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);                    }                }            }        }    }}