新特性：Andorid-foreground 解析

Andorid-foreground 解析

---

foreground 前景色

　　foreground 也就是前景色，它与background相对应，顾名思义，它指定的drawable是在view视图的上方绘制的。

开发实例

1.实现遮罩层：

<FrameLayout
        android:id="@+id/id_frameLayout"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:foreground="#aaff0000"
        android:foregroundTintMode="src_in"
        >
        <TextView
            android:id="@+id/id_text"
            android:layout_width="100dp"
            android:layout_height="100dp"
            android:text="NO.1"
            android:gravity="center_horizontal|center_vertical"
            />
    FrameLayout>

实现效果

2.实现点击效果

　属性能设置为drawable，我们自然就想到了也可以使用 selector drawable，在点击时套上drawable来实现类似点击效果的功能

xml代码:

 <FrameLayout
        android:id="@+id/id_frameLayout"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@android:color/transparent"
        android:foreground="@drawable/selector_foreground"
        android:foregroundTintMode="src_in"
        >
        <TextView
            android:id="@+id/id_text"
            android:layout_width="100dp"
            android:layout_height="100dp"
            android:text="NO.1"
            android:gravity="center_horizontal|center_vertical"
            />
    FrameLayout>

drawable:

<selector xmlns:android="http://schemas.android.com/apk/res/android"
    android:enterFadeDuration="@android:integer/config_shortAnimTime"
    android:exitFadeDuration="@android:integer/config_mediumAnimTime"
    >
    <item android:state_pressed="true">
        <shape
            android:shape="rectangle">
            <solid android:color="#15000000"/>
        shape>
    item>
    <item android:drawable="@android:color/transparent">

    item>

selector>

当然别忘记了在代码中给 frameLayout 设置点击事件

实现效果：

缺陷

　　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。 

case R.styleable.View_foreground:
                    if (targetSdkVersion >= VERSION_CODES.M || this instanceof FrameLayout) {
                        setForeground(a.getDrawable(attr));
                    }
                    break;
                case R.styleable.View_foregroundGravity:
                    if (targetSdkVersion >= VERSION_CODES.M || this instanceof FrameLayout) {
                        setForegroundGravity(a.getInt(attr, Gravity.NO_GRAVITY));
                    }
                    break;

解决方案

　　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);
  }
}

原文地址：http://blog.csdn.net/zhuoxiuwu/article/details/50976145