Android简单的View注入

本来是查找Android sdk samples里面有哪些使用annotation的例子，无意中在VoicemailProviderDemo中发现了这个View的注入实现

像其他的注入框架一样先声明一下

// Mark the views as injectable. These objects are instantiated automatically during
// onCreate() by finding the appropriate view that matches the specified resource_id.
@InjectView(R.id.start_recording_btn)
private Button mStartRec;
@InjectView(R.id.save_btn)
private Button mSaveButton;

然后在onCreate中调用

Injector.get(this).inject();

然后看下InjectView

/**
* Use this annotation to mark the fields of your Activity as being injectable.
* <p>
* See the {@link Injector} class for more details of how this operates.
*/
@Target({ ElementType.FIELD })
@Retention(RetentionPolicy.RUNTIME)
public @interface InjectView {
/**
* The resource id of the View to find and inject.
*/
public int value();
}

也跟其他自定义注解的方式一样

最后贴一下完整的Injector类

/*
* Copyright (C) 2011 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*     http://www.apache.org/licenses/LICENSE-2.0 *
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package com.example.android.voicemail.common.inject;

import android.app.Activity;

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;

/**
* Very lightweight form of injection, inspired by RoboGuice, for injecting common ui elements.
* <p>
* Usage is very simple. In your Activity, define some fields as follows:
*
* <pre class="code">
* @InjectView(R.id.fetch_button)
* private Button mFetchButton;
* @InjectView(R.id.submit_button)
* private Button mSubmitButton;
* @InjectView(R.id.main_view)
* private TextView mTextView;
* </pre>
* <p>
* Then, inside your Activity's onCreate() method, perform the injection like this:
*
* <pre class="code">
* setContentView(R.layout.main_layout);
* Injector.get(this).inject();
* </pre>
* <p>
* See the {@link #inject()} method for full details of how it works. Note that the fields are
* fetched and assigned at the time you call {@link #inject()}, consequently you should not do this
* until after you've called the setContentView() method.
*/
public final class Injector {
private final Activity mActivity;

private Injector(Activity activity) {
mActivity = activity;
}

/**
* Gets an {@link Injector} capable of injecting fields for the given Activity.
*/
public static Injector get(Activity activity) {
return new Injector(activity);
}

/**
* Injects all fields that are marked with the {@link InjectView} annotation.
* <p>
* For each field marked with the InjectView annotation, a call to
* {@link Activity#findViewById(int)} will be made, passing in the resource id stored in the
* value() method of the InjectView annotation as the int parameter, and the result of this call
* will be assigned to the field.
*
* @throws IllegalStateException if injection fails, common causes being that you have used an
*             invalid id value, or you haven't called setContentView() on your Activity.
*/
public void inject() {
for (Field field : mActivity.getClass().getDeclaredFields()) {//遍历所有声明的字段
for (Annotation annotation : field.getAnnotations()) {//遍历所有注解
if (annotation.annotationType().equals(InjectView.class)) {
try {
Class<?> fieldType = field.getType();//获取声明的类型
int idValue = InjectView.class.cast(annotation).value();//获取注解中的vaule 既@InjectView后面括号中的值，cast这种强转方式只是看起来更优雅些
field.setAccessible(true);
Object injectedValue = fieldType.cast(mActivity.findViewById(idValue));
if (injectedValue == null) {
throw new IllegalStateException("findViewById(" + idValue
+ ") gave null for " +
field + ", can't inject");
}
field.set(mActivity, injectedValue);
field.setAccessible(false);
} catch (IllegalAccessException e) {
throw new IllegalStateException(e);
}
}
}
}
}
}