Android输入法键盘弹出问题

Question

我自定义了一个alertDialog,带背景图的，可是总是不能弹出软键盘
dialog.xml<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="312dip"
    android:layout_height="208dip"
    android:orientation="vertical" 
    android:background="@drawable/login_bg"
    android:paddingLeft="20dip"
    
    >
<ImageButton 
    android:id="@+id/login_close_btn"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@drawable/dlg_close_btn"
    android:layout_alignParentTop="true"
    android:layout_alignParentRight="true"
    android:layout_marginTop="5dip"
    android:layout_marginRight="4dip"
    />
    <TextView 
        android:id="@+id/login_tip_text1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/login_tip"
        />
    <ImageView
        android:id="@+id/login_imageView1"
        android:layout_width="250dip"
        android:layout_height="1dip"
        android:background="#a1a19f"
        android:layout_below="@id/login_tip_text1"
        android:layout_marginTop="5dip"
        />
<LinearLayout
    android:id="@+id/login_linearLayout1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:layout_below="@id/login_imageView1"
    android:layout_marginTop="15dip"
    android:layout_centerHorizontal="true"
    >
    <TextView 
        android:id="@+id/login_username_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/username"
        />
    <EditText 
        android:id="@+id/login_username_et"
        android:layout_width="150dip"
        android:layout_height="35dip"
        android:textSize="15dip"
        />
</LinearLayout>
<LinearLayout
    android:id="@+id/login_linearLayout2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:layout_below="@id/login_linearLayout1"
    android:layout_marginTop="15dip"
    android:layout_centerHorizontal="true"
    >
    <TextView 
        android:id="@+id/login_psw_text"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/password"
        />
    <EditText 
        android:id="@+id/login_psw_et"
        android:layout_width="150dip"
        android:layout_height="35dip"
        android:textSize="15dip"
        />
</LinearLayout>

<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@id/login_linearLayout2"
    android:orientation="horizontal"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="20dip"
    >

    <RelativeLayout 
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginRight="20dip"
        >
        <ImageView 
            android:id="@+id/login_btn_image"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="@drawable/dlg_btn_bg"
            />
        <TextView 
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/login_btn"
            android:layout_centerInParent="true"
            />
    </RelativeLayout>
    
    <RelativeLayout 
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginRight="20dip"
        >
        <ImageView 
            android:id="@+id/register_btn_image"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:background="@drawable/dlg_btn_bg"
            />
        <TextView 
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/register_btn"
            android:layout_centerInParent="true"
            />
    </RelativeLayout>
    
    <TextView 
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/find_psw"
        />
</LinearLayout>
</RelativeLayout>java方法final AlertDialog dlg = new AlertDialog.Builder(context).create();
dlg.show();
Window window = dlg.getWindow();
// *** 主要就是在这里实现这种效果的.
// 设置窗口的内容页面,shrew_exit_dialog.xml文件中定义view内容
window.setContentView(R.layout.login_dlg);

usernameText = (EditText) window.findViewById(R.id.login_username_et);
usernameText.setOnTouchListener(new OnTouchListener() {

@Override
public boolean onTouch(View v, MotionEvent event) {
InputMethodManager m = (InputMethodManager)usernameText.getContext().getSystemService(Context.INPUT_METHOD_SERVICE);
                m.toggleSoftInput(0, InputMethodManager.SHOW_FORCED);
return false;
}
});