双击back返回键退出app功能的实现方式

有两种实现思路，看个人喜好吧：

方式1：

public class MainActivity extends Activity {

    private Toast toast;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        toast = Toast.makeText(getApplicationContext(), "确定退出？", 0);
    }

    public void onBackPressed() {
        quitToast();
    }

    private void quitToast() {
        if(null == toast.getView().getParent()) {
            toast.show();
        }else{
            System.exit(0);
        }
    }
}

方式2：

public class MainActivity extends Activity {

    private Toast toast;
    rotected void onCreate(Bundle savedInstanceState) {
        ...
                toast = Toast.makeText(this, "再按一次退出应用", Toast.LENGTH_SHORT);
            toast.setGravity(Gravity.BOTTOM, 0, ConversionUtil.dip2px(this, 150));
    }

    @Override 
    public void onBackPressed() { 
        if (doubleBackToExitPressedOnce) { 
            if(toast!=null){
                toast.cancel();
            }
            super.onBackPressed(); 
            return; 
        } 

        this.doubleBackToExitPressedOnce = true;
        toast.show();

        new Handler().postDelayed(new Runnable() {

            @Override 
            public void run() { 
                doubleBackToExitPressedOnce=false;                        
            } 
        }, 2000); 
    } 
}