n");
} else {
builder.append("finder.findRequiredView(source, ")
.append(bindings.getId())
.append(", \"");
emitHumanDescription(builder, requiredViewBindings);
builder.append("\");\n");
}
}

//字段看这里，进去
emitFieldBindings(builder, bindings);
emitMethodBindings(builder, bindings);
}

static void emitHumanDescription(StringBuilder builder,
Collection<? extends ViewBinding> bindings) {
Iterator<? extends ViewBinding> iterator = bindings.iterator();
switch (bindings.size()) {
case 1:
builder.append(iterator.next().getDescription());//View变量名称
break;
case 2:
builder.append(iterator.next().getDescription())

@Override public String getDescription() {
return "field '" + name + "'";
}

从上面的代码可以知道，动态的部分通过BindingClass这个类来获取的，那这是类怎么来的，看之前的代码：

@Override public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {
Map<TypeElement, BindingClass> targetClassMap = findAndParseTargets(env);

原来这个类是从findAndParseTargets里来的，刚刚我们知道，里面做了定位Bind，OnClick注解的操作，

定位的同时也把注解的值，例如id值，变量名称存在到BindingClass对象中了，很符合面向对象的思想

private void parseBindOne(Element element, Map<TypeElement, BindingClass> targetClassMap,
Set<String> erasedTargetNames) {
boolean hasError = false;
TypeElement enclosingElement = (TypeElement) element.getEnclosingElement();

...

BindingClass bindingClass = targetClassMap.get(enclosingElement);
if (bindingClass != null) {
ViewBindings viewBindings = bindingClass.getViewBinding(id);
if (viewBindings != null) {
Iterator<FieldViewBinding> iterator = viewBindings.getFieldBindings().iterator();
if (iterator.hasNext()) {
FieldViewBinding existingBinding = iterator.next();
error(element, "Attempt to use @%s for an already bound ID %d on '%s'. (%s.%s)",
Bind.class.getSimpleName(), id, existingBinding.getName(),
enclosingElement.getQualifiedName(), element.getSimpleName());
return;
}
}
} else {
bindingClass = getOrCreateTargetClass(targetClassMap, enclosingElement);
}

String name = element.getSimpleName().toString();
String type = elementType.toString();
boolean required = isRequiredBinding(element);

FieldViewBinding binding = new FieldViewBinding(name, type, required);
bindingClass.addField(id, binding);

// Add the type-erased version to the valid binding targets set.
erasedTargetNames.add(enclosingElement.toString());
}

private BindingClass getOrCreateTargetClass(Map<TypeElement, BindingClass> targetClassMap,
TypeElement enclosingElement) {
BindingClass bindingClass = targetClassMap.get(enclosingElement);
if (bindingClass == null) {
String targetType = enclosingElement.getQualifiedName().toString();
String classPackage = getPackageName(enclosingElement);
String className = getClassName(enclosingElement, classPackage) + SUFFIX;

bindingClass = new BindingClass(classPackage, className, targetType);
targetClassMap.put(enclosingElement, bindingClass);
}
return bindingClass;
}

最后，还有一点的是：ButterKnife通过注解获取id值并没有使用到反射，获取到变量也是通过Activity.view的形式

不同与一些反射获取注解的框架，使用反射会增加IO操作，增加了时间操作，多了会变得卡顿

反射的方式成员变量可使用private，而ButterKnife不可以，必须public或者protected

因为ButterKnife没有使用反射，需要Activity.view这样去获取一些对象赋值

具体可以去看源码，总的来说：

1, ButterKnife将View的id值放到@Bind注解中

2, ButterKnife通过extends AbstractProcessor编译时自动调用process方法来定位和存在注解与注解上的id值

3, 找到所有带注解与值的对象，存储在集合中，一个for循环一顿狂写，把java文件写到build目录下

4, 当调用ButterKnife.bind(this)的时候，最终会调用生成的viewBinder类里的bind方法

5, viewBinder里的bind方法，找已动态生成好了finfViewById的过程，通过Activity.view的形式初始化所有view

分析就到这里了

11/9/2016 11:33:29 PM