关于foreground与background的区别

关于foreground与background的区别

先看字面翻译,background是背景色foreground 也就是前景色,也就是说foreground与background相对应，顾名思义，foreground指定的drawable是在view视图的上方绘制的。

先写一个简单的布局

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:gravity="center"
        android:text="QWERTYUIOPASDFGHJKLZXCVBNM" />
</FrameLayout>

我们在FrameLayout中包了一个TextView这时候FrameLayout既没有设置background，也没设置foreground。显示是这样的

这时候我们给FrameLayout加上android:background=”@color/colorPrimary”效果如下

我们再给FrameLayout加上android:foreground=”@color/colorAccent”效果变成这样：

这时候连TextView都不见了,所以可以发现一般可以用foreground来实现一些遮罩层效果.

对了,忘记说了,虽然Android在所有布局的基类 View 类中 就定义了 Foreground 这个属性，但是测试发现 运行时，只有FrameLayout布局上设置该属性才会生效。观察View的代码发现这样一段。它只针对是FrameLayout的实例做获取该styleable的操作。

解决方案如下:

模拟，FrameLayout的相关实现为TextView添加foreGround的代码功能

public class ForegroundTextView extends TextView {

  // UI
  private Drawable foreground;

  // Controller/logic fields
  private final Rect rectPadding = new Rect();
  private boolean foregroundPadding = false;
  private boolean foregroundBoundsChanged = false;
  private boolean backgroundAsForeground = false;

  // Constructors
  public ForegroundTextView(Context context) {
    super(context);
  }

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

  public ForegroundTextView(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);

    final TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.ForegroundLayout, defStyle, 0);

    final Drawable d = a.getDrawable(R.styleable.ForegroundLayout_foreground);
    foregroundPadding = a.getBoolean(R.styleable.ForegroundLayout_foregroundInsidePadding, false);

    backgroundAsForeground = a.getBoolean(R.styleable.ForegroundLayout_backgroundAsForeground, false);

    // Apply foreground padding for ninepatches automatically
    if (!foregroundPadding && getBackground() instanceof NinePatchDrawable) {
      final NinePatchDrawable npd = (NinePatchDrawable) getBackground();
      if (npd != null && npd.getPadding(rectPadding)) {
        foregroundPadding = true;
      }
    }

    final Drawable b = getBackground();
    if (backgroundAsForeground && b != null) {
      setForeground(b);
    } else if (d != null) {
      setForeground(d);
    }

    a.recycle();
  }

  /**
   * Supply a Drawable that is to be rendered on top of all of the child views in the layout.
   *
   * @param drawable The Drawable to be drawn on top of the children.
   */
  public void setForeground(Drawable drawable) {
    if (foreground != drawable) {
      if (foreground != null) {
        foreground.setCallback(null);
        unscheduleDrawable(foreground);
      }

      foreground = drawable;

      if (drawable != null) {
        setWillNotDraw(false);
        drawable.setCallback(this);
        if (drawable.isStateful()) {
          drawable.setState(getDrawableState());
        }
      } else {
        setWillNotDraw(true);
      }
      requestLayout();
      invalidate();
    }
  }

  /**
   * Returns the drawable used as the foreground of this layout. The foreground drawable,
   * if non-null, is always drawn on top of the children.
   *
   * @return A Drawable or null if no foreground was set.
   */
  public Drawable getForeground() {
    return foreground;
  }

  @Override
  protected void drawableStateChanged() {
    super.drawableStateChanged();

    if (foreground != null && foreground.isStateful()) {
      foreground.setState(getDrawableState());
    }
  }

  @Override
  protected boolean verifyDrawable(Drawable who) {
    return super.verifyDrawable(who) || (who == foreground);
  }

  @Override
  public void jumpDrawablesToCurrentState() {
    super.jumpDrawablesToCurrentState();

    if (foreground != null) {
      foreground.jumpToCurrentState();
    }
  }

  @Override protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    super.onSizeChanged(w, h, oldw, oldh);
    foregroundBoundsChanged = true;
  }

  @Override
  public void draw(Canvas canvas) {
    super.draw(canvas);

    if (foreground != null) {
      final Drawable foreground = this.foreground;

      if (foregroundBoundsChanged) {
        foregroundBoundsChanged = false;

        final int w = getRight() - getLeft();
        final int h = getBottom() - getTop();

        if (foregroundPadding) {
          foreground.setBounds(rectPadding.left, rectPadding.top, w - rectPadding.right, h - rectPadding.bottom);
        } else {
          foreground.setBounds(0, 0, w, h);
        }
      }
      foreground.draw(canvas);
    }
  }

  @TargetApi(Build.VERSION_CODES.LOLLIPOP) @Override public boolean onTouchEvent(MotionEvent e) {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
      if (e.getActionMasked() == MotionEvent.ACTION_DOWN) {
        if (foreground != null) {
          foreground.setHotspot(e.getX(), e.getY());
        }
      }
    }
    return super.onTouchEvent(e);
  }
}

以上代码来自:http://blog.csdn.net/zhuoxiuwu/article/details/50976145