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<Action, ImageView> 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<UpdateFlags> 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

<?xml version="1.0" encoding="utf-8"?><merge xmlns:android="http://schemas.android.com/apk/res/android"    android:layout_width="match_parent"    android:layout_height="match_parent">    <ImageView        android:id="@+id/profile_image"        android:layout_width="@dimen/tweet_profile_image_size"        android:layout_height="@dimen/tweet_profile_image_size"        android:layout_marginRight="@dimen/tweet_content_margin"        android:scaleType="centerCrop"/>    <TextView        android:id="@+id/author_text"        android:layout_width="fill_parent"        android:layout_height="wrap_content"        android:layout_toRightOf="@id/profile_image"        android:layout_alignTop="@id/profile_image"        android:textColor="@color/tweet_author_text_color"        android:textSize="@dimen/tweet_author_text_size"        android:singleLine="true"/>    <TextView        android:id="@+id/message_text"        android:layout_width="fill_parent"        android:layout_height="wrap_content"        android:layout_below="@id/author_text"        android:layout_alignLeft="@id/author_text"        android:textColor="@color/tweet_message_text_color"        android:textSize="@dimen/tweet_message_text_size"/>    <ImageView        android:id="@+id/post_image"        android:layout_width="fill_parent"        android:layout_height="@dimen/tweet_post_image_height"        android:layout_below="@id/message_text"        android:layout_alignLeft="@id/message_text"        android:layout_marginTop="@dimen/tweet_content_margin"        android:scaleType="centerCrop"/>    <LinearLayout android:layout_width="fill_parent"        android:layout_height="wrap_content"        android:layout_below="@id/post_image"        android:layout_alignLeft="@id/message_text"        android:layout_marginTop="@dimen/tweet_content_margin"        android:orientation="horizontal">        <ImageView            android:id="@+id/reply_action"            android:layout_width="0dp"            android:layout_height="@dimen/tweet_icon_image_size"            android:layout_weight="1"            android:src="@drawable/tweet_reply"            android:scaleType="fitStart"/>        <ImageView            android:id="@+id/retweet_action"            android:layout_width="0dp"            android:layout_height="@dimen/tweet_icon_image_size"            android:layout_weight="1"            android:src="@drawable/tweet_retweet"            android:scaleType="fitStart"/>        <ImageView            android:id="@+id/favourite_action"            android:layout_width="0dp"            android:layout_height="@dimen/tweet_icon_image_size"            android:layout_weight="1"            android:src="@drawable/tweet_favourite"            android:scaleType="fitStart"/>    </LinearLayout></merge>

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<Action, View> 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<UpdateFlags> 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。