【Android】@ContentView注解与Fragment/Activity的第二构造函数

偏执的太偏执、 2023-07-21 14:29 14阅读 0赞

`androidx.annotation:annotation-1.1.0-alpha01`起引入了 `@ContentView`注解指定fragment或activity的默认布局文件，但时隔不久，API又调整为通过构造函数指定布局文件。下面详细对比一下几种方式的不同：

# 不使用@ContentView注解 #

--------------------

以前，我们需要在Fragment的`onCrateView`中调用inflate或者在Activity的`onCrate`中调用`setContentView`来inflate布局文件：

## Java (Fragment) ##

import android.os.Bundle;
    import android.view.LayoutInflater;
    import android.view.View;
    import android.view.ViewGroup;
    import androidx.fragment.app.Fragment;
    // ...
    
    public class HomeFragment extends Fragment { 
      // ...
      @Override
      public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { 
        return inflater.inflate(R.layout.frag_home, container, false);
      }
    }

## Kotlin (Fragment) ##

import android.os.Bundle
    import android.view.*
    import androidx.fragment.app.Fragment
    // ...
    
    class HomeFragment : Fragment() { 
      override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? { 
        return inflater.inflate(R.layout.frag_home, container, false)
      }
      // ...
    }

## Java (Activity) ##

import android.os.Bundle;
    import androidx.appcompat.app.AppCompatActivity;
    
    public class MainActivity extends AppCompatActivity { 
      @Override
      protected void onCreate(Bundle savedInstanceState) { 
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
      }
    }

## Kotlin (Activity) ##

import android.os.Bundle
    import androidx.appcompat.AppCompatActivity
    
    class MainActivity : AppCompatActivity() { 
      override fun onCreate(savedInstanceState: Bundle?) { 
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
      }
    }

# 使用@ContentView修饰class #

--------------------

将`@ContentView`添加到Fragment或者Activity的class定义上，指定其layout文件

## Java (Fragment) ##

import androidx.annotation.ContentView;
    import androidx.fragment.app.Fragment;
    // ...
    
    @ContentView(R.layout.frag_home)
    public class HomeFragment extends Fragment { 
      // ...
    }

## Kotlin (Fragment) ##

import androidx.annotation.ContentView
    import androidx.fragment.app.Fragment
    // ...
    
    @ContentView(R.layout.frag_home)
    class HomeFragment: Fragment() { 
      // ...
    }

## Java (Activity) ##

import androidx.annotation.ContentView;
    import androidx.appcompat.app.AppCompatActivity;
    
    @ContentView(R.layout.activity_main)
    public class MainActivity extends AppCompatActivity { 
      // ...
    }

## Kotlin (Activity) ##

import androidx.annotation.ContentView
    import androidx.appcompat.app.AppCompatActivity
    
    @ContentView(R.layout.activity_main)
    class MainActivity : AppCompatActivity() { 
      // ...
    }

# 使用带参数的构造函数 #

--------------------

好景不长, `androidx.activity:activity -1.0.0-alpha06`以及 `androidx.fragment:fragment-1.1.0-alpha05` 起，`@ContentView`从class注解改为第二构造函数注解。

官方的 Release Note的说明如下：

> Fragment and FragmentActivity now contain a second constructor that takes a @LayoutRes int, which replaces the previous behavior of annotating your class with @ContentView. This approach works in both app and library modules.

相应的，Fragment和FragmentActivity都增加了第二构造函数，例如：

/** * Alternate constructor that can be used to provide a default layout * that will be inflated by {@link #onCreateView(LayoutInflater, ViewGroup, Bundle)}. * * @see #Fragment() * @see #onCreateView(LayoutInflater, ViewGroup, Bundle) */
        @ContentView
        public Fragment(@LayoutRes int contentLayoutId) { 
            this();
            mContentLayoutId = contentLayoutId;
        }

因此，最新的布局指定变为如下方式：

## Java (Fragment) ##

import androidx.fragment.app.Fragment;
    // ...
    
    public class HomeFragment extends Fragment { 
      // ...
      public HomeFragment() { 
        // Pass the layout resource ID to the super constructor
        super(R.layout.frag_home);
      }
    }

## Kotlin (Fragment) ##

import androidx.fragment.app.Fragment
    // ...
    
    class HomeFragment: Fragment(R.layout.frag_home) { 
      // ...
    }

## Java (Activity) ##

import androidx.appcompat.app.AppCompatActivity;
    
    public class MainActivity extends AppCompatActivity { 
      // ...
      public MainActivity() { 
        // Pass the layout resource ID to the super constructor
        super(R.layout.activity_main);
      }
    }

## Kotlin (Activity) ##

import androidx.appcompat.app.AppCompatActivity
    
    class MainActivity : AppCompatActivity(R.layout.activity_main) { 
      // ...
    }