Andoid笔记

名词解释：
- View: usually UI widgets such as a button or text field
- ViewGroup: invisible view container that define how the child views are laid out, such as in a grid or a vertical list
- Activity: a single screen in your app
- Intent: runtime binding between separate components (such as two activities). An Intent can be explicit in order to start a specific component (a specific Activity instance) or implicit in order to start any component that can handle the intended action (such as “capture a photo”).
- Bundle: a blob of key-value pairs
- Fragment: a nested activity that can define its own layout and manage its own lifecycle
Layout：
- layout_file.xml中的属性：
  - xmlns:android=xxx: 声明xml名称空间android
  - android:id="@+id/edit_message": 声明id属性，使代码中可引用它
    @: resource object, an unique integer for an resource (bitmap, layout file, string) defined in your projects’ gen/R.java file
    +: create
    id: resource type
    edit_message: resource name, other resource type can use the same name
  - 引用resource object时还可加上namespace，如android:hint="@**android:**string/edit_message"，这时引用的是android.R类
- 每个layout_file.xml会被编译成一个View，可在Activity的onCreate()中load它：setContentView(R.layout.layout_file)
- 可在layout_file.xml中定义view并赋予id：
```
  <Button android:id="@+id/my_button" android:layout_width="wrap_content"
      android:layout_height="wrap_content" android:text="@string/my_button_text" />
```
  然后在代码中create它：Button myButton = (Button) findViewById(R.id.my_button);
- View的属性layout_something定义其在ViewGroup中的layout。layout_width和layout_height是所有View都须定义的，可定义值：
  - wrap_content: tells your view to size itself to the dimensions required by its content
  - fill_parent/match_parent(rename in API Level 8): tells your view to become as big as its parent view group will allow
  - n dp: density-independent pixel units
Activity lifecycle：
- Created/Started是临时状态，Resumed/Paused/Stopped是可停留状态。Resumed指正常running，Paused指失去焦点被其他组件部分遮挡，Stopped指被其他组件完全遮挡。
- 在onDestroy()前都会调onPause()和onStop()，除非在onCreate()直接调finish()

AndroidManifest.xml：

所有Activity得在AndroidManifest.xml中用<activity>元素声明

启动app时进入的main activity声明时需要<intent-filter>:

  <intent-filter>
      <action android:name="android.intent.action.MAIN" />
      <category android:name="android.intent.category.LAUNCHER" />
  </intent-filter>

android版本分布
android icon size: 36×36 (ldpi), 48×48 (mdpi), 72×72 (hdpi), 96×96 (xhdpi)

读取AndroidManifest.xml中application的meta-data:

  try {
      ApplicationInfo ai = this.getPackageManager().getApplicationInfo(this.getPackageName(), PackageManager.GET_META_DATA);
      String channelName = ai.metaData.getString("UMENG_CHANNEL");
      if (channelName == null) return "";
      return channelName;
  } catch (NameNotFoundException ex) {
      Log.e("META", "Can not find the application info of" + mActivity.getPackageName());
      return "";
  }

参考：

Android Training