Android自定义View的用法总结

本文参考了：http://greenrobot.me/devpost/android-custom-layout/

Android SDK中提供了很多UI组件，如RelativeLayout， LinearLayout等，使用自定义控件有两大优点：

1、通过减少View的使用来增加UI的显示效率
2、构建SDK中没有的控件

原文总结了4种自定义View，分别是Composite View, Custom Composite View, Flat Custom View和Async Custom Views。示例代码在https://github.com/lucasr/android-layout-samples，可以直接运行。该工程依赖两个工程： Picasso  和 Smoothie .Picasso

 Picasso是一个异步图片加载库，Smoothie提供了异步加载ListView和GridView数据项的接口，使列表数据的加载更加顺滑。

本文只介绍Composite Vew 和 Custom Composite View的方法，这两种方式足够我们使用了，剩余两种方法需要自定义一套控制视图的框架，维护代价高，建议只用在app的核心且稳定的UI中，感兴趣的读者可自行研究。

Composite View
此方法是将多个View结合成一个可重用View的最简单方法，过程如下：
1、自定义控件，继承相应的控件。
2、在构造函数中填充一个merge布局
3、初始化自定义控件中的内部View

4、提供刷新View的接口

下面介绍了一个用法，该View的布局如下图所示：

首先是定义一个类文件TweetCompositeView.java

public class TweetCompositeView extends RelativeLayout implements TweetPresenter {
    private final ImageView mProfileImage;
    private final TextView mAuthorText;
    private final TextView mMessageText;
    private final ImageView mPostImage;
    private final EnumMap mActionIcons;


    public TweetCompositeView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }


    public TweetCompositeView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);


        LayoutInflater.from(context).inflate(R.layout.tweet_composite_view, this, true);
        //初始化内部成员变量
        mProfileImage = (ImageView) findViewById(R.id.profile_image);
        mAuthorText = (TextView) findViewById(R.id.author_text);
        mMessageText = (TextView) findViewById(R.id.message_text);
        mPostImage = (ImageView) findViewById(R.id.post_image);


        mActionIcons = new EnumMap(Action.class);
        for (Action action : Action.values()) {
            final ImageView icon;
            switch (action) {
                case REPLY:
                    icon = (ImageView) findViewById(R.id.reply_action);
                    break;


                case RETWEET:
                    icon = (ImageView) findViewById(R.id.retweet_action);
                    break;


                case FAVOURITE:
                    icon = (ImageView) findViewById(R.id.favourite_action);
                    break;


                default:
                    throw new IllegalArgumentException("Unrecognized tweet action");
            }


            mActionIcons.put(action, icon);
        }
    }


    @Override
    public boolean shouldDelayChildPressedState() {
        return false;
    }


    //提供更新UI的接口
    @Override
    public void update(Tweet tweet, EnumSet flags) {
        mAuthorText.setText(tweet.getAuthorName());
        mMessageText.setText(tweet.getMessage());


        final Context context = getContext();
        ImageUtils.loadImage(context, mProfileImage, tweet.getProfileImageUrl(), flags);


        final boolean hasPostImage = !TextUtils.isEmpty(tweet.getPostImageUrl());
        mPostImage.setVisibility(hasPostImage ? View.VISIBLE : View.GONE);
        if (hasPostImage) {
            ImageUtils.loadImage(context, mPostImage, tweet.getPostImageUrl(), flags);
        }
    }
}

该类继承自RelativeLayout，实现了TweetPresenter的接口以更新UI。构造函数中初始化内部的View

布局文件tweet_composite_view.xml中的merge tag减少了布局的层次

这种方法自定义的View用法简单，维护也方便。但这种方式自定义的View的UI子View较多，对于复杂的View，将影响遍历效率。打开手机设置中的显示布局边界选项，效果图如下所示：

Android某些控件如RelativeLayout，LinearLayout等容器控件，需要多次遍历子View来确定自身的属性，如LinearLayout的weight属性。如果能针对自己的App自定义子View的计算和定位逻辑，则可以极大的优化UI的遍历。这种做法便是接下来介绍的Custom Composite View

Custom Composite View

相比Composite View的方法，一个Custom Composite View继承自一个ViewGroup，并实现了onMeasure和onLayout方法。下面的TweetLayoutView便是一个Custom Composite View.

TweetLayoutView.java

public class TweetLayoutView extends ViewGroup implements TweetPresenter {
    private final ImageView mProfileImage;
    private final TextView mAuthorText;
    private final TextView mMessageText;
    private final ImageView mPostImage;
    private final EnumMap mActionIcons;

    public TweetLayoutView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public TweetLayoutView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);

        LayoutInflater.from(context).inflate(R.layout.tweet_layout_view, this, true);
        mProfileImage = (ImageView) findViewById(R.id.profile_image);
        mAuthorText = (TextView) findViewById(R.id.author_text);
        mMessageText = (TextView) findViewById(R.id.message_text);
        mPostImage = (ImageView) findViewById(R.id.post_image);

        mActionIcons = new EnumMap(Action.class);
        for (Action action : Action.values()) {
            final int viewId;
            switch (action) {
                case REPLY:
                    viewId = R.id.reply_action;
                    break;

                case RETWEET:
                    viewId = R.id.retweet_action;
                    break;

                case FAVOURITE:
                    viewId = R.id.favourite_action;
                    break;

                default:
                    throw new IllegalArgumentException("Unrecognized tweet action");
            }

            mActionIcons.put(action, findViewById(viewId));
        }
    }

    private void layoutView(View view, int left, int top, int width, int height) {
        MarginLayoutParams margins = (MarginLayoutParams) view.getLayoutParams();
        final int leftWithMargins = left + margins.leftMargin;
        final int topWithMargins = top + margins.topMargin;

        view.layout(leftWithMargins, topWithMargins,
                    leftWithMargins + width, topWithMargins + height);
    }

    private int getWidthWithMargins(View child) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
        return child.getWidth() + lp.leftMargin + lp.rightMargin;
    }

    private int getHeightWithMargins(View child) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
        return child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin;
    }

    private int getMeasuredWidthWithMargins(View child) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
        return child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin;
    }

    private int getMeasuredHeightWithMargins(View child) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
        return child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin;
    }

