Android 自定义View实践之EditText字数的简单计

这是继承View类的子类来实现自定义View的例子。

写一个类似于微博的140字限制的简单文本计算器，实现的思路是继承TextView,绑定一个EditText，通过添加EditText的addTextChangedListener，对调用者提供当前自定义View的回调接口，实现超出字数的回调。

自定义属性如下：

<?xml version="1.0" encoding="utf-8"?>
  <resources>
    <declare-styleable name="CountCharTextView">
      <!--用户可输入的最大字数-->
      <attr name="maxChars" format="integer"></attr>
      <!--溢出字数提示-->
      <attr name="exceedTextColor" format="color"></attr>
    </declare-styleable>
  </resources>

继承TextView，实现以下CountCharTextView：

/**
 * 文本字数的简单计算器
 * Created by echoMu on 2017/2/27.
 */
public class CountCharTextView extends TextView {

  private int maxChars;
  private int defaultMaxChars = 500;
  private int exceedTextColor;
  private int defaultTextColor = Color.BLACK;

  private CountCharChangedListener listener;

  public interface CountCharChangedListener {
    void onCountChanged(int countRemaining, boolean hasExceededLimit);
  }

  public CountCharTextView(Context context) {
    this(context, null);
  }

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

  public CountCharTextView(Context context, AttributeSet attrs, int defStyleAttr) {
    super(context, attrs, defStyleAttr);

    init(context, attrs);
  }

  private void init(Context context, AttributeSet attrs) {
    setText(String.valueOf(maxLength));

    TypedArray t = context.obtainStyledAttributes(attrs, R.styleable.CountCharTextView);
    exceedTextColor = t.getColor(R.styleable.CountCharTextView_exceedTextColor, Color.RED);
    maxChars = t.getInteger(R.styleable.CountCharTextView_maxChars, defaultMaxChars);

    t.recycle();
  }

  public void setEditText(EditText targetEt) {
    targetEt.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence charSequence, int i, int i1, int i2) {

        }

        @Override
        public void onTextChanged(CharSequence charSequence, int i, int i1, int i2) {

        }

        @Override
        public void afterTextChanged(Editable editable) {
            String inputText = editable.toString();
            int leftChars = maxChars - inputText.length();
            boolean hasExceeded = inputText.length() > maxChars;
//                if (leftChars <= (Math.round(maxChars * 0.1))) {
            //剩下10%的字数，变色提示
            if (leftChars <= 0) {
                //达到最大字数限制，变色提示
                setTextColor(exceedTextColor);
            } else {
                setTextColor(defaultTextColor);
            }
            setText(String.valueOf(leftChars));
            if (listener != null) {
                listener.onCountChanged(leftChars, hasExceeded);
            } else {
                throw new NullPointerException("A CountCharChangedListener has not been set!");
            }

        }
    });
}

public void setCharCountChangedListener(CountCharChangedListener listener) {
    this.listener = listener;
}

public int getMaxChars() {
    return maxChars;
}

/**
 * 设置最大输入字符
 *
 * @param maxChars
 */
public void setMaxChars(int maxChars) {
    this.maxChars = maxChars;
}

public int getExceedTextColor() {
    return exceedTextColor;
}

/**
 * 设置溢出字符数文本颜色
 *
 * @param exceedTextColor
 */
public void setExceedTextColor(int exceedTextColor) {
    this.exceedTextColor = exceedTextColor;
}
}