Andorid-foreground 解析

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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:

<?xml version="1.0" encoding="utf-8"?><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);  }}