activity的生命周期

最新推荐文章于 2020-08-06 22:53:04 发布

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本文通过一个简单实例详细解析了Android中Activity的生命周期，包括其不同状态的转换过程，以及如何通过重写特定方法来控制这些状态。

前言：

Activity的生命周期不是自身控制的，而是由Android系统控制的。系统通过调用不同的方法，完成Activity不同状态的转换。本文将通过一个实例来帮助理解Activity生命周期。

本文要点：

1.理解生命周期

2.Activity状态扭转

生命周期概述

先上图：

这幅图已经很完整的描述了Activity的生命周期，以及状态转换。但这里针对上图做几点说明：

（1）Activity生命周期中，有很多种状态。其中包括Resumed（运行态,可见且可交互）,Paused（暂停，部分可见但不可交互）,Stoped（停止态，不可见）,Destroyed/Killed（销毁）等；

（2）Entire Lifetime，完整的生命周期从调用onCreate()开始，直至调用onDestroy()结束；

（3）Visible Lifetime，生命周期中可见的阶段，发生在调用onStart()之后，onStop()之前。处于Resumed，或Paused态，在这期间Activity处于屏幕上；

（4）Foreground Lifetime，生命周期可交互的阶段，在调用onResume()之后，onPause()之前。在这期间，Activity显示在所有其他Activity之前，并获取输入焦点。

（5）Android系统通过调用onCreate()方法创建Activity实例.onDestroy()方法释放,关闭Activity；

（6）Android系统通过栈管理Activities，屏幕最前端的Activity位于栈顶。不在栈顶的Activity一般处于Paused状态，Stoped状态，或者Destroyed中的一种；

（7）实际应用中，我们通过重载Android生命周期中不同的方法，实现对不同状态下Activity的控制。如：我们可以重载onCreate()方法，实现对数据库的初始化;重写onDestroy()，执行删除数据库操作等。

状态转换实例

为了便于理解，我简单的写了一个Demo,不明白Activity周期的朋友们，可以亲手实践一下，大家按照我的步骤来。

第一步:新建一个Android工程，我这里命名为MainActivity.再创建一个OtherActivity继承activity。

[java] view plain copy print ?

public class MainActivityextends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
System.out.println("MainActivity:----------------onCreate--");
Button button=(Button) this.findViewById(R.id.button);
button.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
// TODO Auto-generated method stub
Intent intent=new Intent(MainActivity.this,OtherActivity.class);
startActivity(intent);
}
});
}
@Override
protected void onStart() {
super.onStart();
// The activity is about to become visible.
System.out.println("MainActivity:----------------onStart--");
}
@Override
protected void onResume() {
super.onResume();
// The activity has become visible (it is now "resumed").
System.out.println("MainActivity:----------------onResume--");
}
@Override
protected void onPause() {
super.onPause();
// Another activity is taking focus (this activity is about to be "paused").
System.out.println("MainActivity:----------------onPause--");
}
@Override
protected void onStop() {
super.onStop();
// The activity is no longer visible (it is now "stopped")
System.out.println("MainActivity:----------------onStop--");
}
@Override
protected void onDestroy() {
super.onDestroy();
// The activity is about to be destroyed.
System.out.println("MainActivity:----------------onDestroy--");
}
}

public class MainActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        System.out.println("MainActivity:----------------onCreate--");
        Button button=(Button) this.findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() {
            
            public void onClick(View v) {
                // TODO Auto-generated method stub
                Intent intent=new Intent(MainActivity.this,OtherActivity.class);
                startActivity(intent);
            }
        });
 }
    
    @Override
    protected void onStart() {
        super.onStart();
        // The activity is about to become visible.
        System.out.println("MainActivity:----------------onStart--");
    }
    @Override
    protected void onResume() {
        super.onResume();
        // The activity has become visible (it is now "resumed").
        System.out.println("MainActivity:----------------onResume--");
    }
    @Override
    protected void onPause() {
        super.onPause();
        // Another activity is taking focus (this activity is about to be "paused").
        System.out.println("MainActivity:----------------onPause--");
    }
    @Override
    protected void onStop() {
        super.onStop();
        // The activity is no longer visible (it is now "stopped")
        System.out.println("MainActivity:----------------onStop--");
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        // The activity is about to be destroyed.
        System.out.println("MainActivity:----------------onDestroy--");
    }

}

[java] view plain copy print ?

public class OtherActivityextends Activity {
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.other);
System.out.println("OtherActivity:----------------onCreate--");
}
@Override
protected void onStart() {
super.onStart();
// The activity is about to become visible.
System.out.println("OtherActivity:----------------onStart--");
}
@Override
protected void onResume() {
super.onResume();
// The activity has become visible (it is now "resumed").
System.out.println("OtherActivity:----------------onResume--");
}
@Override
protected void onPause() {
super.onPause();
// Another activity is taking focus (this activity is about to be "paused").
System.out.println("OtherActivity:----------------onPause--");
}
@Override
protected void onStop() {
super.onStop();
// The activity is no longer visible (it is now "stopped")
System.out.println("OtherActivity:----------------onStop--");
}
@Override
protected void onDestroy() {
super.onDestroy();
// The activity is about to be destroyed.
System.out.println("OtherActivity:----------------onDestroy--");
}
}

public class OtherActivity extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.other);
        System.out.println("OtherActivity:----------------onCreate--");
    }
    
    @Override
    protected void onStart() {
        super.onStart();
        // The activity is about to become visible.
        System.out.println("OtherActivity:----------------onStart--");
    }
    @Override
    protected void onResume() {
        super.onResume();
        // The activity has become visible (it is now "resumed").
        System.out.println("OtherActivity:----------------onResume--");
    }
    @Override
    protected void onPause() {
        super.onPause();
        // Another activity is taking focus (this activity is about to be "paused").
        System.out.println("OtherActivity:----------------onPause--");
    }
    @Override
    protected void onStop() {
        super.onStop();
        // The activity is no longer visible (it is now "stopped")
        System.out.println("OtherActivity:----------------onStop--");
    }
    @Override
    protected void onDestroy() {
        super.onDestroy();
        // The activity is about to be destroyed.
        System.out.println("OtherActivity:----------------onDestroy--");
    }

}

第三步:运行上述工程,效果图如下(没什么特别的):