Android Widgets 之 Space 使用详解

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简介

Space是一个轻量级View子类，可以用在布局中创建组件之间的间隙。

用法

  <android.support.v4.widget.Space      android:layout_width="match_parent"      android:layout_height="50dp" />

说明

/** * Space is a lightweight {@link View} subclass that may be used to create gaps between components * in general purpose layouts. */public class Space extends View {    public Space(Context context, AttributeSet attrs, int defStyle) {        super(context, attrs, defStyle);        if (getVisibility() == VISIBLE) {            setVisibility(INVISIBLE);        }    }    public Space(Context context, AttributeSet attrs) {        this(context, attrs, 0);    }    public Space(Context context) {        this(context, null);    }    /**     * Draw nothing.     *     * @param canvas an unused parameter.     */    @Override    public void draw(Canvas canvas) {    }    /**     * Compare to: {@link View#getDefaultSize(int, int)}     * If mode is AT_MOST, return the child size instead of the parent size     * (unless it is too big).     */    private static int getDefaultSize2(int size, int measureSpec) {        int result = size;        int specMode = MeasureSpec.getMode(measureSpec);        int specSize = MeasureSpec.getSize(measureSpec);        switch (specMode) {            case MeasureSpec.UNSPECIFIED:                result = size;                break;            case MeasureSpec.AT_MOST:                result = Math.min(size, specSize);                break;            case MeasureSpec.EXACTLY:                result = specSize;                break;        }        return result;    }    @Override    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {        setMeasuredDimension(                getDefaultSize2(getSuggestedMinimumWidth(), widthMeasureSpec),                getDefaultSize2(getSuggestedMinimumHeight(), heightMeasureSpec));    }}

查看源码会发现Space控件onDraw方法没有实现。

Space控件在布局中只占位置，而不去绘制渲染。所以在组件之前填充间隙效率会很高，有助于提高性能。正是因为onDraw()为空，对该 view 没有做任务绘制渲染，所以不能对 Space 设置背景色。

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