自定义控件(自定义属性、合理设计onMeasure、合理设计onDraw等)(转)
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在Android开发中,自定义控件是提升应用独特性和功能扩展性的重要手段。自定义控件涉及的关键知识点包括自定义属性、合理的`onMeasure`设计以及`onDraw`方法的优化。下面将详细阐述这些核心内容。 **自定义属性**是自定义控件时常常需要考虑的部分。Android提供了XML属性扩展机制,允许我们在自定义控件时添加特有的属性,以方便在布局文件中配置。这通常通过创建一个`attrs.xml`文件来定义自定义属性,例如: ```xml <resources> <declare-styleable name="CustomView"> <attr name="custom_color" format="color"/> <attr name="custom_size" format="dimension"/> </declare-styleable> </resources> ``` 接着,在自定义控件类中,我们需要通过`Context.obtainStyledAttributes()`方法获取到这些属性值,并进行解析和使用: ```java TypedArray attributes = context.obtainStyledAttributes(attrs, R.styleable.CustomView); int customColor = attributes.getColor(R.styleable.CustomView_custom_color, Color.TRANSPARENT); float customSize = attributes.getDimension(R.styleable.CustomView_custom_size, 0); attributes.recycle(); ``` **合理的`onMeasure`设计**对于自定义控件的尺寸控制至关重要。`onMeasure()`方法用于确定控件的大小,遵循MeasureSpec规则。MeasureSpec包含了父视图对子视图尺寸的要求。我们需要根据MeasureSpec提供的模式(EXACTLY, AT_MOST, UNSPECIFIED)和大小来计算自己的尺寸,并通过`setMeasuredDimension()`设置。例如: ```java @Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { int widthSize = MeasureSpec.getSize(widthMeasureSpec); int heightSize = MeasureSpec.getSize(heightMeasureSpec); // 计算并设置控件的实际尺寸 setMeasuredDimension(widthSize, heightSize); } ``` **优化`onDraw`方法**是确保自定义控件流畅显示的关键。`onDraw()`方法负责绘制控件的内容,需要高效地执行。尽量避免在此方法中进行复杂的计算或耗时的操作。我们可以使用`canvas`对象进行绘制,如绘制文本、线条、形状等。同时,注意使用`canvas.save()`和`canvas.restore()`来管理绘图状态,减少不必要的重绘。 ```java @Override protected void onDraw(Canvas canvas) { super.onDraw(canvas); // 在这里进行绘制操作 canvas.drawColor(customColor); canvas.drawText("Hello, Custom View!", 50, 50, paint); } ``` 为了进一步优化,还可以考虑实现`View`的`onDirty()`方法,当控件内容发生变化时手动调用此方法,触发重绘,而不是依赖系统的定时刷新。 总结,自定义控件涉及到自定义属性的定义与获取、`onMeasure`方法的正确实现以及`onDraw`方法的优化。理解并掌握这些知识点,能帮助开发者更好地创建满足需求的自定义视图,提升应用的用户体验。在实际项目中,还需要考虑性能优化、兼容性等问题,以确保自定义控件的稳定性和效率。
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