背景

需要实现一个 号码脱敏的EditText，在编辑时动态脱敏。该功能可以用 TransformationMethod实现

TransformationMethod 使用注意点

public interface TransformationMethod
{
    /**
     * Returns a CharSequence that is a transformation of the source text --
     * for example, replacing each character with a dot in a password field.
     * Beware that the returned text must be exactly the same length as
     * the source text, and that if the source text is Editable, the returned
     * text must mirror it dynamically instead of doing a one-time copy.
     * The method should not return {@code null} unless {@code source}
     * is {@code null}.
     */
    public CharSequence getTransformation(CharSequence source, View view);

    /**
     * This method is called when the TextView that uses this
     * TransformationMethod gains or loses focus.
     */
    public void onFocusChanged(View view, CharSequence sourceText,
                               boolean focused, int direction,
                               Rect previouslyFocusedRect);
}

TransformationMethod 方法用于转换TextView 的显示，需要注意的一点，转换后的长度必须与 source 长度相等.，如果长度不等，会在绘制Text时崩溃情况。

java.lang.IndexOutOfBoundsException
        at android.view.GLES20Canvas.drawText(GLES20Canvas.java:888)
        at android.text.Layout.drawText(Layout.java:367)
        at android.widget.Editor.drawHardwareAccelerated(Editor.java:1514)
        at android.widget.Editor.onDraw(Editor.java:1439)
        at android.widget.TextView.onDraw(TextView.java:5780)
        at android.view.View.draw(View.java:15488)
        at android.view.View.updateDisplayListIfDirty(View.java:14377)