3

随着新外观的 Android ADT(Android 开发者工具)有一些新的项目向导,它们比我习惯的旧的 hello world 模板更进一步,我试图理解它们,但发现它真的很困难。

我正在查看全屏项目向导。

enter code here当您使用此向导创建项目时,它将创建一个包含文件 SystemUIHider.java 的 util 库。

这是问题。

我懂了:

protected OnVisibilityChangeListener mOnVisibilityChangeListener = sDummyListener;

它调用了类文件底部的接口:

public interface OnVisibilityChangeListener {

        public void onVisibilityChange(boolean visible);
    }

我的问题是:

1)我认为必须扩展接口,那么这段代码有什么实际用途?

2)如果 OnVisibilityChangeListener 是一个接口而不是严格来说是一个类,这怎么可能?

 private static OnVisibilityChangeListener sDummyListener = new OnVisibilityChangeListener() {
        @Override
        public void onVisibilityChange(boolean visible) {
        }
    };

谢谢 !

package com.example.test2.util;

import android.app.Activity;
import android.os.Build;
import android.view.View;

public abstract class SystemUiHider {
    /**
     * When this flag is set, the
     * {@link android.view.WindowManager.LayoutParams#FLAG_LAYOUT_IN_SCREEN}
     * flag will be set on older devices, making the status bar "float" on top
     * of the activity layout. This is most useful when there are no controls at
     * the top of the activity layout.
     * <p>
     * This flag isn't used on newer devices because the <a
     * href="http://developer.android.com/design/patterns/actionbar.html">action
     * bar</a>, the most important structural element of an Android app, should
     * be visible and not obscured by the system UI.
     */
    public static final int FLAG_LAYOUT_IN_SCREEN_OLDER_DEVICES = 0x1;

    /**
     * When this flag is set, {@link #show()} and {@link #hide()} will toggle
     * the visibility of the status bar. If there is a navigation bar, show and
     * hide will toggle low profile mode.
     */
    public static final int FLAG_FULLSCREEN = 0x2;

    /**
     * When this flag is set, {@link #show()} and {@link #hide()} will toggle
     * the visibility of the navigation bar, if it's present on the device and
     * the device allows hiding it. In cases where the navigation bar is present
     * but cannot be hidden, show and hide will toggle low profile mode.
     */
    public static final int FLAG_HIDE_NAVIGATION = FLAG_FULLSCREEN | 0x4;

    /**
     * The activity associated with this UI hider object.
     */
    protected Activity mActivity;

    /**
     * The view on which {@link View#setSystemUiVisibility(int)} will be called.
     */
    protected View mAnchorView;

    /**
     * The current UI hider flags.
     * 
     * @see #FLAG_FULLSCREEN
     * @see #FLAG_HIDE_NAVIGATION
     * @see #FLAG_LAYOUT_IN_SCREEN_OLDER_DEVICES
     */
    protected int mFlags;

    /**
     * The current visibility callback.
     */
    protected OnVisibilityChangeListener mOnVisibilityChangeListener = sDummyListener;

    /**
     * Creates and returns an instance of {@link SystemUiHider} that is
     * appropriate for this device. The object will be either a
     * {@link SystemUiHiderBase} or {@link SystemUiHiderHoneycomb} depending on
     * the device.
     * 
     * @param activity The activity whose window's system UI should be
     *            controlled by this class.
     * @param anchorView The view on which
     *            {@link View#setSystemUiVisibility(int)} will be called.
     * @param flags Either 0 or any combination of {@link #FLAG_FULLSCREEN},
     *            {@link #FLAG_HIDE_NAVIGATION}, and
     *            {@link #FLAG_LAYOUT_IN_SCREEN_OLDER_DEVICES}.
     */
    public static SystemUiHider getInstance(Activity activity, View anchorView, int flags) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
            return new SystemUiHiderHoneycomb(activity, anchorView, flags);
        } else {
            return new SystemUiHiderBase(activity, anchorView, flags);
        }
    }

    protected SystemUiHider(Activity activity, View anchorView, int flags) {
        mActivity = activity;
        mAnchorView = anchorView;
        mFlags = flags;
    }

    /**
     * Sets up the system UI hider. Should be called from
     * {@link Activity#onCreate}.
     */
    public abstract void setup();

    /**
     * Returns whether or not the system UI is visible.
     */
    public abstract boolean isVisible();

    /**
     * Hide the system UI.
     */
    public abstract void hide();

    /**
     * Show the system UI.
     */
    public abstract void show();

    /**
     * Toggle the visibility of the system UI.
     */
    public void toggle() {
        if (isVisible()) {
            hide();
        } else {
            show();
        }
    }

    /**
     * Registers a callback, to be triggered when the system UI visibility
     * changes.
     */
    public void setOnVisibilityChangeListener(OnVisibilityChangeListener listener) {
        if (listener == null) {
            listener = sDummyListener;
        }

        mOnVisibilityChangeListener = listener;
    }

    /**
     * A dummy no-op callback for use when there is no other listener set.
     */
    private static OnVisibilityChangeListener sDummyListener = new OnVisibilityChangeListener() {
        @Override
        public void onVisibilityChange(boolean visible) {
        }
    };

    /**
     * A callback interface used to listen for system UI visibility changes.
     */
    public interface OnVisibilityChangeListener {
        /**
         * Called when the system UI visibility has changed.
         * 
         * @param visible True if the system UI is visible.
         */
        public void onVisibilityChange(boolean visible);
    }
}
4

1 回答 1

0
private static OnVisibilityChangeListener sDummyListener = new OnVisibilityChangeListener() {
        @Override
        public void onVisibilityChange(boolean visible) {
        // Implementation here.
        }
};

上面的代码片段创建并实例化了一个匿名类,该类OnVisibilityChangeListener通过提供接口中定义的函数的实现来实现。

匿名类旨在用于您需要不会被重用的特定实现的地方。也就是说,如果上面的类只在一个地方实例化,就没有理由给它一个名字,只需使用一个匿名类。通常,匿名类在使用它们的位置被内联实例化,例如:

protected OnVisibilityChangeListener mOnVisibilityChangeListener = new OnVisibilityChangeListener() {
        @Override
        public void onVisibilityChange(boolean visible) { }
};

或作为函数的参数,如以下代码片段:

something.setOnVisibilityChangeListener(new OnVisibilityChangeListener() {
        @Override
        public void onVisibilityChange(boolean visible) { }
});
于 2013-01-15T00:50:39.757 回答