学习注解(@)笔记

1.@在Android中的作用?
(1)和编译器一起给你一些提示警告信息。
(2)快捷有效的编译Java代码,谷歌出的support-annotations这个库 就是主要干这个的。
(3)和反射一起 提供一些类似于spring 可配置的功能，方便简洁.

http://tools.android.com/tech-docs/support-annotations (谷歌官方注解库文档)

2.一个简单的用法,@Nullable (可为空) 和 @NonNull(不能为空)。

private string hello(@NonNull String s){
return " " ;
}
调用hello这个方法;
String s = null;
hello(s);
运行后会出现这个警告;
1 Argument 'a' might be null less... (Ctrl+F1)
2 This inspection analyzes method control and data flow to report possible conditions that are always true or false, expressions whose value is statically proven to be constant, and situations that can lead to nullability contract violations.
3 Variables, method parameters and return values marked as @Nullable or @NotNull are treated as nullable (or not-null, respectively) and used during the analysis to check nullability contracts, e.g. report possible NullPointerException errors.
4 More complex contracts can be defined using @Contract annotation, for example:
5 @Contract(", null -> null") — method returns null if its second argument is null @Contract(", null -> null; _, !null -> !null") — method returns null if its second argument is null and not-null otherwise @Contract("true -> fail") — a typical assertFalse method which throws an exception if true is passed to it
6 The inspection can be configured to use custom @Nullable
7 @NotNull annotations (by default the ones from annotations.jar will be used)

这里出现警告是因为你所传入的值为空,但在注解里面已经处理,所以你看到的只是警告,而不是NullPointerException errors.
从这里就可以看出来@的方便性了吧.

http://www.cnblogs.com/punkisnotdead/p/3384464.html(反射的链接)

3.一个仿照Butterknife案例

(1).首先定义一个InjectView注解

  package com.example.administrator.testfab;
 
  import java.lang.annotation.ElementType;
  import java.lang.annotation.Retention;
  import java.lang.annotation.RetentionPolicy;
  import java.lang.annotation.Target;
 
 /**
  * Created by Administrator on 2015/8/5.
  */
 
 @Target(ElementType.FIELD)
 @Retention(RetentionPolicy.RUNTIME)
 public @interface InjectView {
     //id就表示哪些控件，-1就表示取不到时候的默认值
     int id() default -1;
 } 

(2)然后定义一个解释器

1 package com.example.administrator.testfab;
 2 
 3 import android.app.Activity;
 4 import android.view.View;
 5 
 6 import java.lang.reflect.Field;
 7 
 8 /**
 9  * Created by Administrator on 2015/8/5.
10  */
11 public class InjectViewParser {
12 
13     public static void inject(Object object) {
14 
15         try {
16             parse(object);
17         } catch (Exception e) {
18             e.printStackTrace();
19         }
20     }
21 
22     public static void parse(Object object) throws Exception {
23         final Class<?> clazz = object.getClass();
24         View view = null;
25         Field[] fields = clazz.getDeclaredFields();
26         for (Field field : fields) {
27             if (field.isAnnotationPresent(InjectView.class)) {
28                 InjectView injectView = field.getAnnotation(InjectView.class);
29                 int id = injectView.id();
30                 if (id < 0) {
31                     throw new Exception("id must not be null");
32                 } else {
33                     field.setAccessible(true);
34                     if (object instanceof View) {
35                         view = ((View) object).findViewById(id);
36                     } else if (object instanceof Activity) {
37                         view = ((Activity) object).findViewById(id);
38                     }
39                     field.set(object, view);
40                 }
41 
42             }
43 
44         }
45 
46     }
47 }

(3)最后在actity里使用即可;

 1 package com.example.administrator.testfab;
 2 
 3 import android.os.AsyncTask;
 4 import android.os.Bundle;
 5 import android.support.annotation.CallSuper;
 6 import android.support.annotation.NonNull;
 7 import android.support.annotation.Nullable;
 8 import android.support.annotation.StringRes;
 9 import android.support.v7.app.AppCompatActivity;
10 import android.util.Log;
11 import android.view.Menu;
12 import android.view.MenuItem;
13 import android.widget.Button;
14 
15 public class MainActivity extends AppCompatActivity {
16 
17     @InjectView(id = R.id.bt)
18     private Button bt;
19 
20 
21     @Override
22     protected void onCreate(Bundle savedInstanceState) {
23         super.onCreate(savedInstanceState);
24         setContentView(R.layout.activity_main);
25         //开始注入
26         InjectViewParser.inject(this);
27         //这个主要是测试注入id 成功没有 成功了就不会报错~
28         bt.setText("inject done");
29 
30 
31     }
32 
33 
34     @Override
35     public boolean onCreateOptionsMenu(Menu menu) {
36         // Inflate the menu; this adds items to the action bar if it is present.
37         getMenuInflater().inflate(R.menu.menu_main, menu);
38         return true;
39     }
40 
41     @Override
42     public boolean onOptionsItemSelected(MenuItem item) {
43         // Handle action bar item clicks here. The action bar will
44         // automatically handle clicks on the Home/Up button, so long
45         // as you specify a parent activity in AndroidManifest.xml.
46         int id = item.getItemId();
47 
48         //noinspection SimplifiableIfStatement
49         if (id == R.id.action_settings) {
50             return true;
51         }
52 
53         return super.onOptionsItemSelected(item);
54     }
55
56 
57 
58 }