    @Override
    public boolean shouldDelayChildPressedState() {
        return false;
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        final int widthSize = MeasureSpec.getSize(widthMeasureSpec);

        int widthUsed = 0;
        int heightUsed = 0;

        measureChildWithMargins(mProfileImage,
                                widthMeasureSpec, widthUsed,
                                heightMeasureSpec, heightUsed);
        widthUsed += getMeasuredWidthWithMargins(mProfileImage);

        measureChildWithMargins(mAuthorText,
                                widthMeasureSpec, widthUsed,
                                heightMeasureSpec, heightUsed);
        heightUsed += getMeasuredHeightWithMargins(mAuthorText);

        measureChildWithMargins(mMessageText,
                                widthMeasureSpec, widthUsed,
                                heightMeasureSpec, heightUsed);
        heightUsed += getMeasuredHeightWithMargins(mMessageText);

        if (mPostImage.getVisibility() != View.GONE) {
            measureChildWithMargins(mPostImage,
                                    widthMeasureSpec, widthUsed,
                                    heightMeasureSpec, heightUsed);
            heightUsed += getMeasuredHeightWithMargins(mPostImage);
        }

        int maxIconHeight = 0;
        for (Action action : Action.values()) {
            final View iconView = mActionIcons.get(action);
            measureChildWithMargins(iconView,
                                    widthMeasureSpec, widthUsed,
                                    heightMeasureSpec, heightUsed);

            final int height = getMeasuredHeightWithMargins(iconView);
            if (height > maxIconHeight) {
                maxIconHeight = height;
            }

            widthUsed += getMeasuredWidthWithMargins(iconView);
        }
        heightUsed += maxIconHeight;

        int heightSize = heightUsed + getPaddingTop() + getPaddingBottom();
        setMeasuredDimension(widthSize, heightSize);
    }

    @Override
    protected void onLayout(boolean changed, int l, int t, int r, int b) {
        final int paddingLeft = getPaddingLeft();
        final int paddingTop = getPaddingTop();

        int currentTop = paddingTop;

        layoutView(mProfileImage, paddingLeft, currentTop,
                   mProfileImage.getMeasuredWidth(),
                   mProfileImage.getMeasuredHeight());

        final int contentLeft = getWidthWithMargins(mProfileImage) + paddingLeft;
        final int contentWidth = r - l - contentLeft - getPaddingRight();

        layoutView(mAuthorText, contentLeft, currentTop,
                   contentWidth, mAuthorText.getMeasuredHeight());
        currentTop += getHeightWithMargins(mAuthorText);

        layoutView(mMessageText, contentLeft, currentTop,
                contentWidth, mMessageText.getMeasuredHeight());
        currentTop += getHeightWithMargins(mMessageText);

        if (mPostImage.getVisibility() != View.GONE) {
            layoutView(mPostImage, contentLeft, currentTop,
                       contentWidth, mPostImage.getMeasuredHeight());

            currentTop += getHeightWithMargins(mPostImage);
        }

        final int iconsWidth = contentWidth / mActionIcons.size();
        int iconsLeft = contentLeft;

        for (Action action : Action.values()) {
            final View icon = mActionIcons.get(action);

            layoutView(icon, iconsLeft, currentTop,
                       iconsWidth, icon.getMeasuredHeight());
            iconsLeft += iconsWidth;
        }
    }

    @Override
    public LayoutParams generateLayoutParams(AttributeSet attrs) {
        return new MarginLayoutParams(getContext(), attrs);
    }

    @Override
    protected LayoutParams generateDefaultLayoutParams() {
        return new MarginLayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
    }

    @Override
    public void update(Tweet tweet, EnumSet flags) {
        mAuthorText.setText(tweet.getAuthorName());
        mMessageText.setText(tweet.getMessage());

        final Context context = getContext();
        ImageUtils.loadImage(context, mProfileImage, tweet.getProfileImageUrl(), flags);

        final boolean hasPostImage = !TextUtils.isEmpty(tweet.getPostImageUrl());
        mPostImage.setVisibility(hasPostImage ? View.VISIBLE : View.GONE);
        if (hasPostImage) {
            ImageUtils.loadImage(context, mPostImage, tweet.getPostImageUrl(), flags);
        }
    }
}

这个类的 布局文件仍然是 tweet_composite_view.xml，构造函数中初始化内部的View,与Composite View的不同之处在于，它通过重载onMeasure和onLayout方法来确定内部View的尺寸和位置。 基本思路是过程通过ViewGroup’s 的measureChildWithMargins() 方法和背后的 getChildMeasureSpec() 方法计算出了每个子视图的 MeasureSpec 。这个自定义View的效果图的布局层次如下图所示，和Composite View的层次一样，但这个View的遍历开销要少于前者。

 如果想进一步优化关键部分的UI，如ListView和GridView，可以考虑把Custom Composite View合成单一的View统一管理，使得到的View的层次如下图所示：

要达到这个效果，需要参考Flat Custom View的自定义View方式，刚兴趣读者可参考源代码。